
                                    
                   THE IBM PC PROGRAMMER'S GUIDE TO C
                                    
                                    
                                    
                               3rd Edition
                                    
                                    
                                    
                             Matthew Probert


                                    
                            COPYRIGHT NOTICE


This publication remains the property of Matthew Probert. License is
hereby given for this work to be freely distibuted in whole under the
proviso that credit is given to the author. Sections of this work may be
used and distributed without payment under the proviso that credit is
given to both this work and the author. Source code occuring in this work
may be used within commercial and non-commercial applications without
charge and without reference to the author.

 BIOGRAPHICAL NOTES


Matthew Probert is a software consultant working for his own firm,
Servile Software. He has been involved with micro-computer software
design and programming since the age of eighteen and has been involved
with the C programming language for the past ten years.

His educational background lies in the non-too distinguished honour of
having a nervous break down during his last year of secondary school
which resulted in a lack of formal qualifications. However, Matthew has
made up for it by achieving a complete recovery and has been studying
Psychology with particular attention to behaviourism and conversation
ever since.

His chequered career spans back twelve years during which time he has
trained people in the design and use of database management applications,
been called upon to design and implement structured methodologies and has
been a "good old fashioned" analyst programmer.

Matthew Probert is currently researching Artificial Intelligence with
particular reference to the application of natural language processing,
whereby a computer software package may decode written human language and
respond to it in an intelligent manner. He is also monitoring the
progress of facilitated communication amongst autistic and children with
severe learning and challenging behaviour and hopes one day to be able to
develope a computer based mechanism for true and reliable communication
between autistic people and the rest of society.


Matthew Probert can be contacted via
     Servile Software
     5 Longcroft Close
     Basingstoke
     Hampshire
     RG21 8XG
     England

     Telephone 01256 478576

                                    
                                 PREFACE

In 1992, an English software house, Servile Software published a paper
entitled "HOW TO C", which sought to introduce computer programmers to
the C programming language. That paper was written by Matthew Probert. A
follow up effort was "HOW TO CMORE", a document that was also published
by Servile Software. Now those two documents have been amalgamated and
thoroughly revamped to create this book. I have included loads of new
source code that can be lifted directly out of the text.

All the program listings have been typed in to the Turbo C compiler,
compiled and executed successfully before being imported into this
document.

I hope you enjoy my work, and more I hope that you learn to program in C.
It really is a great language, there can be no other language that gives
the computer the opportunity to live up to the old saying;


"To err is human, to make a complete balls up requires a computer!"



Warning!

This document is the result of over ten years experience as a software
engineer. This document contains professional source code that is not
intended for beginers.


                                    
                              INTRODUCTION


The major distinguishing features of the C programming language are;

 ú    block-structured flow-control constructs (typical of most high-level
      languages);
 ú    freedom to manipulate basic machine objects (eg: bytes) and to refer
      to them using any particular object view desired (typical of assembly-
      languages);
 ú    both high-level operations (eg: floating-point arithmetic) and low-
      level operations (which map closely onto machine-language instructions,
      thereby offering the means to code in an optimal, yet portable, manner).

This book sets out to describe the C programming language, as commonly
found with compilers for the IBM PC, to enable a computer programmer with
no previous knowledge of the C programming language to program in C using
the IBM PC including the ROM facilities provided by the PC and facilities
provided DOS.

It is assumed that the reader has access to a C compiler, and to the
documentation that accompanies it regarding library functions.

The example programs were written with Borland's Turbo C, most of the non-
standard facilities provided by Turbo C should be found in later releases
of Microsoft C.



Differences Between the Various Versions of C

The original C (prior to the definitive book by K&R) defined the
combination assignment operators (eg: +=, *=, etc.) backwards (ie: they
were written =+, =*, etc.).  This caused terrible confusion when a
statement such as

 x=-y;
was compiled - it could have meant

 x = x - y;
or

 x = (-y);
Ritchie soon spotted this ambiguity and changed the language to have
these operators written in the now-familiar manner (+=, *=, etc.).

The major variations, however, are between K&R C and ANSI C.  These can
be summarized as follows:

 ú    introduction of function prototypes in declarations; change of
      function definition preamble to match the style of prototypes;
 ú    introduction of the ellipsis ("...") to show variable-length
      function argument lists;
 ú    introduction of the keyword `void' (for functions not returning a
      value) and the type `void *' for generic pointer variables;
 ú    addition of string-merging, token-pasting and stringizing functions
      in the preprocessor;
 ú    addition of trigraph translation in the preprocessor;
 ú    addition of the `#pragma' directive and formalization of the
      `declared()' pseudofunction in the preprocessor;
 ú    introduction of multi-byte strings and characters to support non-
      English languages;
 ú    introduction of the `signed' keyword (to complement the `unsigned'
      keyword when used in integer declarations) and the unary plus (`+')
      operator.



C is a medium level language

The powerful facilities offered by C to allow manipulation of direct
memory addresses and data, even down to the bit level, along with C's
structured approach to programming cause C to be classified as a "medium
level" programming language. It possesses fewer ready made facilities
than a high level language, such as BASIC, but a higher level of
structure than low level Assembler.


Key words

The original C language as described in; "The C programming language", by
Kernighan and Ritchie, provided 27 key words. To those 27 the ANSI
standards committee on C have added five more. This confusingly results
in two standards for the C language. However, the ANSI standard is
quickly taking over from the old K & R standard.


The 32 C key words are;

auto              double             int                struct
break             else               long               switch
case              enum               register           typedef
char              extern             return             union
const             float              short              unsigned
continue          for                signed             void
default           goto               sizeof             volatile
do                if                 static             while

Some C compilers offer additional key words specific to the hardware
environment that they operate on. You should be aware of your own C
compilers additional key words. Most notably on the PC these are;


near      far        huge



Structure

C programs are written in a structured manner. A collection of code
blocks are created that call each other to comprise the complete program.
As a structured language C provides various looping and testing commands
such as;


           do-while,  for, while, if

and the use of jumps, while provided for, are rarely used.

A C code block is contained within a pair of curly braces "{ }", and may
be a complete procedure, in C terminology called a "function", or a
subset of code within a function. For example the following is a code
block. The statements within the curly braces are only executed upon
satisfaction of the condition that "x < 10";


if (x < 10)
{
     a = 1;
     b = 0;
}

while this, is a complete function code block containing a sub code block
as a do-while loop;

int GET_X()
{
     int x;

     do
     {
          printf("\nEnter a number between 0 and 10 ");
          scanf("%d",&x);
     }
     while(x < 0 || x > 10);
     return(x);
}

Notice how every statement line is terminated in a semicolon, unless that
statement marks the start of a code block, in which case it is followed
by a curly brace. C is a case sensitive but free flow language, spaces
between commands are ignored, and therefore the semicolon delimiter is
required to mark the end of the command line.

Having a freeflow structure the following commands are recognised as the
same by the C compiler;


     x = 0;
     x      =0;
     x=0;


The general form of a C program is as follows;

     compiler preprocessor statements
     global data declarations
     
     
     
     
     return-type main(parameter list)
     {
          statements
     }
     
     return-type f1(parameter list)
     {
          statements
     }
     
     return-type f2(parameter list)
     {
          statements
     }
     .
     .
     .
     return-type fn(parameter list)
     {
          statements
     }
     


Comments

C allows comments to be included in the program. A comment line is
defined by being enclosed within "/*" and "*/". Thus the following is a
comment;


/* This is a legitimate C comment line */


Libraries

C programs are compiled and combined with library functions provided with
the C compiler. These libraries are of generally standard functions, the
functionality of which are defined in the ANSI standard of the C
language, but are provided by the individual C compiler manufacturers to
be machine dependant. Thus, the standard library function "printf()"
provides the same facilities on a DEC VAX as on an IBM PC, although the
actual machine language code in the library is quite different for each.
The C programmer however, does not need to know about the internals of
the libraries, only that each library function will behave in the same
way on any computer.


                                    
                               DATA TYPES


There are four basic types of data in the C language; character, integer,
floating point, and valueless that are referred to by the C key words;

"char", "int", "float" and "void" respectively.

To the basic data types may be added the type modifiers; signed,
unsigned, long and short to produce further data types. By default data
types are assumed signed, and the signed modifier is rarely used, unless
to overide a compiler switch defaulting a data type to unsigned.

The size of each data type varies from one hardware platform to another,
but the least range of values that can be held is described in the ANSI
standard as follows;


Type                     Size                     Range
                                                  
char                     8                        -127 to 127
unsigned char            8                        0 to 255
int                      16                       -32767 to 32767
unsigned int             16                       0 to 65535
long int                 32                       -2147483647 to
                                                  2147483647
unsigned long int        32                       0 to 4294967295
float                    32                       Six digit precision
double                   64                       Ten digit precision
long double              80                       Ten digit precision


In practice, this means that the data type `char' is particularly
suitable for storing flag type variables, such as status codes, which
have a limited range of values. The `int' data type can be used, but if
the range of values does not exceed 127 (or 255 for an unsigned char),
then each declared variable would be wasting storage space.

Which real number data type to use: `float', `double' or `long double' is
another tricky question. When numeric accuracy is required, for example
in an accounting application, the instinct would be to use the `long
double', but this requires at least 10 bytes of storage space for each
variable. And real numbers are not as precise as integers anyway, so
perhaps one should use integer data types instead and work around the
problem. The data type `float' is worse than useless since its six digit
precision is too inaccurate to be relied upon. Generally, then, you
should use integer data types where ever possible, but if real numbers
are required use at least a `double'.


Declaring a variable

All variables in a C program must be declared before they can be used.
The general form of a variable definition is;


     type name;

So, for example to declare a variable "x", of data type "int" so that it
may store a value in the range -32767 to 32767, you use the statement;


     int x;

Character strings may be declared, which are in reality arrays of
characters. They are declared as follows;


     char name[number_of_elements];

So, to declare a string thirty characters long, and called `name' you
would use the declaration;


     char name[30];


Arrays of other data types also may be declared in one, two or more
dimensions in the same way. For example to declare a two dimensional
array of integers;


     int x[10][10];

The elements of this array are then accessed as;

     x[0][0]
     x[0][1]
     x[n][n]

There are three levels of access to variable; local, module and global. A
variable declared within a code block is only known to the statements
within that code block. A variable declared outside any function code
blocks but prefixed with the storage modifier "static" is known only to
the statements within that source module. A variable declared outside any
functions and not prefixed with the static storage type modifier may be
accessed by any statement within any source module of the program.


For example;

     int error;
     static int a;
     
     main()
     {
          int x;
          int y;
     
     }
     
     funca()
     {
          /* Test variable 'a' for equality with 0 */
          if (a == 0)
          {
               int b;
               for(b = 0; b < 20; b++)
                    printf("\nHello World");
          }
     
     }
     

In this example the variable `error' is accessible by all source code
modules compiled together to form the finished program. The variable `a'
is accessible by statements in both functions `main()' and `funca()', but
is invisible to any other source module. Variables `x' and `y' are only
accessible by statements within function `main()'. The variable `b' is
only accessible by statements within the code block following the `if'
statement.

If a second source module wished to access the variable `error' it would
need to declare `error' as an `extern' global variable thus;


     extern int error;
     
     funcb()
     {
     }
     
C will quite happily allow you, the programmer, to assign different data
types to each other. For example, you may declare a variable to be of
type `char' in which case a single byte of data will be allocated to
store the variable. To this variable you can attempt to allocate larger
values, for example;


     main()
     {
     
          x = 5000;
     
     }
     
In this example the variable `x' can only store a value between -127 and
128, so the figure 5000 will NOT be assigned to the variable `x'. Rather
the value 136 will be assigned!


Often you may wish to assign different data types to each other, and to
prevent the compiler from warning you of a possible error you can use a
cast to tell the compiler that you know what you're doing. A cast
statement is a data type in parenthesis preceding a variable or
expression;


     main()
     {
          float x;
          int y;
     
          x = 100 / 25;
     
          y = (int)x;
     }

In this example the (int) cast tells the compiler to convert the value of
the floating point variable x to an integer before assigning it to the
variable y.



Formal parameters

A C function may receive parameters from a calling function. These
parameters are declared as variables within the parentheses of the
function name, thus;


     int MULT(int x, int y)
     {
          /* Return parameter x multiplied by parameter y */
          return(x * y);
     }
     
     main()
     {
          int a;
          int b;
          int c;
     
          a = 5;
          b = 7;
          c = MULT(a,b);
     
          printf("%d multiplied by %d equals %d\n",a,b,c);
     }
     

Access modifiers

There are two access modifiers; `const' and `volatile'. A variable
declared to be `const' may not be changed by the program, whereas a
variable declared as type  as type `volatile' may be changed by the
program. In addition, declaring a variable to be volatile prevents the C
compiler from allocating the variable to a register, and reduces the
optimization carried out on the variable.



Storage class types
C provides four storage types; `extern', `static', `auto' and `register'.

The extern storage type is used to allow a source module within a C
program to access a variable declared in another source module.

Static variables are only accessible within the code block that declared
them, and additionally if the variable is local, rather than global, they
retain their old value between subsequent calls to the code block.

Register variables are stored within CPU registers where ever possible,
providing the fastest possible access to their values.

The auto type variable is only used with local variables, and declares
the variable to retain its value locally only. Since this is the default
for local variables the auto storage type is very rarely used.
                                    
                                    
                                    
                                OPERATORS

Operators are tokens that cause a computation to occur when applied to
variables. C provides the following operators;


&                  Address
*                  Indirection
+                  Unary plus
-                  Unary minus
~                  Bitwise compliment
!                  Logical negation
++                 As a prefix;
                   preincrement
                   As a suffix;
                   postincrement
--                 As a prefix;
                   predecrement
                   As a suffix;
                   postdecrement
+                  Addition
-                  Subtraction
*                  Multiply
/                  Divide
%                  Remainder
<<                 Shift left
>>                 Shift right
&                  Bitwise AND
|                  Bitwise OR
^                  Bitwise XOR
&&                 Logical AND
||                 Logical OR
=                  Assignment
*=                 Assign product
/=                 Assign quotient
%=                 Assign remainder
                   (modulus)
+=                 Assign sum
-=                 Assign difference
<<=                Assign left shift
>>=                Assign right shift
&=                 Assign bitwise AND
|=                 Assign bitwise OR
^=                 Assign bitwise XOR
<                  Less than
>                  Greater than
<=                 Less than or equal
                   to
>=                 Greater than or
                   equal to
==                 Equal to
!=                 Not equal to
.                  Direct component
                   selector
->                 Indirect component
                   selector
a ? x:y            "If a is true then
                   x else y"
[]                 Define arrays
()                 Parenthesis
                   isolate conditions
                   and expressions
...                Ellipsis are used
                   in formal
                   parameter lists of
                   function
                   prototypes to show
                   a variable number
                   of
                   parameters or
                   parameters of
                   varying types.

To illustrate some more commonly used operators consider the following
short program;


     main()
     {
          int a;
          int b;
          int c;
          a = 5;           /* Assign a value of 5 to variable 'a' */
          b = a / 2;       /* Assign the value of 'a' divided by two to
     variable 'b' */
          c = b * 2;       /* Assign the value of 'b' multiplied by two
     to variable 'c' */
     
          if (a == c)     /* Test if 'a' holds the same value as 'c' */
     
               puts("Variable 'a' is an even number");
          else
               puts("Variable 'a' is an odd number");
     }
     
     
Normally when incrementing the value of a variable you would write
something like;

          x = x + 1

C provides the incremental operator '++' as well so that you can write;

          x++

Similarly you can decrement the value of a variable using '--' as;

          x--

All the other mathematical operators may be used the same, so in a C
program you can write in shorthand;


NORMAL                               C
                                     
x = x + 1                            x++
x = x - 1                            x--
x = x * 2                            x *= 2
x = x / y                            x /= y
x = x % 5                            x %= 5

and so on.
                                    
                                    
                                    
                                FUNCTIONS

Functions are the source code procedures that comprise a C program. They
follow the general form;

     return_type function_name(parameter_list)
     {
          statements
     }


The return_type specifies the data type that will be returned by the
function; char, int, double, void &c.

The code within a C function is invisible to any other C function, and
jumps may not be made from one function into the middle of another,
although functions may call other functions. Also, functions cannot be
defined within functions, only within source modules.

Parameters may be passed to a function either by value, or by reference.
If a parameter is passed by value, then only a copy of the current value
of the parameter is passed to the function. A parameter passed by
reference however, is a pointer to the actual parameter that may then be
changed by the function.


The following example passes two parameters by value to a function,
funca(), which attempts to change the value of the variables passed to
it. And then passes the same two parameters by reference to funcb() which
also attempts to modify their values.


     #include <stdio.h>
     
     int funca(int x, int y)
     {
          /* This function receives two parameters by value, x and y */
     
          x = x * 2;
          y = y * 2;
     
          printf("\nValue of x in funca() %d value of y in funca()
     %d",x,y);
     
          return(x);
     }
     
     
     int funcb(int *x, int *y)
     {
          /* This function receives two parameters by reference, x and y
     */
     
          *x = *x * 2;
          *y = *y * 2;
     
          printf("\nValue of x in funcb() %d value of y in funcb()
     %d",*x,*y);
     
          return(*x);
     }
     
     main()
     {
          int x;
          int y;
          int z;
     
          x = 5;
          y = 7;
     
          z = funca(x,y);
          z = funcb(&x,&y);
     
          printf("\nValue of x %d value of y %d value of z %d",x,y,z);
     }
     

Actually funcb() does not change the values of the parameters it
receives.  Rather it changes the contents of the memory addresses pointed
to by the received parameters. While funca() receives the values of
variables `x' and `y' from function main(), funcb() receives the memory
addresses of the variables `x' and `y' from function main().



Passing an array to a function

The following program passes an array to a function, funca(), which
initialises the array elements;

     #include <stdio.h>
     
     void funca(int x[])
     {
          int n;
     
          for(n = 0; n < 100; n++)
          x[n] = n;
     }
     
     main()
     {
          int array[100];
          int counter;
     
          funca(array);
     
          for(counter = 0; counter < 100; counter++)
               printf("\nValue of element %d is
     %d",counter,array[counter]);
     }

The parameter of funca() `int x[]' is declared to be an array of any
length.  This works because the compiler passes the address of the start
of the array parameter to the function, rather than the value of the
individual elements.  This does of course mean that the function can
change the value of the array elements. To prevent a function from
changing the values you can specify the parameter as type `const';


     funca(const int x[])
     {
     }

This will then generate a compiler error at the line that attempts to
write a value to the array. However, specifying a parameter to be const
does not protect the parameter from indirect assignment as the following
program illustrates;


     #include <stdio.h>
     
     int funca(const int x[])
     {
          int *ptr;
          int n;
     
          /* This line gives a 'suspicious pointer conversion warning' */
          /* because x is a const pointer, and ptr is not */
          ptr = x;
     
          for(n = 0; n < 100; n++)
          {
               *ptr = n;
               ptr++;
          }
     }
     
     main()
     {
          int array[100];
          int counter;
     
          funca(array);
     
          for(counter = 0; counter < 100; counter++)
               printf("\nValue of element %d is
     %d",counter,array[counter]);
     }
     


Passing parameters to main()

C allows parameters to be passed from the operating system to the program
when it starts executing through two parameters; argc and argv[], as
follows;


     #include <stdio.h>
     
     main(int argc, char *argv[])
     {
          int n;
     
          for(n = 0; n < argc; n++)
          printf("\nParameter %d equals %s",n,argv[n]);
     }
     

Parameter argc holds the number of parameters passed to the program, and
the array argv[] holds the addresses of each parameter passed. argv[0] is
always the program name. This feature may be put to good use in
applications that need to access system files. Consider the following
scenario:

A simple database application stores its data in a single file called
"data.dat". The application needs to be created so that it may be stored
in any directory on either a floppy diskette or a hard disk, and executed
both from within the host directory and through a DOS search path. To
work correctly the application must always know where to find the data
file; "data.dat". This is solved by assuming that the data file will be
in the same directory as the executable module, a not unreasonable
restriction to place upon the operator. The following code fragment then
illustrates how an application may apply this algorithm into practice to
be always able to locate a desired system file:


     #include <string.h>
     
     char system_file_name[160];
     
     void main(int argc,char *argv[])
     {
          char *data_file = "DATA.DAT";
          char *p;
     
          strcpy(system_file_name,argv[0]);
          p = strstr(system_file_name,".EXE");
          if (p == NULL)
          {
               /* The executable is a .COM file */
             p = strstr(system_file_name,".COM");
          }
     
          /* Now back track to the last '\' character in the file name */
          while(*(p - 1) != '\\')
               p--;
     
          strcpy(p,data_file);
     }

In practice this code creates a string in system_file_name that is
comprised of path\data.dat, so if for example the executable file is
called "test.exe" and resides in the directory \borlandc, then
system_file_name will be assigned with: \borlandc\data.dat



Returning from a function

The command `return' is used to return immediately from a function. If
the function was declared with a return data type, then return should be
used with a parameter of the same data type.



Function prototypes

Prototypes for functions allow the C compiler to check that the type of
data being passed to and from functions is correct. This is very
important to prevent data overflowing its allocated storage space into
other variables areas.

A function prototype is placed at the beginning of the program, after any
preprocessor commands, such as #include <stdio.h>, and before the
declaration of any functions.
                                    
                           THE C PREPROCESSOR

C allows for commands to the compiler to be included in the source code.
These commands are then called preprocessor commands and are defined by
the ANSI standard to be;


    #if
    #ifdef
    #ifndef
    #else
    #elif
    #include
    #define
    #undef
    #line
    #error
    #pragma

All preprocessor commands start with a hash symbol, "#", and must be on a
line on their own (although comments may follow).



#define

The #define command specifies an identifier and a string that the
compiler will substitute every time it comes accross the identifier
within that source code module. For example;


#define FALSE 0
#define TRUE !FALSE

The compiler will replace any subsequent occurence of `FALSE' with `0'
and any subsequent occurence of `TRUE' with `!0'. The substitution does
NOT take place if the compiler finds that the identifier is enclosed by
quotation marks, so


     printf("TRUE");

would NOT be replaced, but

     printf("%d",FALSE);

would be.

The #define command also can be used to define macros that may include
parameters. The parameters are best enclosed in parenthesis to ensure
that correct substitution occurs.


This example declares a macro `larger()' that accepts two parameters and
returns the larger of the two;

     #include <stdio.h>
     
     #define larger(a,b) (a > b) ? (a) : (b)
     
     int main()
     {
          printf("\n%d is largest",larger(5,7));
     
     }


#error

The #error command causes the compiler to stop compilation and to display
the text following the #error command. For example;


#error REACHED MODULE B

will cause the compiler to stop compilation and display;

     REACHED MODULE B



#include

The #include command tells the compiler to read the contents of another
source file. The name of the source file must be enclosed either by
quotes or by angular brackets thus;


     #include "module2.c"
     #include <stdio.h>

Generally, if the file name is enclosed in angular brackets, then the
compiler will search for the file in a directory defined in the
compiler's setup. Whereas if the file name is enclosed in quotes then the
compiler will look for the file in the current directory.



#if, #else, #elif, #endif

The #if set of commands provide conditional compilation around the
general form;

     #if constant_expression
          statements
     #else
          statements
     #endif

#elif stands for '#else if' and follows the form;

     #if expression
          statements
     #elif expression
          statements
     #endif



#ifdef, #ifndef

These two commands stand for '#if defined' and '#if not defined'
respectively and follow the general form;

     #ifdef macro_name
          statements
     #else
          statements
     #endif

     #ifndef macro_name
          statements
     #else
          statements
     #endif

where 'macro_name' is an identifier declared by a #define statement.



#undef

Undefines a macro previously defined by #define.


#line

Changes the compiler declared global variables __LINE__ and __FILE__. The
general form of #line is;

     #line number "filename"

where number is inserted into the variable '__LINE__' and 'filename' is
assigned to '__FILE__'.


#pragma

This command is used to give compiler specific commands to the compiler.
The compiler's manual should give you full details of any valid options
to go with the particular implementation of #pragma that it supports.
                                    
                       PROGRAM CONTROL STATEMENTS

As with any computer language, C includes statements that test the
outcome of an expression. The outcome of the test is either TRUE or
FALSE. The C language defines a value of TRUE as non-zero, and FALSE as
zero.



Selection statements

The general purpose selection statement is "if" that follows the general
form;

          if (expression)
               statement
          else
               statement

Where "statement" may be a single statement, or a code block enclosed in
curly braces. The "else" is optional. If the result of the expression
equates to TRUE, then the statement(s) following the if() will be
evaluated.  Otherwise the statement(s) following the else, if there is
one, will be evaluated.


An alternative to the if....else combination is the ?: command that takes
the form;


           expression ? true_expression : false_expression

Where if the expression evaluates to TRUE, then the true_expression will
be evaluated, otherwise the false_expression will be evaluated. Thus we
get;


     #include <stdio.h>
     
     main()
     {
          int x;
     
          x = 6;
     
          printf("\nx is an %s number", x % 2 == 0 ? "even" : "odd");
     }
     
C also provides a multiple branch selection statement, switch, which
successively tests a value of an expression against a list of values and
branches program execution to the first match found. The general form of
switch is;


     switch (expression)
     {
          case value1 :  statements
                    break;
          case value2 :  statements
                    break;
          .
          .
          .
          .
          case valuen :  statements
                    break;
          default :      statements
     }

The break statement is optional, but if omitted, program execution will
continue down the list.

     #include <stdio.h>
     
     main()
     {
          int x;
     
          x = 6;
     
          switch(x)
          {
               case 0 : printf("\nx equals zero");
                     break;
               case 1 : printf("\nx equals one");
                     break;
               case 2 : printf("\nx equals two");
                     break;
               case 3 : printf("\nx equals three");
                     break;
               default : printf("\nx is larger than three");
          }
     }

Switch statements may be nested within one another. This is a
particularly useful feature for confusing people who read your source
code!


Iteration statements
C provides three looping or iteration statements; for, while, and do-
while. The for loop has the general form;


     for(initialization;condition;increment)

and is useful for counters such as in this example that displays the
entire ascii character set;

     #include <stdio.h>
     
     main()
     {
          int x;
     
          for(x = 32; x < 128; x++)
               printf("%d\t%c\t",x,x);
     }
     
An infinite for loop is also quite valid;

     for(;;)
     {
          statements
     }

Also, C allows empty statements. The following for loop removes leading
spaces from a string;

     for(; *str == ' '; str++)
          ;

Notice the lack of an initializer, and the empty statement following the
loop.

The while loop is somewhat simpler than the for loop and follows the
general form;

     while (condition)
          statements

The statement following the condition, or statements enclosed in curly
braces will be executed until the condition is FALSE. If the condition is
false before the loop commences, the loop statements will not be
executed. The do-while loop on the other hand is always executed at least
once. It takes the general form;


     do
     {
          statements
     }
     while(condition);


Jump statements

The "return" statement is used to return from a function to the calling
function. Depending upon the declared return data type of the function it
may or may not return a value;


     int MULT(int x, int y)
     {
          return(x * y);
     }

or;

     void FUNCA()
     {
          printf("\nHello World");
          return;
     }

The "break" statement is used to break out of a loop or from a switch
statement. In a loop it may be used to terminate the loop prematurely, as
shown here;


     #include <stdio.h>
     
     main()
     {
          int x;
     
          for(x = 0; x < 256; x++)
          {
               if (x == 100)
                    break;
     
               printf("%d\t",x);
          }
     }

In contrast to "break" is "continue", which forces the next iteration of
the loop to occur, effectively forcing program control back to the loop
statement.


C provides a function for terminating the program prematurely, "exit()".
Exit() may be used with a return value to pass back to the calling
program;


     exit(return_value);


More About ?:


A powerful, but often misunderstood feature of the C programming language
is ?:. This is an operator that acts upon a boolean expression, and
returns one of two values dependant upon the result of the expression;


     <boolean expression> ? <value for true> : <value for false>

It can be used almost anywhere, for example it was used in the binary
search demonstration program;

     printf("\n%s\n",(result == 0) ? "Not found" : "Located okay");

Here it passes either "Not found" or "Located okay" to the printf()
function dependant upon the outcome of the boolean expression `result ==
0'. Alternatively it can be used for assigning values to a variable;

     x = (a  == 0) ? (b) : (c);

Which will assign the value of b to variable x if a is equal to zero,
otherwise it will assign the value of c to variable x.

This example returns the name of the executing program, without any path
description;

     #include <stdio.h>
     #include <stddef.h>
     #include <string.h>
     
     char *progname(char *pathname)
     {
          /* Return name of running program */
          unsigned l;
          char *p;
          char *q;
          static char bnbuf[256];
     
          return pathname? p = strrchr (pathname, '\\'),
                     q = strrchr (pathname, '.'),
                     l = (q == NULL? strchr (pathname, '\0'): q)
                       - (p == NULL? p = pathname: ++p),
                     strncpy (bnbuf, p, l),
                     bnbuf[l] = '\0',
                     strlwr (bnbuf)
                  : NULL;
     }
     
     void main(int argc, char *argv[])
     {
          printf("\n%s",progname(argv[0]));
     }
     
     

Continue

The continue keyword forces control to jump to the test statement of the
innermost loop (while, do...while()). This can be useful for terminating
a loop gracefuly as this program that reads strings from a file until
there are no more illustrates;


     #include <stdio.h>
     
     void main()
     {
          FILE *fp;
          char *p;
          char buff[100];
     
          fp = fopen("data.txt","r");
          if (fp == NULL)
          {
               fprintf(stderr,"Unable to open file data.txt");
               exit(0);
          }
     
          do
          {
               p = fgets(buff,100,fp);
               if (p == NULL)
                    /* Force exit from loop */
                    continue;
               puts(p);
          }
          while(p);
     }
     
With a for() loop however, continue passes control back to the third
parameter!
                                    
                            INPUT AND OUTPUT



Input
Input to a C program may occur from the console, the standard input
device (unless otherwise redirected this is the console), from a file or
from a data port.

The general input command for reading data from the standard input stream
`stdin' is scanf(). Scanf() scans a series of input fields, one character
at a time. Each field is then formatted according to the appropriate
format specifier passed to the scanf() function as a parameter. This
field is then stored at the ADDRESS passed to scanf() following the
format specifiers list.

For example, the following program will read a single integer from the
stream stdin;


     main()
     {
          int x;
     
          scanf("%d",&x);
     }

Notice the address operator & prefixing the variable name `x' in the
scanf() parameter list. This is because scanf() stores values at
ADDRESSES rather than assigning values to variables directly.

The format string is a character string that may contain three types of
data:

whitespace characters (space, tab and newline), non-whitespace characters
(all ascii characters EXCEPT %) and format specifiers.

Format specifiers have the general form;


     %[*][width][h|l|L]type_character

After the % sign the format specifier is comprised of:

 an optional assignment suppression character, *, which suppresses
 assignment of the next input field.
 
 an optional width specifier, width, which declares the maximum number
 of characters to be read.
 
 an optional argument type modifier, h or l or L, where:
           h is a short integer
           l is a long
           L is a long double

     the data type character that is one of;


d      Decimal integer
D      Decimal long integer
o      Octal integer
O      Octal long integer
i      Decimal, octal or hexadecimal integer
I      Decimal, octal or hexadecimal long
       integer
u      Decimal unsigned integer
U      Decimal unsigned long integer
x      Hexadecimal integer
X      Hexadecimal long integer
e      Floating point
f      Floating point
g      Floating point
s      Character string
c      Character
%      % is stored

An example using scanf();


     #include <stdio.h>
     
     main()
     {
          char name[30];
          int age;
     
          printf("\nEnter your name and age ");
          scanf("%30s%d",name,&age);
          printf("\n%s %d",name,age);
     }

Notice the include line, "#include <stdio.h>", this is to tell the
compiler to also read the file stdio.h that contains the function
prototypes for scanf() and printf().


If you type in and run this sample program you will see that only one
name can be entered, that is you can't enter;


     JOHN SMITH

because scanf() detects the whitespace between "JOHN" and "SMITH" and
moves on to the next input field, which is age, and attempts to assign
the value "SMITH" to the age field! The limitations of scanf() as an
input function are obvious.

An alternative input function is gets() that reads a string of characters
from the stream stdin until a newline character is detected. The newline
character is replaced by a null (0 byte) in the target string. This
function has the advantage of allowing whitespace to be read in. The
following program is a modification to the earlier one, using gets()
instead of scanf().


     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>
     
     main()
     {
          char data[80];
          char *p;
          char name[30];
          int age;
     
          printf("\nEnter your name and age ");
          /* Read in a string of data */
          gets(data);
     
     
          /* P is a pointer to the last character in the input string */
          p = &data[strlen(data) - 1];
     
          /* Remove any trailing spaces by replacing them with null bytes
     */
          while(*p == ' '){
               *p = 0;
               p--;
          }
     
          /* Locate last space in the string */
          p = strrchr(data,' ');
     
          /* Read age from string and convert to an integer */
          age = atoi(p);
     
          /* Terminate data string at start of age field */
          *p = 0;
     
          /* Copy data string to name variable */
          strcpy(name,data);
     
          /* Display results */
          printf("\nName is %s age is %d",name,age);
     }


Output
The most common output function is printf() that is very similar to
scanf() except that it writes formatted data out to the standard output
stream stdout.  Printf() takes a list of output data fields and applies
format specifiers to each and outputs the result. The format specifiers
are the same as for scanf() except that flags may be added to the format
specifiers. These flags include;


     - Which left justifies the output padding to the right with spaces.
     + Which causes numbers to be prefixed by their sign

The width specifier is also slightly different for printf(). In its most
useful form is the precision specifier;


     width.precision

So, to print a floating point number to three decimal places you would
use;

     printf("%.3f",x);

And to pad a string with spaces to the right or truncate the string to
twenty characters if it is longer, to form a fixed twenty character
width;

     printf("%-20.20s",data);

Special character constants may appear in the printf() parameter list.
These are;

\n         Newline
\r         Carriage return
\t         Tab
\b         Sound the computer's bell
\f         Formfeed
\v         Vertical tab
\\         Backslash character
\'         Single quote
\"         Double quote
\?         Question mark
\O         Octal string
\x         Hexadecimal string

Thus, to display "Hello World", surrounded in quotation marks and
followed by a newline you would use;

     printf("\"Hello World\"\n");

The following program shows how a decimal integer may be displayed as a
decimal, hexadecimal or octal integer. The 04 following the % in the
printf() format tells the compiler to pad the displayed figure to a width
of at least four digits padded with leading zeroes if required.


     /* A simple decimal to hexadecimal and octal conversion program */
     
     #include <stdio.h>
     
     main()
     {
          int x;
     
          do
          {
               printf("\nEnter a number, or 0 to end ");
               scanf("%d",&x);
               printf("%04d  %04X  %04o",x,x,x);
          }
          while(x != 0);
     
     }
     
There are associated functions to printf() that you should be aware of.
fprintf() has the prototype;

     fprintf(FILE *fp,char *format[,argument,...]);

This variation on printf() simply sends the formatted output to the
specified file stream.

sprintf() has the function prototype;

     sprintf(char *s,char *format[,argument,...]);

and writes the formatted output to a string. You should take care using
sprintf() that the target string has been declared long enough to hold
the result of the sprintf() output. Otherwise other data will be
overwritten in memory.

An example of using sprintf() to copy multiple data to a string;

     #include<stdio.h>
     
     int main()
     {
          char buffer[50];
     
          sprintf(buffer,"7 * 5 == %d\n",7 * 5);
     
          puts(buffer);
     }
     

An alternative to printf() for outputting a simple string to the stream
stdout is puts(). This function sends a string to the stream stdout
followed by a newline character. It is faster than printf(), but far less
flexible. Instead of;

     printf("Hello World\n");

You can use;

     puts("Hello World");



Direct Console I/O
Data may be sent to, and read directly from the console (keyboard and
screen) using the direct console I/O functions. These functions are
prefixed `c'. The direct console I/O equivalent of printf() is then
cprintf(), and the equivalent of puts() is cputs(). Direct console I/O
functions differ from standard I?o functions:

  1. They do not make use of the predefined streams, and hence may not be
     redirected
  2. They are not portable across operating systems (for example you cant
     use direct console I/O functions in a Windows programme).
  3. They are faster than their standard I/O equivalents
  4. They may not work with all video modes (especially VESA display
     modes).
                                    
                                    
                                    
                                POINTERS

A pointer is a variable that holds the memory address of an item of data.
Therefore it points to another item. A pointer is declared like an
ordinary variable, but its name is prefixed by `*', thus;


     char *p;

This declares the variable `p' to be a pointer to a character variable.
Pointers are very powerful, and similarly dangerous! If only because a
pointer can be inadvertently set to point to the code segment of a
program and then some value assigned to the address of the pointer!

The following program illustrates a simple, though fairly useless
application of a pointer;


     #include <stdio.h>
     
     main()
     {
          int a;
          int *x;
     
          /* x is a pointer to an integer data type */
     
          a = 100;
          x = &a;
     
          printf("\nVariable 'a' holds the value %d at memory address
     %p",a,x);
     }

Here is a more useful example of a pointer illustrating how because the
compiler knows the type of data pointed to by the pointer, when the
pointer is incremented it is incremented the correct number of bytes for
the data type.  In this case two bytes;

     #include <stdio.h>
     
     main()
     {
          int n;
          int a[25];
          int *x;
     
          /* x is a pointer to an integer data type */
     
          /* Assign x to point to array element zero */
          x = a;
     
          /* Assign values to the array */
          for(n = 0; n < 25; n++)
               a[n] = n;
     
     
          /* Now print out all array element values */
          for(n = 0; n < 25; n++ , x++)
               printf("\nElement %d holds %d",n,*x);
     }
     
To read or assign a value to the address held by a pointer you use the
indirection operator `*'. Thus in the above example, to print the value
at the memory address pointed to by variable x I have used `*x'.

Pointers may be incremented and decremented and have other mathematics
applied to them also. For example in the above program to move variable x
along the array one element at a time we put the statement `x++' in the
for loop. We could move x along two elements by saying `x += 2'. Notice
that this doesn't mean "add 2 bytes to the value of x", but rather it
means "add 2 of the pointer's data type size units to the value of x".

Pointers are used extensively in dynamic memory allocation. When a
program is running it is often necessary to temporarily allocate a block
of data, say a table, in memory. C provides the function malloc() for
this purpose that follows the general form;


     any pointer type = malloc(number_of_bytes);

malloc() actually returns a void pointer type, which means it can be any
type; integer, character, floating point or whatever. This example
allocates a table in memory for 1000 integers;

     #include <stdio.h>
     #include <stdlib.h>
     
     main()
     {
          int *x;
          int n;
     
          /* x is a pointer to an integer data type */
     
          /* Create a 1000 element table, sizeof() returns the compiler
     */
          /* specific number of bytes used to store an integer */
     
          x = malloc(1000 * sizeof(int));
     
     
          /* Check to see if the memory allocation succeeded */
          if (x == NULL)
          {
               printf("\nUnable to allocate a 1000 element integer
     table");
               exit(0);
          }
     
          /* Assign values to each table element */
          for(n = 0; n < 1000; n++)
          {
               *x = n;
               x++;
          }
     
          /* Return x to the start of the table */
          x -= 1000;
     
          /* Display the values in the table */
          for(n = 0; n < 1000; n++){
               printf("\nElement %d holds a value of %d",n,*x);
               x++;
          }
          /* Deallocate the block of memory now it's no longer required
     */
          free(x);
     }
     
Pointers are also extensively used with character arrays, called strings.
Since all C program strings are terminated by a zero byte we can count
the letters in a string using a pointer;

     #include <stdio.h>
     #include <string.h>
     
     main()
     {
          char *p;
          char text[100];
          int len;
     
          /* Initialise variable 'text' with some writing */
          strcpy(text,"This is a string of data");
     
          /* Set variable p to the start of variable text */
          p = text;
     
          /* Initialise variable len to zero */
          len = 0;
     
          /* Count the characters in variable text */
          while(*p)
          {
               len++;
               p++;
          }
     
          /* Display the result */
          printf("\nThe string of data has %d characters in it",len);
     }
     

The 8088/8086 group of CPUs, as used in the IBM PC, divide memory into
64K segments. To address all 1Mb of memory a 20 bit number is used
comprised of an `offset' to and a 64K `segment'. The IBM PC uses special
registers called "segment registers" to record the segments of addresses.

This leads the C language on the IBM PC to have three new keywords; near,
far and huge.

near pointers are 16 bits wide and access only data within the current
segment.

far pointers are comprised of an offset and a segment address allowing
them to access data any where in memory, but arithmetic with far pointers
is fraught with danger! When a value is added or subtracted from a far
pointer it is only actualy the segment part of the pointer that is
affected, thus leading to the situation where a far pointer being
incremented wraps around on its own offset 64K segment.

huge pointers are a variation on the far pointer that can be successfuly
incremented and decremented through the entire 1Mb range since the
compiler generates code to amend the offset as applicable.

It will come as no surprise that code using near pointers is executed
faster than code using far pointers, which in turn is faster than code
using huge pointers.

to give a literal address to a far pointer IBM PC C compilers provide a
macro MK-FP() that has the prototype;


     void far *MK_FP(unsigned segment, unsigned offset);

For example, to create a far pointer to the start of video memory on a
colour IBM PC system you could use;

          screen = MK_FP(0xB800,0);

Two coressponding macros provided are;

FP_SEG() and FP_OFF()

Which return the segment and offset respectively of a far pointer. The
following example uses FP_SEG() and FP_OFF() to send segment and offset
addresses to a DOS call to create a new directory path;


     #include <dos.h>
     
     int makedir(char *path)
     {
          /* Make sub directory, returning non zero on success */
     
          union REGS inreg,outreg;
          struct SREGS segreg;
     
          inreg.h.ah = 0x39;
          segreg.ds = FP_SEG(path);
          inreg.x.dx = FP_OFF(path);
          intdosx(&inreg,&outreg,&segreg);
     
          return(1 - outreg.x.cflag);
     }
     

Finally, the CPU's segment registers can be read with the function
`segread()'. This is a function unique to C compilers for the the 8086
family of processors. segread() has the function prototype:


     void segread(struct SREGS *segp);

It returns the current values of the segment registers in the SREGS type
structure pointed to by the pointer `segp'. For example:

     #include <dos.h>
     
     main()
     {
          struct SREGS sregs;
     
          segread(&sregs);
          printf("\nCode segment is currently at %04X, Data segment is at
     %04X",sregs.cs,sregs.ds);
          printf("\nExtra segment is at %04X, Stack segment is at
     %04X",sregs.es,sregs.ss);
     }
                                    
                               STRUCTURES


C provides the means to group together variables under one name providing
a convenient means of keeping related information together and providing
a structured approach to data.

The general form for a structure definition is;


     typedef struct
     {
          variable_type variable_name;
          variable_type variable_name;
     }
     structure_name;


When accessing data files, with a fixed record structure, the use of a
structure variable becomes essential. The following example shows a
record structure for a very simple name and address file. It declares a
data structure called `data', and comprised of six fields; `name',
`address', `town', `county' , `post' and `telephone'.


     typedef struct
     {
          char name[30];
          char address[30];
          char town[30];
          char county[30];
          char post[12];
          char telephone[15];
     }
     data;

The structure 'data' may then be used in the program as a variable data
type for declaring variables;

     data record;

Thus declares a variable called 'record' to be of type 'data'.

The individual fields of the structure variable are accessed by the
general form;

     structure_variable.field_name;

Thus, the name field of the structure variable record is accessed with;

     record.name

There is no limit to the number of fields that may comprise a structure,
nor do the fields have to be of the same types, for example;


     typedef struct
     {
          char name[30];
          int age;
          char *notes;
     }
     dp;

Declares a structure 'dp' that is comprised of a character array field,
an integer field and a character pointer field.

A structure variable may be passed as a parameter by passing the address
of the variable as the parameter with the & operator thus;

     func(&my_struct)


The following is an example program that makes use of a structure to
provide the basic access to the data in a simple name and address file;


/* A VERY simple address book written in ANSI C */

#include <stdio.h>
#include <stdlib.h>
#include <io.h>
#include <string.h>
#include <fcntl.h>
#include <sys\stat.h>

/* num_lines is the number of screen display lines */
#define num_lines   25


typedef struct
{
     char name[30];
     char address[30];
     char town[30];
     char county[30];
     char post[12];
     char telephone[15];
}
data;

data record;
int handle;


/* Function prototypes */

void ADD_REC(void);
void CLS(void);
void DISPDATA(void);
void FATAL(char *);
void GETDATA(void);
void MENU(void);
void OPENDATA(void);
int SEARCH(void);

void CLS()
{
     int n;

     for(n = 0; n < num_lines; n++)
          puts("");
}

void FATAL(char *error)
{
     printf("\nFATAL ERROR: %s",error);
     exit(0);
}

void OPENDATA()
{
     /* Check for existence of data file and if not create it */
     /* otherwise open it for reading/writing at end of file */

     handle = open("address.dat",O_RDWR|O_APPEND,S_IWRITE);

     if (handle == -1)
     {
          handle = open("address.dat",O_RDWR|O_CREAT,S_IWRITE);
          if (handle == -1)
               FATAL("Unable to create data file");
     }
}

void GETDATA()
{
     /* Get address data from operator */

     CLS();

     printf("Name ");
     gets(record.name);
     printf("\nAddress ");
     gets(record.address);
     printf("\nTown ");
     gets(record.town);
     printf("\nCounty ");
     gets(record.county);
     printf("\nPost Code ");
     gets(record.post);
     printf("\nTelephone ");
     gets(record.telephone);
}

void DISPDATA()
{
     /* Display address data */
     char text[5];

     CLS();

     printf("Name %s",record.name);
     printf("\nAddress %s",record.address);
     printf("\nTown %s",record.town);
     printf("\nCounty %s",record.county);
     printf("\nPost Code %s",record.post);
     printf("\nTelephone %s\n\n",record.telephone);

     puts("Press RETURN to continue");
     gets(text);
}

void ADD_REC()
{
     /* Insert or append a new record to the data file */
     int result;

     result = write(handle,&record,sizeof(data));

     if (result == -1)
          FATAL("Unable to write to data file");
}

int SEARCH()
{
     char text[100];
     int result;

     printf("Enter data to search for ");
     gets(text);
     if (*text == 0)
          return(-1);

     /* Locate start of file */
     lseek(handle,0,SEEK_SET);

     do
     {
          /* Read record into memory */
          result = read(handle,&record,sizeof(data));
          if (result > 0)
          {
               /* Scan record for matching data */
               if (strstr(record.name,text) != NULL)
                    return(1);
               if (strstr(record.address,text) != NULL)
                    return(1);
               if (strstr(record.town,text) != NULL)
                    return(1);
               if (strstr(record.county,text) != NULL)
                    return(1);
               if (strstr(record.post,text) != NULL)
                    return(1);
               if (strstr(record.telephone,text) != NULL)
                    return(1);
          }
     }
     while(result > 0);
     return(0);
}

void MENU()
{
     int option;
     char text[10];

     do
     {
          CLS();
          puts("\n\t\t\tSelect Option");
          puts("\n\n\t\t\t1 Add new record");
          puts("\n\n\t\t\t2 Search for data");
          puts("\n\n\t\t\t3 Exit");
          puts("\n\n\n\n\n");
          gets(text);
          option = atoi(text);

          switch(option)
          {
               case 1 : GETDATA();
                     /* Go to end of file to append new record */
                     lseek(handle,0,SEEK_END);
                     ADD_REC();
                     break;

               case 2 : if (SEARCH())
                          DISPDATA();
                     else
                     {
                          puts("NOT FOUND!");
                          puts("Press RETURN to continue");
                          gets(text);
                     }
                     break;

               case 3 : break;
          }
     }
     while(option != 3);
}

void main()
{
     CLS();
     OPENDATA();
     MENU();
}

Bit Fields

C allows the inclusion of variables with a size of less than eight bits
to be included in structures. These variables are known as bit fields,
and may be any declared size from one bit upwards.

The general form for declaring a bit field is;


     type name : number_of_bits;


For example, to declare a set of status flags, each occupying one bit;

typedef struct
{
     unsigned carry  : 1;
     unsigned zero   : 1;
     unsigned over   : 1;
     unsigned parity : 1;
}
df;

df flags;

The variable `flags' then occupies only four bits in memory, and yet is
comprised of four variables that may be accessed like any other structure
field.



Uunions

Another facility provided by C for efficient use of available memory is
the union structure. A union structure is a collection of variables that
all share the same memory storage address. As such only one of the
variables is ever accessible at a time.

The general form of a union definition is;


     union name
     {
          type variable_name;
          type variable_name;
          .
          .
          .
          type variable_name;
     };

Thus, to declare a union structure for two integer variables;

     union data
     {
          int vara;
          int varb;
     };

and to declare a variable of type 'data';

     data my_var;

The variable 'my_var' is then comprised of the two variables 'vara' and
'varb'  that are accessed like with any form of structure, eg;

     my_var.vara = 5;

Assigns a value of 5 to the variable 'vara' of union 'my_var'.


Enumerations

An enumeration assigns ascending integer values to a list of symbols. An
enumeration declaration takes the general form;


     enum name { enumeration list } variable_list;

Thus to define a symbol list of colours, you can say;

     enum COLOURS
     {
          BLACK,
          BLUE,
          GREEN,
          CYAN,
          RED,
          MAGENTA,
          BROWN,
          LIGHTGRAY,
          DARKGRAY,
          LIGHTBLUE,
          LIGHTGREEN,
          LIGHTCYAN,
          LIGHTRED,
          LIGHTMAGENTA,
          YELLOW,
          WHITE
     };

Then, the number zero may be referred to by the symbol BLACK, the number
one by the symbol BLUE, the number two by the symbol GREEN and so on.

The following program illustrates the use of an enumeration list to
symbolise integers;

     #include <stdio.h>
     
     enum COLOURS
     {
          BLACK,
          BLUE,
          GREEN,
          CYAN,
          RED,
          MAGENTA,
          BROWN,
          LIGHTGRAY,
          DARKGRAY,
          LIGHTBLUE,
          LIGHTGREEN,
          LIGHTCYAN,
          LIGHTRED,
          LIGHTMAGENTA,
          YELLOW,
          WHITE
     };
     
     
     void main()
     {
          int x;
     
          x = RED;
     
          printf("\nVariable 'x' holds %d",x);
     
     }
                                    
                                FILE I/O


C provides buffered file streams for file access. Some C platforms, such
as Unix and DOS provide unbuffered file handles as well.



Buffered streams

Buffered streams are accessed through a variable of type 'file pointer'.
The data type FILE is defined in the header file stdio.h. Thus to declare
a file pointer;

     #include <stdio.h>

     FILE *ptr;

To open a stream C provides the function fopen(), which accepts two
parameters, the name of the file to be opened, and the access mode for
the file to be opened with. The access mode may be any one of;


Mode   Description
       
r      Open for reading
w      Create for writing, destroying any
       existing file
a      Open for append, creating a new file
       if it doesn't
        exist
r+      Open an existing file for reading
       and writing
w+      Create for reading and writing,
       destroying any
        existing file
a+      Open for append, creating a new file
       if it doesn't exist.


Optionaly either `b' or `t' may be appended for binary or text mode. If
neither is appended, then the file stream will be opened in the mode
described by the global variable _fmode. Data read or written from file
streams opened in text mode undergoes conversion, that is the characters
CR and LF are converted to CR LF pairs on writing, and the CR LF pair is
converted to a single LF on reading. File streams opened in binary mode
do not undergo conversion.


If fopen() fails to open the file, it returns a value of NULL (defined in
stdio.h) to the file pointer.

Thus, the following program will create a new file called "data.txt" and
open it for reading and writing;

     #include <stdio.h>
     
     void main()
     {
          FILE *fp;
     
          fp = fopen("data.txt","w+");
     
     }

To close a stream C provides the function fclose(), which accepts the
stream's file pointer as a parameter;

     fclose(fp);

If an error occurs in closing the file stream, fclose() returns non zero.

There are four basic functions for receiving and sending data to and from
streams; fgetc(), fputc(), fgets() and fputs().

fgetc() simply reads a single character from the specified input stream;

     char fgetc(FILE *fp);

Its opposite number is fputc(), which simply writes a single character to
the specified input stream;

     char fputc(char c, FILE *fp);

fgets() reads a string from the input stream;

     char *fgets(char s, int numbytes, FILE *fp);

It stops reading when either numbytes - 1 bytes have been read, or a
newline character is read in. A null terminating byte is appended to the
read string, s. If an error occurs, fgets() returns NULL.


fputs() writes a null terminated string to a stream;

     int fputs(char *s, FILE *fp);

Excepting fgets(), which returns a NULL pointer if an error occurs, all
the other functions described above return EOF (defined in stdio.h) if an
error occurs during the operation.


The following program creates a copy of the file "data.dat" as "data.old"
and illustrates the use of fopen(), fgetc(), fputc() and fclose();


     #include <stdio.h>
     
     int main()
     {
          FILE *in;
          FILE *out;
     
          in = fopen("data.dat","r");
     
          if (in == NULL)
          {
               puts("\nUnable to open file data.dat for reading");
               return(0);
          }
     
          out = fopen("data.old","w+");
     
          if (out == NULL)
          {
               puts("\nUnable to create file data.old");
               return(0);
          }
     
          /* Loop reading and writing one byte at a time until end-of-
     file */
          while(!feof(in))
               fputc(fgetc(in),out);
     
          /* Close the file streams */
          fclose(in);
          fclose(out);
     
          return(0);
     }

Example program using fputs() to copy text from stream stdin (usually
typed in at the keyboard) to a new file called "data.txt".

     #include <stdio.h>
     
     int main()
     {
          FILE *fp;
          char text[100];
     
          fp = fopen("data.txt","w+");
     
          do
          {
               gets(text);
               fputs(text,fp);
          }
          while(*text);
     
          fclose(fp);
     }


Random access using streams

Random file access for streams is provided for by the fseek() function
that has the following prototype;

     int fseek(FILE *fp, long numbytes, int fromwhere);

fseek() repositions a file pointer associated with a stream previously
opened by a call to fopen(). The file pointer is positioned `numbytes'
from the location `fromwhere', which may be the file beginning, the
current file pointer position, or the end of the file, symbolised by the
constants SEEK_SET, SEEK_CUR and SEEK_END respectively. If a call to
fseek() succeeds, a value of zero is returned.


Associated with fseek() is ftell(), which reports the current file
pointer position of a stream, and has the following function prototype;


     long int ftell(FILE *fp);

ftell() returns either the position of the file pointer, measured in
bytes from the start of the file, or -1 upon an error occurring.



Handles

File handles are opened with the open() function that has the prototype;

     int open(char *filename,int access[,unsigned mode]);

If open() is successful, the number of the file handle is returned.
Otherwise open() returns -1.

The access integer is comprised from bitwise oring together of the
symbolic constants declared in fcntl.h. These vary from compiler to
compiler but may be;


     O_APPEND       If set, the file pointer will be set to the end of
the
                    file prior to each write.
     O_CREAT        If the file does not exist it is created.
     O_TRUNC        Truncates the existing file to a length of zero
bytes.
     O_EXCL          Used with O_CREAT
     O_BINARY       Opens the file in binary mode
     O_TEXT         Opens file in text mode

The optional mode parameter is comprised by bitwise oring of the symbolic
constants defined in stat.h. These vary from C compiler to C compiler but
may be;

     S_IWRITE       Permission to write
     S_IREAD        Permission to read


Once a file handle has been assigned with open(), the file may be
accessed with read() and write().

Read() has the function prototype;

     int read(int handle, void *buf, unsigned num_bytes);

It attempts to read 'num_bytes' and returns the number of bytes actually
read from the file handle 'handle', and stores these bytes in the memory
block pointed to by 'buf'.

Write() is very similar to read() and has the same function prototype and
return values, but writes 'num_bytes' from the memory block pointed to by
'buf'.

Files opened with open() are closed using close() that has the function
prototype;

     int close(int handle);

close() returns zero on success, and -1 if an error occurs trying to
close the handle.

Random access is provided by lseek(), which is very similar to fseek(),
except that it accepts an integer file handle as the first parameter
rather than a stream FILE pointer.

This example uses file handles to read data from stdin (usually the
keyboard) and copy the text to a new file called "data.txt".

     #include <io.h>
     #include <fcntl.h>
     #include <sys\stat.h>
     
     int main()
     {
          int handle;
          char text[100];
     
          handle = open("data.txt",O_RDWR|O_CREAT|O_TRUNC,S_IWRITE);
     
          do
          {
               gets(text);
               write(handle,&text,strlen(text));
          }
          while(*text);
     
          close(handle);
     }



Advanced File I/O

The ANSI standard on C defines file IO as by way of file streams, and
defines various functions for file access;


fopen() has the prototype;

     FILE *fopen(const char *name,const char *mode);

fopen() attempts to open a stream to a file name in a specified mode. If
successful a FILE type pointer is returned to the file stream, if the
call fails NULL is returned. The mode string can be one of the following;


Mode      Description
 r        Open for reading only
 w        Create for writing, overwriting any existing file with the same
          name.
 a        Open for append (writing at end of file) or create the file if
it
          does not exist.
 r+       Open an existing file for reading and writing.
 w+       Create a new file for reading and writing.
 a+       Open for append with read and write access.


fclose() is used to close a file stream previously opened by a call to
fopen(). It has the prototype;

     int fclose(FILE *fp);

When a call to fclose() is successful, all buffers to the stream are
flushed and a value of zero is returned. If the call fails fclose()
returns EOF.

Many host computers, and the IBM PC is no exception, use buffered file
access, that is when writing to a file stream the data is stored in
memory and only written to the stream when it exceeds a predefined number
of bytes. A power failure occurring before the data has been written to
the stream will result in the data never being written, so the function
fflush() can be called to force all pending data to be written. fflush()
has the prototype;

     int fflush(FILE *fp);

When a call to fflush() is successful, the buffers connected with the
stream are flushed and a value of zero is returned. On failure fflush()
returns EOF.

The location of the file pointer connected with a stream can be
determined with the function ftell(). ftell() has the prototype;


     long int ftell(FILE *fp);

and returns the offset of the file pointer in bytes from the start of the
file, or -1L if the call fails.

Similarly, you can move the file pointer to a new position with fseek().
fseek() has the prototype;

     int fseek(FILE *fp, long offset, int from_what_place);

fseek() attempts to move the file pointer, 'fp' 'offset' bytes from the
position 'from_what_place'. 'from_what_place' is predefined as one of;

     SEEK_SET       The file's beginning
     SEEK_CUR       The file pointer's current position
     SEEK_END       End of file

The offset may be a positive value to move the file pointer on through
the file, or negative to move backwards.

To move a file pointer quickly back to the start of a file, and clear any
references to errors that have occurred C provides the function rewind()
that has the prototype;

     void rewind(FILE *fp);

rewind(fp) is similar to fseek(fp,0L,SEEK_SET) in that they both set the
file pointer to the start of the file, but whereas fseek() clears the EOF
error marker, rewind() clears all error indicators.

Errors occurring with file functions can be checked with the function
ferror() that has the prototype;

     int ferror(FILE *fp);

ferror() returns a nonzero value if an error has occurred on the
specified stream. After checking ferror() and reporting any errors you
should clear the error indicators, and this can be done by a call to
clearerr() that has the prototype;

     void clearerr(FILE *fp);

The condition of reaching end of file (EOF) can be tested for with the
predefined macro feof() that has the prototype;

     int feof(FILE *fp);

feof() returns a nonzero value if an end of file error indicator was
detected on the specified file stream, and zero if the end of file has
not yet been reached.

Reading data from a file stream can be achieved using several functions;
A single character can be read with fgetc() that has the prototype;


     int fgetc(FILE *fp);

fgetc() returns either the character read converted to an integer, or EOF
if an error occurred.

Reading a string of data is achieved with fgets(). fgets() attempts to
read a string terminated by a newline character and of no more than a
specified number of bytes from the file stream. It has the prototype;


     char *fgets(char s, int n, FILE *fp);

A successful call to fgets() results in a string being stored in `s' that
is either terminated by a newline character, or that is `n' - 1
characters long, which ever came first. The newline character is retained
by fgets(), and a null bytes is appended to the string. If the call fails
a NULL pointer is returned.


Strings may be written to a stream using fputs() that has the prototype;

     int fputs(const char *s,FILE *fp);

fputs() writes all the characters in the string `s' to the stream `fp'
except the null terminating byte. On success, fputs() returns the last
character written, on failure it returns EOF.

To write a single character to a stream use fputc() that has the
prototype;

     int fputc(int c,FILE *fp);

If successful, fputc() returns the character written, otherwise it
returns EOF.

To read a large block of data, or a record from a stream you can use
fread() that has the prototype;

     size_t fread(void *ptr,size_t size, size_t n, FILE *fp);

fread() attempts to read 'n' items, each of length 'size' from the file
stream 'fp' into the block of memory pointed to by 'ptr'. To check the
success or failure status of fread() use ferror().

The sister function to fread() is fwrite() that has the prototype;

     size_t fwrite(const void *ptr,size_t size, size_t n,FILE *fp);

that writes 'n' items each of length 'size' from the memory area pointed
to by 'ptr' to the specified stream 'fp'.

Formatted input from a stream is achieved with fscanf() that has the
prototype;

     int fscanf(FILE *fp, const char *format[,address ...]);

fscanf() returns the number of fields successfully stored, and EOF on end
of file. This short example shows how fscanf() is quite useful for
reading numbers from a stream, but hopeless when it comes to strings!

     #include <stdio.h>
     
     void main()
     {
          FILE *fp;
          int a;
          int b;
          int c;
          int d;
          int e;
          char text[100];
     
          fp = fopen("data.txt","w+");
     
          if(!fp)
          {
               perror("Unable to create file");
               exit(0);
          }
     
          fprintf(fp,"1 2 3 4 5 \"A line of numbers\"");
     
          fflush(fp);
     
          if (ferror(fp))
          {
               fputs("Error flushing stream",stderr);
               exit(1);
          }
     
          rewind(fp);
          if (ferror(fp))
          {
               fputs("Error rewind stream",stderr);
               exit(1);
          }
     
          fscanf(fp,"%d %d %d %d %d %s",&a,&b,&c,&d,&e,text);
          if (ferror(fp))
          {
               fputs("Error reading from stream",stderr);
               exit(1);
          }
     
          printf("\nfscanf() returned %d %d %d %d %d %s",a,b,c,d,e,text);
     }
     
As you can see from the example, fprintf() can be used to write formatted
data to a stream.

If you wish to store the position of a file pointer on a stream, and then
later restore it to the same position you can use the functions fgetpos()
and fsetpos(). fgetpos() reads the current location of the file pointer
and has the prototype;

     int fgetpos(FILE *fp, fpos_t *pos);

fsetpos() repositions the file pointer and has the prototype;

     int fsetpos(FILE *fp, const fpos_t *fpos);

fpos_t is defined in stdio.h.

These functions are more convenient than doing an ftell() followed by an
fseek().

An open stream can have a new file associated with it in place of the
existing file by using the function freopen() that has the prototype;

     FILE *freopen(const char *name,const char *mode,FILE *fp);

freopen() closes the existing stream and then attempts to reopens it with
the specified file name. This is useful for redirecting the predefined
streams stdin, stdout and stderr to a file or device.

For example; if you wish to redirect all output intended to stdout
(usually the host computer's display device) to a printer you might use;

     freopen("LPT1","w",stdout);

where LPT1 is the name of the printer device (on a PC host, LPT1 is the
name of the parallel port).


Predefined I/O Streams

There are three predefined I/O streams; stdin, stdout, and stderr. The
streams stdin and stdout default to the keyboard and display
respectively, but can be redirected on some hardware platforms, such as
the PC and under UNIX. The stream stderr defaults to the display and is
not usually redirected by the operator. Thus it can be used for the
display of error messages even when program output has been redirected,
such as with;

      fputs("Error message",stderr);

The functions printf() and puts(), output data to the stream stdout, and
can therefore be redirected by the operator of the program. scanf() and
gets() accept input from the stream stdin.

As an example of file i/o with the PC consider the following short
program that does a hex dump of a specified file to the predefined stream
stdout, which may be redirected to a file using;

          dump filename.ext > target.ext

     #include <stdio.h>
     #include <fcntl.h>
     #include <io.h>
     #include <string.h>
     
     main(int argc, char *argv[])
     {
          unsigned counter;
          unsigned char v1[20];
          int f1;
          int x;
          int n;
     
          if (argc != 2)
          {
               fputs("\nERROR: Syntax is dump f1\n",stderr);
               return(1);
          }
     
          f1 = open(argv[1],O_RDONLY);
     
          if (f1 == -1)
          {
               fprintf(stderr,"\nERROR: Unable to open %s\n",argv[1]);
               return(1);
          }
     
          fprintf(stdout,"\nDUMP OF FILE %s\n\n",strupr(argv[1]));
     
          counter = 0;
     
          while(1)
          {
               /* Set buffer to zero bytes */
               memset(v1,0,20);
     
               /* Read buffer from file */
               x = _read(f1,&v1,16);
     
               /* x will be 0 on EOF or -1 on error */
               if (x < 1)
                    break;
     
               /* Print file offset to stdout */
               fprintf(stdout,"%06d(%05x) ",counter,counter);
     
               counter += 16;
     
               /* print hex values of buffer to stdout */
               for(n = 0; n < 16; n++)
                    fprintf(stdout,"%02x ",v1[n]);
     
               /* Print ascii values of buffer to stdout */
               for(n = 0; n < 16; n++)
               {
                    if ((v1[n] > 31) && (v1[n] < 128))
                         fprintf(stdout,"%c",v1[n]);
                    else
                         fputs(".",stdout);
               }
     
               /* Finish the line with a new line */
               fputs("\n",stdout);
          }
     
          /* successful termination */
          return(0);
     }
                                    
                                 STRINGS

The C language has one of the most powerful string handling capabilities
of any general purpose computer language.

A string is a single dimension array of characters terminated by a zero
byte.

Strings may be initialised in two ways. Either in the source code where
they may be assigned a constant value, as in;

     int main()
     {
          char *p = "System 5";
          char name[] = "Test Program" ;
     }

or at run time by the function strcpy() that has the function prototype;

     char *strcpy(char *destination, char *source);

strcpy() copies the string pointed to by source into the location pointed
to by destination as in the following example;


     #include<stdio.h>
     
     int main()
     {
          char name[50];
     
          strcpy(name,"Servile Software");
     
          printf("\nName equals %s",name);
     }

C also allows direct access to each individual byte of the string, so the
following is quite permissible;

     #include<stdio.h>
     
     int main()
     {
          char name[50];
     
          strcpy(name,"Servile Software");
     
          printf("\nName equals %s",name);
     
          /* Replace first byte with lower case 's' */
          name[0] = 's';
     
          printf("\nName equals %s",name);
     }
     
The ANSI standard on the C programming language defines the following
functions for use with strings;

char *strcat(char *dest, char *source)            Appends string source
to the end of string destination.

char *strchr(char *s, int c)            Returns a pointer to the first
occurence of character 'c' within s.

int strcmp(char *s1, char *s2)               Compares strings s1 and s2
returning        < 0 if s1 is less than s2
                                                       == 0 if s1 and s2
are the same
                                                       > 0 if s1 is
greater than s2

int strcoll(char *s1, char *s2)              Compares strings s1 and s2
                    according to the collating sequence set by
                              setlocale() returning    < 0 if s1 is less
than s2
                                             == 0 if s1 and s2 are the
same
                                             > 0 if s1 is greater than s2

char *strcpy(char *dest, char *src)          Copies string src into
string dest.

unsigned strcspn(char *s1, char *s2)         Returns the length of string
s1 that consists entirely of characters not in
string s2.

unsigned strlen(char *s)                Returns the length of string s.

char *strncat(char *dest, char *src, unsigned len)     Copies at most
'len' characters from string src into string dest.

int strncmp(char *s1, char *s2, unsigned len)     Compares at most 'len'
characters from
                              string s1 with string s2 returning      < 0
if s1 is less than s2
                                                  == 0 if s1 and s2 are
the same
                                                  > 0 if s1 is greater
than s2

char *strncpy(char *dest, char *src, unsigned len)     Copies 'len'
characters from string  src into string dest, truncating or
                              padding with zero bytes as required.

char *strpbrk(char *s1, char *s2)            Returns a pointer to the
first character in string s1 that occurs in
                              string s2.

char *strrchr(char *s, int c)           Returns a pointer to the last
occurence of 'c' within string s.

unsigned strspn(char *s1, char *s2)          Returns the length of the
initial segment of string s1 that consists
                              entirely of characters in string s2.

char *strstr(char *s1, char *s2)             Returns a pointer to the
first occurence of string s2 within string
                              s1, or NULL if string s2 is not found in
string s1.

char *strtok(char *s1, char *s2)             Returns a pointer to the
token found in string s1 that is defined by
                              delimiters in string s2. Returns NULLif no
tokens are found.

The ANSI standard also defines various functions for converting strings
into numbers and numbers into strings.

Some C compilers include functions to convert strings to upper and lower
case, but these functions are not defined in the ANSI standard. However,
the ANSI standard does define the functions; toupper() and tolower() that
return an

integer parameter converted to upper and lowercase respectively. By using
these functions we can create our own ANSI compatible versions;


     #include<stdio.h>
     
     void strupr(char *source)
     {
          char *p;
     
          p = source;
          while(*p)
          {
               *p = toupper(*p);
               p++;
          }
     }
     
     void strlwr(char *source)
     {
          char *p;
     
          p = source;
          while(*p)
          {
               *p = tolower(*p);
               p++;
          }
     }


     int main()
     {
          char name[50];
     
          strcpy(name,"Servile Software");
     
          printf("\nName equals %s",name);
     
          strupr(name);
     
          printf("\nName equals %s",name);
     
          strlwr(name);
     
          printf("\nName equals %s",name);
     }

C does not impose a maximum length that a string may be, unlike other
computer languages. However, some CPUs impose restrictions on the maximum
size a block of memory can be. For example, the 8088 family of CPUs, as
used by the IBM PC, impose a limit of 64K bytes on a segment of memory.

An example program to reverse all the characters in a string.

     #include <stdio.h>
     #include <string.h>
     
     char *strrev(char *s)
     {
          /* Reverses the order of all characters in a string except the
     null */
          /* terminating byte */
     
          char *start;
          char *end;
          char tmp;
     
          /* Set pointer 'end' to last character in string */
          end = s + strlen(s) - 1;
     
          /* Preserve pointer to start of string */
          start = s;
     
          /* Swop characters */
          while(end >= s)
          {
               tmp = *end;
               *end = *s;
               *s = tmp;
               end--;
               s++;
          }
          return(start);
     }
     
     
     main()
     {
          char text[100];
          char *p;
     
          strcpy(text,"This is a string of data");
     
          p = strrev(text);
     
          printf("\n%s",p);
     }
     

Strtok()

The function strtok() is a very powerful standard C feature for
extracting substrings from within a single string. It is used where the
substrings are separated by known delimiters, such as commas in the
following example;

     #include <stdio.h>
     #include <string.h>
     
     main()
     {
          char data[50];
          char *p;
     
          strcpy(data,"RED,ORANGE,YELLOW,GREEN,BLUE,INDIGO,VIOLET");
     
          p = strtok(data,",");
          while(p)
          {
               puts(p);
               p = strtok(NULL,",");
          };
     }
     
Or this program can be written with a for() loop thus;

     #include <stdio.h>
     #include <string.h>
     
     main()
     {
          char data[50];
          char *p;
     
          strcpy(data,"RED,ORANGE,YELLOW,GREEN,BLUE,INDIGO,VIOLET");
     
          for(strtok(data,","); p; p = strtok(NULL,","))
          {
               puts(p);
          };
     }
     
They both compile to the same code but follow different programming
styles.

Initially, you call strtok() with the name of the string variable to be
parsed, and a second string that contains the known delimiters. Strtok()
then returns a pointer to the start of the first substring and replaces
the first token with a zero delimiter. Subsequent calls to strtok() can
be made in a loop passing NULL as the string to be parsed, and strtok()
will return the subsequent substrings.


Since strtok() can accept many delimiter characters in the second
parameter string we can use it as the basis of a simple word counting
program;

     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>
     
     void main(int argc, char *argv[])
     {
          FILE *fp;
          char buffer[256];
          char *p;
          long count;
     
          if (argc != 2)
          {
               fputs("\nERROR: Usage is wordcnt <file>\n",stderr);
               exit(0);
          }
     
          /* Open file for reading */
          fp = fopen(argv[1],"r");
     
          /* Check the open was okay */
          if (!fp)
          {
               fputs("\nERROR: Cannot open source file\n",stderr);
               exit(0);
          }
     
          /* Initialise word count */
          count = 0;
     
          do
          {
               /* Read a line of data from the file */
               fgets(buffer,255,fp);
     
               /* check for an error in the read or EOF */
               if (ferror(fp) || feof(fp))
                    continue;
     
               /* count words in received line */
               /* Words are defined as separated by the characters */
               /* \t(tab) \n(newline) , ; : . ! ? ( ) - and [space] */
               p = strtok(buffer,"\t\n,;:.!?()- ");
               while(p)
               {
                    count++;
                    p = strtok(NULL,"\t\n,;:.!?()- ");
               }
          }
          while(!ferror(fp) && !feof(fp));
     
          /* Finished reading. Was it due to an error? */
          if (ferror(fp))
          {
               fputs("\nERROR: Reading source file\n",stderr);
               fclose(fp);
               exit(0);
          }
     
          /* Reading finished due to EOF, quite valid so print count */
          printf("\nFile %s contains %ld words\n",argv[1],count);
          fclose(fp);
     }
     



Converting Numbers To And From Strings

All C compilers provide a facility for converting numbers to strings.
This being sprintf(). However, as happens sprintf() is a multi-purpose
function that is therefore large and slow. The following function ITOS()
accepts two parameters, the first being a signed integer and the second
being a pointer to a character string. It then copies the integer into
the memory pointed to by the character pointer. As with sprintf() ITOS()
does not check that the target string is long enough to accept the result
of the conversion. You should then ensure that the target string is long
enough.

Example function for copying a signed integer into a string;

     void ITOS(long x, char *ptr)
     {
          /* Convert a signed decimal integer to a string */
     
          long pt[9] = { 100000000, 10000000, 1000000, 100000, 10000,
     1000, 100, 10, 1 };
          int n;
     
          /* Check sign */
          if (x < 0)
          {
               *ptr++ = '-';
               /* Convert x to absolute */
               x = 0 - x;
          }
     
          for(n = 0; n < 9; n++)
          {
               if (x > pt[n])
               {
                    *ptr++ = '0' + x / pt[n];
                    x %= pt[n];
               }
          }
          return;
     }
     
To convert a string to a floating point number, C provides two functions;
atof() and strtod(). atof() has the prototype;

     double atof(const char *s);

strtod has the prototype;

     double strtod(const char *s,char **endptr);

Both functions scan the string and convert it as far as they can, until
they come across a character they don't understand. The difference
between the two functions is that if strtod() is passed a character
pointer for parameter `endptr', it sets that pointer to the first
character in the string that terminated the conversion. Because of its
better error reporting, by way of endptr, strtod() is often preferred to
atof().

To convert a string to an integer use atoi() that has the prototype;


     int atoi(const char *s);

atoi() does not check for an overflow, and the results are undefined!

atol() is a similar function but returns a long. Alternatively, you can
use strtol() and stroul() instead that have better error checking.

                                    
                              TEXT HANDLING

Human languages write information down as `text'. This is comprised of
words, figures and punctuation. The words being made up of upper case and
lower case letters. Processing text with a computer is a commonly
required task, and yet quite a difficult one.

The ANSI C definitions include string processing functions that are by
their nature sensitive to case. That is the letter `A' is seen as
distinct from the letter `a'. This is the first problem that must be
overcome by the programmer. Fortunately both Borland's Turbo C compilers
and Microsoft's C compilers include case insensitive forms of the string
functions.

stricmp() for example is the case insensitive form of strcmp(), and
strnicmp() is the case insensitive form of strncmp().

If you are concerned about writing portable code, then you must restrict
yourself to the ANSI C functions, and write your own case insensitive
functions using the tools provided.

Here is a simple implementation of a case insensitive version of
strstr().  The function simply makes a copy of the parameter strings,
converts those copies both to upper case and then does a standard
strstr() on the copies.  The offset of the target string within the
source string will be the same for the copy as the original, and so it
can be returned relative to the parameter string.


     char *stristr(char *s1, char *s2)
     {
          char c1[1000];
          char c2[1000];
          char *p;
     
          strcpy(c1,s1);
          strcpy(c2,s2);
     
          strupr(c1);
          strupr(c2);
     
          p = strstr(c1,c2);
          if (p)
               return s1 + (p - c1);
          return NULL;
     }
     
This function scans a string, s1 looking for the word held in s2. The
word must be a complete word, not simply a character pattern, for the
function to return true. It makes use of the stristr() function described
previously.

     int word_in(char *s1,char *s2)
     {
          /* return non-zero if s2 occurs as a word in s1 */
          char *p;
          char *q;
          int ok;
     
          ok = 0;
          q = s1;
     
          do
          {
               /* Locate character occurence s2 in s1 */
               p = stristr(q,s2);
               if (p)
               {
                    /* Found */
                    ok = 1;
     
                    if (p > s1)
                    {
                         /* Check previous character */
                         if (*(p - 1) >= 'A' && *(p - 1) <= 'z')
                              ok = 0;
                    }
     
                    /* Move p to end of character set */
                    p += strlen(s2);
                    if (*p)
                    {
                         /* Check character following */
                         if (*p >= 'A' && *p <= 'z')
                              ok = 0;
                    }
               }
               q = p;
          }
          while(p && !ok);
          return ok;
     }


Some more useful functions for dealing with text are truncstr() that
truncates a string;

     void truncstr(char *p,int num)
     {
          /* Truncate string by losing last num characters */
          if (num < strlen(p))
               p[strlen(p) - num] = 0;
     }
     
trim() that removes trailing spaces from the end of a string;

     void trim(char *text)
     {
          /* remove trailing spaces */
          char *p;
     
          p = &text[strlen(text) - 1];
          while(*p == 32 && p >= text)
               *p-- = 0;
     }
     
strlench() that changes the length of a string by adding or deleting
characters;

     void strlench(char *p,int num)
     {
          /* Change length of string by adding or deleting characters */
     
          if (num > 0)
               memmove(p + num,p,strlen(p) + 1);
          else
          {
               num = 0 - num;
               memmove(p,p + num,strlen(p) + 1);
          }
     }
     
strins() that inserts a string into another string;

     void strins(char *p, char *q)
     {
          /* Insert string q into p */
          strlench(p,strlen(q));
          strncpy(p,q,strlen(q));
     }
     
strchg() that replaces all occurences of one sub-string with another
within a target string;

     void strchg(char *data, char *s1, char *s2)
     {
          /* Replace all occurences of s1 with s2 */
          char *p;
          char changed;
     
          do
          {
               changed = 0;
               p = strstr(data,s1);
               if (p)
               {
                    /* Delete original string */
                    strlench(p,0 - strlen(s1));
     
                    /* Insert replacement string */
                    strins(p,s2);
                    changed = 1;
               }
          }
          while(changed);
     }
                                    
                                  TIME

C provides a function, time(), which reads the computer's system clock to
return the system time as a number of seconds since midnight on January
the first, 1970. However, this value can be converted to a useful string
by the function ctime() as illustrated in the following example;

     #include <stdio.h>
     #include <time.h>
     
     int main()
     {
          /* Structure to hold time, as defined in time.h  */
          time_t t;
     
          /* Get system date and time from computer */
          t = time(NULL);
          printf("Today's date and time: %s\n",ctime(&t));
     }
     
The string returned by ctime() is comprised of seven fields;

     Day of the week,
     Month of the year,
     Date of the day of the month,
     hour,
     minutes,
     seconds,
     century of the year

terminated by a newline character and null terminating byte. Since the
fields always occupy the same width, slicing operations can be carried
out on the string with ease. The following program defines a structure
`time' and a function gettime() that extracts the hours, minutes and
seconds of the current time and places them in the structure;


     #include <stdio.h>
     #include <time.h>
     
     struct time
     {
          int ti_min;         /* Minutes */
          int ti_hour;        /* Hours */
          int ti_sec;         /* Seconds */
     };
     
     void gettime(struct time *now)
     {
          time_t t;
          char temp[26];
          char *ts;
     
          /* Get system date and time from computer */
          t = time(NULL);
     
          /* Translate dat and time into a string */
          strcpy(temp,ctime(&t));
     
          /* Copy out just time part of string */
          temp[19] = 0;
          ts = &temp[11];
     
          /* Scan time string and copy into time structure */
          sscanf(ts,"%2d:%2d:%2d",&now->ti_hour,&now->ti_min,&now-
     >ti_sec);
     }
     
     int main()
     {
          struct time now;
     
          gettime(&now);
     
          printf("\nThe time is
     %02d:%02d:%02d",now.ti_hour,now.ti_min,now.ti_sec);
     
     }

The ANSI standard on C does actually provide a function ready made to
convert the value returned by time() into a structure;

     #include <stdio.h>
     #include <time.h>
     
     int main()
     {
          time_t t;
          struct tm *tb;
     
          /* Get time into t */
          t = time(NULL);
     
          /* Convert time value t into structure pointed to by tb */
          tb = localtime(&t);
     
          printf("\nTime is %02d:%02d:%02d",tb->tm_hour,tb->tm_min,tb-
     >tm_sec);
     }
     
The structure 'tm' is defined in time.h as;

     struct tm
     {
          int tm_sec;
          int tm_min;
          int tm_hour;
          int tm_mday;
          int tm_mon;
          int tm_year;
          int tm_wday;
          int tm_yday;
          int tm_isdst;
     };


Timers
Often a program must determine the date and time from the host computer's
non-volatile RAM. There are several time functions provided by the ANSI
standard on C that allow a program to retrieve, from the host computer,
the current date and time;

time() returns the number of seconds that have elapsed since midnight on
January the 1st 1970. It has the prototype;

     time_t time(time_t *timer);

time() fills in the time_t variable sent as a parameter and returns the
same value. You can call time() with a NULL parameter and just collect
the return value thus;

     #include <time.h>
     
     void main()
     {
          time_t now;
     
          now = time(NULL);
     }
     
asctime() converts a time block to a twenty six character string of the
format;

                Wed Oct 14 10:23:45 1992\n\0

asctime() has the prototype;

               char *asctime(const struct tm *tblock);

ctime() converts a time value (as returned by time()) into a twenty six
chracter string of the same format as asctime(). For example;

     #include <stdio.h>
     #include <time.h>
     
     void main()
     {
          time_t now;
          char date[30];
     
          now = time(NULL);
          strcpy(date,ctime(&now));
     }
     
difftime() returns the difference, in seconds, between two values (as
returned by time()). This can be useful for testing the elapsed time
between two events, the time a function takes to execute, and for
creating consistent delays that are irrelevant of the host computer.

An example delay program;

     #include <stdio.h>
     #include <time.h>
     
     
     void DELAY(int period)
     {
          time_t start;
     
          start = time(NULL);
          while(time(NULL) < start + period)
               ;
     }
     
     void main()
     {
          printf("\nStarting delay now....(please wait 5 seconds)");
     
          DELAY(5);
     
          puts("\nOkay, I've finished!");
     }

gmtime() converts a local time value (as returned by time()) to the GMT
time and stores it in a time block. This function depends upon the global
variable timezone being set.


The time block is a predefined structure (declared in time.h) as follows;

     struct tm
     {
          int tm_sec;
          int tm_min;
          int tm_hour;
          int tm_mday;
          int tm_mon;
          int tm_year;
          int tm_wday;
          int tm_yday;
          int tm_isdst;
     };

tm_mday records the day of the month, ranging from 1 to 31; tm_wday is
the day of the week with Sunday being represented by 0; the year is
recorded less 1900; tm_isdst is a flag to show whether daylight saving
time is in effect. The actual names of the structure and its elements may
vary from compiler to compiler, but the structure should be the same in
essence.

mktime() converts a time block to a calendar format. It follows the
prototype;

                time_t mktime(struct tm *t);

The following example allows entry of a date, and uses mktime() to
calculate the day of the week appropriate to that date. Only dates from
the first of January 1970 are recognisable by the time functions.

     #include <stdio.h>
     #include <time.h>
     #include <string.h>
     
     void main()
     {
          struct tm tsruct;
          int okay;
          char data[100];
          char *p;
          char *wday[] = {"Sunday", "Monday", "Tuesday", "Wednesday",
     "Thursday", "Friday", "Saturday" ,
                    "prior to 1970, thus not known" };
          do
          {
               okay = 0;
               printf("\nEnter a date as dd/mm/yy ");
               p = fgets(data,8,stdin);
               p = strtok(data,"/");
     
               if (p != NULL)
                    tsruct.tm_mday = atoi(p);
               else
                    continue;
     
               p = strtok(NULL,"/");
               if (p != NULL)
                    tsruct.tm_mon = atoi(p);
               else
                    continue;
     
               p = strtok(NULL,"/");
     
               if (p != NULL)
                    tsruct.tm_year = atoi(p);
               else
                    continue;
               okay = 1;
          }
          while(!okay);
     
          tsruct.tm_hour = 0;
          tsruct.tm_min = 0;
          tsruct.tm_sec = 1;
          tsruct.tm_isdst = -1;
     
          /* Now get day of the week */
          if (mktime(&tsruct) == -1)
          tsruct.tm_wday = 7;
     
          printf("That was %s\n",wday[tsruct.tm_wday]);
     }
     
mktime() also makes the neccessary adjustments for values out of range,
this can be utilised for discovering what the date will be in n number of
days time thus;


     #include <stdio.h>
     #include <time.h>
     #include <string.h>
     
     void main()
     {
          struct tm *tsruct;
          time_t today;
     
          today = time(NULL);
          tsruct = localtime(&today);
     
          tsruct->tm_mday += 10;
          mktime(tsruct);
     
          printf("In ten days it will be %02d/%02d/%2d\n", tsruct-
     >tm_mday,tsruct->tm_mon + 1,tsruct->tm_year);
     
     }
     

This program uses Julian Dates to decide any day of the week since the
1st of October 1582 when the Gregorian calendar was introduced.

     char *WDAY(int day, int month, int year)
     {
          /* Returns a pointer to a string representing the day of the
     week */
     
          static char *cday[] = { "Saturday","Sunday","Monday","Tuesday",
     "Wednesday","Thursday","Friday" };
          double yy;
          double yt;
          double j;
          int y1;
          int y4;
          int x;
     
          yy = year / 100;
          y1 = (int)(yy);
          yt = year / 400;
          y4 = (int)(yt);
          x = 0;
     
          if (month < 3)
          {
               year--;
               x = 12;
          }
     
          j = day + (int)(365.25*year);
     
          j += (int)(30.6001 * (month + 1 + x)) - y1 + y4;
     
          if (yy == y1 && yt != y4 && month < 3)
               j++;
     
          j = 1 + j - 7 * (int)(j/7);
     
          if (j > 6)
               j -= 7;
     
          return(cday[j]);
     }
     
     
With time() and difftime() we can create a timer for testing the
execution times of functions thus;

     #include <stdio.h>
     #include <time.h>
     
     main()
     {
          time_t now;
          time_t then;
          double elapsed;
     
          int n;
     
          now = time(NULL);
     
          /* This loop is adjustable for multiple passes */
          for(n = 0; n < 5000; n++)
               /* Call the function to test */
               func();
     
          then = time(NULL);
     
          elapsed = difftime(then,now);
          printf("\nElapsed seconds==%lf\n",elapsed);
     }
     
By way of time() and ctime() the current system date and time can be
retrieved from the host computer thus;

     #include <stdio.h>
     #include <time.h>
     
     main()
     {
          time_t now;
          char *date;
          int n;
     
          /* Get system time */
          now = time(NULL);
     
          /* Convert system time to a string */
          date = ctime(&now);
     
          /*Display system time */
          printf("\nIt is %s",date);
     }
     
time_t is a type defined in time.h as the type of variable returned by
time(). This type may vary from compiler to compiler, and therefore is
represented by the type "time_t".
                                    
                              HEADER FILES

Function prototypes for library functions supplied with the C compiler,
and standard macros are declared in header files.

The ANSI standard on the C programming language lists the following
header files;

Header file    Description
               
assert.h       Defines the assert debugging macro
ctype.h        Character classification and
               conversion macros
errno.h        Constant mnemonics for error codes
float.h        Defines implementation specific
               macros for dealing with floating
               point mathematics
limits.h       Defines implementation specific
               limits on type values
locale.h       Country specific parameters
math.h         Prototypes for mathematics functions
setjmp.h       Defines typedef and functions for
               setjmp/longjmp
signal.h       Constants and declarations for use by
               signal() and raise()
stdarg.h       Macros for dealing with argument
               lists
stddef.h       Common data types and macros
stdio.h        Types and macros required for
               standard I/O
stdlib.h       Prototypes of commonly used functions
               and miscellany
string.h       String manipulation function
               prototypes
time.h         Structures for time conversion
               routines
                                    
                                DEBUGGING

The ANSI standard on C includes a macro function for debugging called
assert(). This expands to an if() statement, which if it returns true
terminates the program and outputs to the standard error stream a message
comprised of:


     Assertion failed: <test>, file <module>, line <line number>
     Abnormal program termination
For example, the following program accidentally assigns a zero value to a
pointer!

     #include <stdio.h>
     #include <assert.h>
     
     main()
     {
          /* Demonstration of assert */
     
          int *ptr;
          int x;
     
          x = 0;
     
          /* Whoops! error in this line! */
          ptr = x;
     
          assert(ptr != NULL);
     }
     
When run, this program terminates with the message:

     Assertion failed: ptr != 0, file TEST.C, line 16
     Abnormal program termination

When a program is running okay, the assert() functions can be removed
from the compiled program by simply adding the line;

     #define NDEBUG

before the #include <assert.h> line. Effectively the assert functions are
commented out in the preprocessed source before compilation, this means
that the assert expressions are not evaluated, and thus cannot cause any
side effects.
                                    
                               FLOAT ERRORS

Floating point numbers are decimal fractions, decimal fractions do not
accurately equate to normal fractions as not every number will divide
precisely by ten. This creates the potential for rounding errors in
calculations that use floating point numbers. The following program
illustrates one such example of rounding error problems;


     #include <stdio.h>
     
     void main()
     {
          float number;
     
          for(number = 1; number > 0.4; number -= 0.01)
               printf("\n%f",number);
     }
     
At about 0.47 (depending upon the host computer and compiler) the program
starts to store an inaccurate value for 'number'.

This problem can be minimised by using longer floating point numbers,
doubles or long doubles that have larger storage space allocated to them.
For really accurate work though, you should use integers and only convert
to a floating point number for display. You also should notice that most
C compilers default floating point numbers to `doubles' and when using
`float' types have to convert the double down to a float!


                                    
                             ERROR HANDLING

When a system error occurs within a program, for example when an attempt
to open a file fails, it is helpful to the program's user to display a
message reporting the failure. Equally it is useful to the program's
developer to know why the error occurred, or at least as much about it as
possible. To this end the ANSI standard on C describes a function,
perror(), which has the prototype;


     void perror(const char *s);

and is used to display an error message. The program's own prefixed error
message is passed to perror() as the string parameter. This error message
is displayed by perror() followed by the host's system error separated by
a colon. The following example illustrates a use for perror();


     #include <stdio.h>
     
     void main()
     {
          FILE *fp;
          char fname[] = "none.xyz";
     
          fp = fopen(fname,"r");
     
          if(!fp)
               perror(fname);
          return;
     }
     
If the fopen() operation fails, a message similar to;

     none.xyz: No such file or directory

is displayed.

You should note, perror() sends its output to the predefined stream
`stderr', which is usually the host computer's display unit.


perror() finds its message from the host computer via the global variable
'errno' that is set by most, but not all system functions.

Unpleasant errors might justify the use of abort(). abort() is a function
that terminates the running program with a message;

     "Abnormal program termination"

and returns an exit code of 3 to the parent process or operating system.



Critical Error Handling With The IBM PC AND DOS

The IBM PC DOS operating system provides a user amendable critical error
handling function. This function is usually discovered by attempting to
write to a disk drive that does not have a disk in it, in which case the
familiar;

     Not ready error writing drive A
     Abort Retry Ignore?

Message is displayed on the screen. Fine when it occurs from within a DOS
program, not so fine from within your own program!

The following example program shows how to redirect the DOS critical
error interrupt to your own function;


     /* DOS critical error handler test */
     
     #include <stdio.h>
     #include <dos.h>
     
     void interrupt new_int();
     void interrupt (*old_int)();
     
     char status;
     
     main()
     {
          FILE *fp;
     
          old_int = getvect(0x24);
     
          /* Set critical error handler to my function */
          setvect(0x24,new_int);
     
          /* Generate an error by not having a disc in drive A */
          fp = fopen("a:\\data.txt","w+");
     
          /* Display error status returned */
          printf("\nStatus ==  %d",status);
     
     }
     
     void interrupt new_int()
     {
          /* set global error code */
          status = _DI;
     
          /* ignore error and return */
          _AL = 0;
     }
     
When the DOS critical error interrupt is called, a status message is
passed in the low byte of the DI register. This message is one of;

Code                     Meaning
                         
00                       Write-protect error
01                       Unknown unit
02                       Drive not ready
03                       Unknown command
04                       Data error, bad CRC
05                       Bad request structure
                         length
06                       Seek error
07                       Unknown media type
08                       Sector not found
09                       Printer out of paper
0A                       Write error
0B                       Read error
0C                       General failure

Your critical error interrupt handler can transfer this status message
into a global variable, and then set the result message held in register
AL to one of;


Code                     Action
                         
00                       Ignore error
01                       Retry
02                       Terminate program
03                       Fail (Available with
                         DOS 3.3 and above)


If you choose to set AL to 02, terminate program, you should ensure ALL
files are closed first since DOS will terminate the program abruptly,
leaving files open and memory allocated, not a pretty state to be in!


The example program shown returns an ignore status from the critical
error interrupt, and leaves the checking of any errors to the program
itself. So, in this example after the call to fopen() we could check the
return value in fp, and if it reveals an error (NULL in this case) we
could then check the global variable status and act accordingly, perhaps
displaying a polite message to the user to put a disk in the floppy drive
and ensure that the door is closed.

The following is a practical function for checking whether a specified
disc drive can be accessed. It should be used with the earlier critical
error handler and global variable `status'.

     int DISCOK(int drive)
     {
          /* Checks for whether a disc can be read */
          /* Returns false (zero) on error */
          /* Thus if(!DISCOK(drive)) */
          /*          error();  */
     
          unsigned char buffer[25];
     
          /* Assume okay */
          status = 0;
     
          /* If already logged to disc, return okay */
          if ('A' + drive == diry[0])
               return(1);
     
          /* Attempt to read disc */
          memset(buffer,0,20);
          sprintf(buffer,"%c:$$$.$$$",'A'+drive);
     
          _open(buffer,O_RDONLY);
     
          /* Check critical error handler status */
          if (status == 0)
               return(1);
     
          /* Disc cannot be read */
          return(0);
     }
                                    
                                  CAST


Casting tells the compiler what a data type is, and can be used to change
a data type. For example, consider the following;

     #include <stdio.h>
     
     void main()
     {
          int x;
          int y;
     
          x = 10;
          y = 3;
     
          printf("\n%lf",x / y);
     }

The printf() function has been told to expect a double. However, the
compiler sees the variables `x' and `y' as integers, and an error occurs!
To make this example work we must tell the compiler that the result of
the expression x / y is a double, this is done with a cast thus;


     #include <stdio.h>
     
     void main()
     {
          int x;
          int y;
     
          x = 10;
          y = 3;
     
          printf("\n%lf",(double)(x / y));
     }

Notice the data type `double' is enclosed by parenthesis, and so is the
expression to convert. But now, the compiler knows that the result of the
expression is a double, but it still knows that the variables `x' and `y'
are integers and so an integer division will be carried out. We have to
cast the constants thus;

     #include <stdio.h>
     
     void main()
     {
          int x;
          int y;
     
          x = 10;
          y = 3;
     
          printf("\n%lf",(double)(x) / (double)(y));
     }

Because both of the constants are doubles, the compiler knows that the
outcome of the expression will also be a double.



                                    
                      THE IMPORTANCE OF PROTOTYPING

Prototyping a function involves letting the compiler know in advance what
type of values a function will receive and return. For example, lets look
at strtok(). This has the prototype;


     char *strtok(char *s1, const char *s2);

This prototype tells the compiler that strtok() will return a character
pointer, the first received parameter will be a pointer to a character
string, and that string can be changed by strtok(), and the last
parameter will be a pointer to a character string that strtok() cannot
change.

The compiler knows how much space to allocate for the return parameter,
sizeof(char *), but without a prototype for the function the compiler
will assume that the return value of strtok() is an integer, and will
allocate space for a return type of int, that is sizeof(int). If an
integer and a character pointer occupy the same number of bytes on the
host computer no major problems will occur, but if a character pointer
occupies more space than an integer, then the compiler wont have
allocated enough space for the return value and the return from a call to
strtok() will overwrite some other bit of memory. If, as so often happens
the return value is returned via the stack, the results of confusing the
compiler can be disastrous!

Thankfully most C compilers will warn the programmer if a call to a
function has been made without a prototype, so that you can add the
required function prototypes.

Consider the following example that will not compile on most modern C
compilers due to the nasty error in it;


     #include <stdio.h>
     
     int FUNCA(int x, int y)
     {
          return(MULT(x,y));
     }
     
     double MULT(double x, double y)
     {
          return(x * y);
     }
     
     
     main()
     {
          printf("\n%d",FUNCA(5,5));
     }
     
When the compiler first encounters the function MULT() it is as a call
from within FUNCA(). In the absence of any prototype for MULT() the
compiler assumes that MULT() returns an integer. When the compiler finds
the definition for function MULT() it sees that a return of type double
has been declared. The compiler then reports an error in the compilation
saying something like;


     "Type mismatch in redclaration of function 'MULT'"

What the compiler is really trying to say is, prototype your functions
before using them! If this example did compile, and was then run it
probably would crash the computer's stack and cause a system hang.

                                    
                          POINTERS TO FUNCTIONS

C allows a pointer to point to the address of a function, and this
pointer to be called rather than specifying the function. This is used by
interrupt changing functions and may be used for indexing functions
rather than using switch statements. For example;

     #include <stdio.h>
     #include <math.h>
     
     double (*fp[7])(double x);
     
     void main()
     {
          double x;
          int p;
     
          fp[0] = sin;
          fp[1] = cos;
          fp[2] = acos;
          fp[3] = asin;
          fp[4] = tan;
          fp[5] = atan;
          fp[6] = ceil;
     
          p = 4;
     
          x = fp[p](1.5);
          printf("\nResult %lf",x);
     }

This example program defines an array of pointers to functions, (*fp[])()
that are then called dependant upon the value in the indexing variable p.
This program could also be written;

     #include <stdio.h>
     #include <math.h>
     
     void main()
     {
          double x;
          int p;
     
          p = 4;
     
          switch(p)
          {
               case 0 :  x = sin(1.5);
                     break;
               case 1 :  x = cos(1.5);
                     break;
               case 2 :  x = acos(1.5);
                     break;
               case 3 :  x = asin(1.5);
                     break;
               case 4 :  x = tan(1.5);
                     break;
               case 5 :  x = atan(1.5);
                     break;
               case 6 :  x = ceil(1.5);
                     break;
          }
          puts("\nResult %lf",x);
     }

The first example, using pointers to the functions, compiles into much
smaller code and executes faster than the second example.

The table of pointers to functions is a useful facility when writing
language interpreters, the program compares an entered instruction
against a table of key words that results in an index variable being set
and then the program simply needs to call the function pointer indexed by
the variable, rather than wading through a lengthy switch() statement.

                                    
                            DANGEROUS PITFALLS

One of the most dangerous pitfalls can occur with the use of gets(). This
function accepts input from the stream stdin until it receives a newline
character, which it does not pass to the program. All the data it
receives is stored in memory starting at the address of the specified
string, and quite happily overflowing into other variables! This danger
can be avoided by using fgets() that allows a maximum number of
characters to be specified, so you can avoid overflow problems. Notice
though that fgets() does retain the newline character scanf() is another
function best avoided. It accepts input from stdin and stores the
received data at the addresses provided to it. If those addresses are not
really addresses where the data ends up is anybodys guess!

This example is okay, since scanf() has been told to store the data at
the addresses occupied by the two variables `x' and `y'.


     void main()
     {
          int x;
          int y;
     
          scanf("%d%d",&x,&y);
     }
     
But in this example scanf() has been told to store the data at the
addresses suggested by the current values of `x' and `y'! An easy and
common mistake to make, and yet one that can have very peculiar effects.


     void main()
     {
          int x;
          int y;
     
          scanf("%d%d",x,y);
     }
     
The answer is, don't use scanf(), use fgets() and parse your string
manually using the standard library functions strtod(), strtol() and
strtoul().

Here is the basis of a simple input string parser that returns the
individual input fields from an entered string;

     #include <stdio.h>
     #include <string.h>
     
     void main()
     {
          char input[80];
          char *p;
     
          puts("\nEnter a string ");
          fgets(input,79,stdin);
     
          /* now parse string for input fields */
          puts("The fields entered are:");
          p = strtok(input,", ");
          while(p)
          {
               puts(p);
               p = strtok(NULL,", ");
          }
     }
     
                                    
                                 SIZEOF

A preprocessor instruction, `sizeof', returns the size of an item, be it
a structure, pointer, string or whatever. However! take care when using
`sizeof'. Consider the following program;


     #include <stdio.h>
     #include <mem.h>
     
     char string1[80]; char *text = "This is a string of data" ;
     
     void main()
     {
          /* Initialise string1 correctly */
          memset(string1,0,sizeof(string1));
     
          /* Copy some text into string1 ? */
          memcpy(string1,text,sizeof(text));
     
          /* Display string1 */
          printf("\nString 1 = %s\n",string1);
     }
     
What it is meant to do is initialise all 80 elements of string1 to
zeroes, which it does alright, and then copy the constant string `text'
into the variable `string1'. However, variable text is a pointer, and so
the sizeof(text) instruction returns the size of the character pointer
(perhaps two bytes) rather than the length of the string pointed to by
the pointer!  If the length of the string pointed to by `text' happened
to be the same as the size of a character pointer then no error would be
noticed.

                                    
                               INTERRUPTS

The IBM PC BIOS and DOS contain functions that may be called by a program
by way of the function's interrupt number. The address of the function
assigned to each interrupt is recorded in a table in RAM called the
interrupt vector table. By changing the address of an interrupt vector a
program can effectively disable the original interrupt function and
divert any calls to it to its own function. This was done by the critical
error handler described in the section on error handling.

Borland's Turbo C provides two library functions for reading and changing
an interrupt vector. These are: setvect() and getvect(). The
corresponding Microsoft C library functions are: _dos_getvect() and
_dos_setvect().

getvect() has the function prototype;

     void interrupt(*getvect(int interrupt_no))();

setvect() has the prototype;

     void setvect(int interrupt_no, void interrupt(*func)());

To read and save the address of an existing interrupt a program uses
getvect() thus;

     /* Declare variable to record old interrupt */
     void interrupt(*old)(void);
     
     main()
     {
          /* get old interrupt vector */
          old = getvect(0x1C);
          .
          .
          .
     }
     
Where 0x1C is the interrupt vector to be retrieved.

To then set the interrupt vector to a new address, our own function, we
use setvect() thus;

     void interrupt new(void)
     {
          .
          .
          /* New interrupt function */
          .
          .
          .
     }
     
     main()
     {
          .
          .
          .
          setvect(0x1C,new);
          .
          .
          .
          .
     }
     
There are two important points to note about interrupts;

First, if the interrupt is called by external events then before changing
the vector you MUST disable the interrupt callers using disable() and
then re-enable the interrupts after the vector has been changed using
enable().  If a call is made to the interrupt while the vector is being
changed ANYTHING could happen!

Secondly, before your program terminates and returns to DOS you must
reset any changed interrupt vectors! The exception to this is the
critical error handler interrupt vector that is restored automatically by
DOS, so your program needn't bother restoring it.

This example program hooks the PC clock timer interrupt to provide a
background clock process while the rest of the program continues to run.
If included with your own program that requires a constantly displayed
clock on screen, you need only amend the display coordinates in the call
to puttext(). Sincle the closk display code is called by a hardware
issued interrupt, your program can start the clock and forget it until it
terminates.

     
     /* Compile in LARGE memory model */
     
     #include <stdio.h>
     #include <dos.h>
     #include <time.h>
     #include <conio.h>
     #include <stdlib.h>
     
     enum { FALSE, TRUE };
     
     #define COLOUR (BLUE << 4) | YELLOW
     
     #define BIOS_TIMER  0x1C
     
     static unsigned installed = FALSE;
     static void interrupt (*old_tick) (void);
     
     static void interrupt tick (void)
     {
          int i;
          struct tm *now;
          time_t this_time;
          char time_buf[9];
          static time_t last_time = 0L;
          static char video_buf[20] =
          {
               ' ', COLOUR, '0', COLOUR, '0', COLOUR, ':', COLOUR, '0',
     COLOUR,
               '0', COLOUR, ':', COLOUR, '0', COLOUR, '0', COLOUR, ' ',
     COLOUR
          };
     
          enable ();
     
          if (time (&this_time) != last_time)
          {
               last_time = this_time;
     
               now = localtime(&this_time);
     
               sprintf(time_buf, "%02d:%02d.%02d",now->tm_hour,now-
     >tm_min,now->tm_sec);
     
               for (i = 0; i < 8; i++)
               {
                    video_buf[(i + 1) << 1] = time_buf[i];
               }
     
               puttext (71, 1, 80, 1, video_buf);
          }
     
          old_tick ();
     }
     
     void stop_clock (void)
     {
          if (installed)
          {
               setvect (BIOS_TIMER, old_tick);
               installed = FALSE;
          }
     }
     
     void start_clock (void)
     {
          static unsigned first_time = TRUE;
     
          if (!installed)
          {
               if (first_time)
               {
                    atexit (stop_clock);
                    first_time = FALSE;
               }
     
               old_tick = getvect (BIOS_TIMER);
               setvect (BIOS_TIMER, tick);
               installed = TRUE;
          }
     }
                                    
                                 SIGNAL

Interrupts raised by the host computer can be trapped and diverted in
several ways. A simple method is to use signal().

Signal() takes two parameters in the form;

     void (*signal (int sig, void (*func) (int))) (int);

The first parameter, `sig' is the signal to be caught. These are often
predefined by the header file `signal.h'.

The second parameter is a pointer to a function to be called when the
signal is raised. This can either be a user function, or a macro defined
in the header file `signal.h' to do some arbitrary task, such as ignore
the signal for example.

On a PC platform, it is often useful to disable the `ctrl-break' key
combination that is used to terminate a running program by the user. The
following PC signal() call replaces the predefined signal `SIGINT', which
equates to the ctrl-break interrupt request, with the predefined macro
`SIG-IGN', ignore the request;


     signal(SIGINT,SIG_IGN);

This example catches floating point errors on a PC, and zero divisions!

     #include <stdio.h>
     #include <signal.h>
     
     void (*old_sig)();
     
     void catch(int sig)
     {
          printf("Catch was called with: %d\n",sig);
     }
     
     
     void main()
     {
          int a;
          int b;
     
          old_sig = signal(SIGFPE,catch);
     
          a = 0;
          b = 10 / a;
     
          /* Restore original handler before exiting! */
          signal(SIGFPE,old_sig);
     }
     
                                    
                          SORTING AND SEARCHING

The ANSI C standard defines qsort(), a function for sorting a table of
data. The function follows the format;

     qsort(void *base,size_t elements,size_t width,int (*cmp)(void *,
void *));

The following short program illustrates the use of qsort() with a
character array.

     #include <string.h>
     
     main()
     {
          int n;
          char data[10][20];
     
          /* Initialise some arbirary data */
     
          strcpy(data[0],"RED");
          strcpy(data[1],"BLUE");
          strcpy(data[2],"GREEN");
          strcpy(data[3],"YELLOW");
          strcpy(data[4],"INDIGO");
          strcpy(data[5],"BROWN");
          strcpy(data[6],"BLACK");
          strcpy(data[7],"ORANGE");
          strcpy(data[8],"PINK");
          strcpy(data[9],"CYAN");
     
          /* Sort the data table */
          qsort(data[0],10,20,strcmp);
     
          /* Print the data table */
          for(n = 0; n < 10; n++)
               puts(data[n]);
     }
     

Here is a program that implements the shell sort algorithm (this one is
based on the routine in K & R), which sorts arrays of pointers based upon
the data pointed to by the pointers;

     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>
     
     #define LINELEN     80
     #define MAXLINES    2000
     
     char *lines[MAXLINES];
     int lastone;
     
     void SHELL(void);
     
     void SHELL()
     {
          /* SHELL Sort Courtesy of K & R */
     
          int gap;
          int i;
          int j;
          char temp[LINELEN];
     
          for(gap = lastone / 2; gap > 0; gap /= 2)
          for(i = gap; i < lastone; i++)
               for(j = i - gap; j >= 0 && strcmp(lines[j] , lines[j +
     gap]) >
                  0; j -= gap)
               {
                    strcpy(temp,lines[j]);
                    strcpy(lines[j] , lines[j + gap]);
                    strcpy(lines[j + gap] , temp);
     
               }
     }
     
     void main(int argc, char *argv[])
     {
          FILE *fp;
          char buff[100];
          int n;

          /* Check command line parameter has been given */
          if (argc != 2)
          {
               printf("\nError: Usage is SERVSORT file");
               exit(0);
          }
     
          /* Attempt to open file for updating */
          fp = fopen(argv[1],"r+");
          if (fp == NULL)
          {
               printf("\nError: Unable to open %s",argv[1]);
               exit(0);
          }
     
          /* Initialise element counter to zero */
          lastone = 0;
     
          /* Read file to be sorted */
          while((fgets(buff,100,fp)) != NULL)
          {
               /* Allocate memory block*/
               lines[lastone] = malloc(LINELEN);
               if (lines[lastone] == NULL)
               {
                    printf("\nError: Unable to allocate memory");
                    fclose(fp);
                    exit(0);
               }
               strcpy(lines[lastone],buff);
               lastone++;
     
               if (lastone > MAXLINES)
               {
                    printf("\nError: Too many lines in source file");
                    exit(0);
               }
          }
          /* Call sort function */
          SHELL();
     
          /* Close file */
          fclose(fp);
     
          /* Reopen file in create mode */
          fp = fopen(argv[1],"w+");
     
          /* Copy sorted data from memory to disk */
          for(n = 0; n < lastone; n++)
               fputs(lines[n],fp);
     
          /* Close file finally */
          fclose(fp);
     
          /* Return to calling program */
          return(1);
     }
     

If we want to use qsort() with a table of pointers we have to be a bit
more clever than usual.

This example uses the colours again, but this time they are stored in
main memory and indexed by a table of pointers. Because we have a table
of pointers to sort there are two differences between this program's
qsort() and the previous one;

First we can't use strcmp() as the qsort() comparison function, secondly
the width of the table being sorted is sizeof(char *), that is the size
of a character pointer.

Notice the comparison function cmp() that receives two parameters, both
are pointers to a pointer. qsort() sends to this function the values held
in data[], which are in turn pointers to the data. So we need to use this
indirection to locate the data, otherwise we would be comparing the
addresses at which the data is held rather than the data itself!

     #include <alloc.h>
     #include <string.h>
     
     /* Function prototype for comparison function */
     int (cmp)(char **,char **);
     
     int cmp(char **s1, char **s2)
     {
          /* comparison function using pointers to pointers */
          return(strcmp(*s1,*s2));
     }
     
     main()
     {
          int n;
          char *data[10];
     
          for(n = 0; n < 10; n++)
               data[n] = malloc(20);
     
          strcpy(data[0],"RED");
          strcpy(data[1],"BLUE");
          strcpy(data[2],"GREEN");
          strcpy(data[3],"YELLOW");
          strcpy(data[4],"INDIGO");
          strcpy(data[5],"BROWN");
          strcpy(data[6],"BLACK");
          strcpy(data[7],"ORANGE");
          strcpy(data[8],"PINK");
          strcpy(data[9],"CYAN");
     
          /* The data table is comprised of pointers */
          /* so the call to qsort() must reflect this */
          qsort(data,10,sizeof(char *),cmp);
     
          for(n = 0; n < 10; n++)
               puts(data[n]);
     }
     
The quick sort is a fast sorting algorithm that works by subdividing the
data table into two sub-tables and then subdividing the sub-tables. As it
subdivides the table, so it compares the elements in the table and swaps
them as required.

The following program implements the quick sort algorithm, which is
usually already used by qsort();


     #include <string.h>
     
     #define MAXELE  2000
     
     char data[10][20];
     int lastone;
     
     void QKSORT()
     {
          /* Implementation of QUICKSORT algorithm */
     
          int i;
          int j;
          int l;
          int p;
          int r;
          int s;
          char temp[100];
          static int sl[MAXELE][2];
     
          /* sl[] is an index to the sub-table */
     
          l = 0;
          r = lastone;
          p = 0;
     
          do
          {
               while(l < r)
               {
                    i = l;
                    j = r;
                    s = -1;
     
                    while(i < j)
                    {
                         if (strcmp(data[i],data[j]) > 0)
                         {
                              strcpy(temp,data[i]);
                              strcpy(data[i],data[j]);
                              strcpy(data[j],temp);
                              s = 0 - s;
                         }
     
                         if (s == 1)
                              i++;
                         else
                              j--;
                    }
     
                    if (i + 1 < r)
                    {
                         p++;
                         sl[p][0] = i + 1;
                         sl[p][1] = r;
                    }
                    r = i - 1;
               }
               if (p != 0)
               {
                    l = sl[p][0];
                    r = sl[p][1];
                    p--;
               }
          }
          while(p > 0);
     }
     
     main()
     {
          int n;
     
          /* Initialise arbitrary data */
          strcpy(data[0],"RED");
          strcpy(data[1],"BLUE");
          strcpy(data[2],"GREEN");
          strcpy(data[3],"YELLOW");
          strcpy(data[4],"INDIGO");
          strcpy(data[5],"BROWN");
          strcpy(data[6],"BLACK");
          strcpy(data[7],"ORANGE");
          strcpy(data[8],"PINK");
          strcpy(data[9],"CYAN");
     
          /* Set last element indicator */
          lastone = 9;
     
          /* Call quick sort function */
          QKSORT();
     
          /* Display sorted list */
          for(n = 0; n < 10; n++)
               puts(data[n]);
     
     }
     
A table sorted into ascending order can be searched with bsearch(), this
takes the format;

     bsearch(key,base,num_elements,width,int (*cmp)(void *, void *));

bsearch() returns a pointer to the first element in the table that
matches the key, or zero if no match is found.

Or you can write your own binary search function thus;

     int BSRCH(char *key, void *data, int numele, int width)
     {
          /* A binary search function returning one if found */
          /* Zero if not found */
     
          int bp;
          int tp;
          int mp;
          int result;
          char *p;
     
          bp = 0;
          tp = numele;
          mp = (tp + bp) / 2;
     
          /* Locate element mp in table by assigning pointer to start */
          /* and incrementing it by width * mp */
          p = data;
          p += width * mp;
     
          while((result = strcmp(p,key)) != 0)
          {
               if (mp >= tp)
                    /* Not found! */
                    return(0);
               if (result < 0)
                    bp = mp + 1;
               else
                    tp = mp - 1;
     
               mp = (bp + tp) / 2;
               p = data;
               p += width * mp;
          }
          return(1);
     }
     
     void main()
     {
          int result;
          char data[10][20];
     
          /* Initialise some arbirary data */
     
          strcpy(data[0],"RED");
          strcpy(data[1],"BLUE");
          strcpy(data[2],"GREEN");
          strcpy(data[3],"YELLOW");
          strcpy(data[4],"INDIGO");
          strcpy(data[5],"BROWN");
          strcpy(data[6],"BLACK");
          strcpy(data[7],"ORANGE");
          strcpy(data[8],"PINK");
          strcpy(data[9],"CYAN");
     
          /* Sort the data table */
          qsort(data[0],10,20,strcmp);
     
          result = BSRCH("CYAN",data[0],10,20);
     
          printf("\n%s\n",(result == 0) ? "Not found" : "Located okay");
     }
     
There are other sorting algorithms as well. This program incorporates the
QUICK SORT, BUBBLE SORT, FAST BUBBLE SORT, INSERTION SORT and SHELL SORT
for comparing how each performs on a random 1000 item string list;

     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>
     
     char data[1000][4];
     char save[1000][4];
     
     int lastone;
     
     void INITDATA(void);
     void QKSORT(void);
     void SHELL(void);
     void BUBBLE(void);
     void FBUBBLE(void);
     void INSERTION(void);
     void MKDATA(void);
     
     void QKSORT()
     {
          /* Implementation of QUICKSORT algorithm */
     
          int i;
          int j;
          int l;
          int p;
          int r;
          int s;
          char temp[20];
          static int sl[1000][2];
     
          l = 0;
          r = lastone;
          p = 0;
     
          do
          {
               while(l < r)
               {
                    i = l;
                    j = r;
                    s = -1;
     
                    while(i < j)
                    {
                         if (strcmp(data[i],data[j]) > 0)
                         {
                              strcpy(temp,data[i]);
                              strcpy(data[i],data[j]);
                              strcpy(data[j],temp);
                              s = 0 - s;
                         }
     
                         if (s == 1)
                              i++;
                         else
                              j--;
                    }
     
                    if (i + 1 < r)
                    {
                         p++;
                         sl[p][0] = i + 1;
                         sl[p][1] = r;
                    }
                    r = i - 1;
               }
               if (p != 0)
               {
                    l = sl[p][0];
                    r = sl[p][1];
                    p--;
               }
          }
          while(p > 0);
     }
     
     void SHELL()
     {
          /* SHELL Sort Courtesy of K & R */
     
          int gap;
          int i;
          int j;
          char temp[20];
     
          for(gap = lastone / 2; gap > 0; gap /= 2)
          for(i = gap; i < lastone; i++)
               for(j = i - gap; j >= 0 && strcmp(data[j] , data[j + gap])
     > 0;
                      j -= gap)
               {
                    strcpy(temp,data[j]);
                    strcpy(data[j] , data[j + gap]);
                    strcpy(data[j + gap] , temp);
               }
     }
     
     void BUBBLE()
     {
          int a;
          int b;
          char temp[20];
     
          for(a = lastone; a >= 0; a--)
          {
               for(b = 0; b < a; b++)
               {
                    if(strcmp(data[b],data[b + 1]) > 0)
                    {
                         strcpy(temp,data[b]);
                         strcpy(data[b] , data[b + 1]);
                         strcpy(data[b + 1] , temp);
                    }
               }
          }
     }
     
     void FBUBBLE()
     {
          /* bubble sort with swap flag*/
     
          int a;
          int b;
          int s;
          char temp[20];
     
          s = 1;
     
          for(a = lastone; a >= 0 && s == 1; a--)
          {
               s = 0;
               for(b = 0; b < a; b++)
               {
                    if(strcmp(data[b],data[b + 1]) > 0)
                    {
                         strcpy(temp,data[b]);
                         strcpy(data[b] , data[b + 1]);
                         strcpy(data[b + 1] , temp);
                         s = 1;
                    }
               }
          }
     }
     
     void INSERTION()
     {
          int a;
          int b;
          char temp[20];
     
          for(a = 0; a < lastone; a++)
          {
               b = a;
               strcpy(temp,data[a + 1]);
               while(b >= 0)
               {
                    if (strcmp(temp,data[b]) < 0)
                    {
                         strcpy(data[b+1],data[b]);
                         b--;
                    }
                    else
                         break;
               }
               strcpy(data[b+1],temp);
          }
     }
     
     void MKDATA()
     {
          /* Initialise arbitrary data */
          /* Uses random(), which is not ANSI C */
          /* Returns a random number between 0 and n - 1 */
     
          int n;
          for(n = 0; n < 1000; n++)
               sprintf(save[n],"%d",random(1000));
     }
     
     void INITDATA()
     {
          int n;
     
          for(n = 0 ; n < 1000; n++)
               strcpy(data[n],save[n]);
     }
     
     void main()
     {
          MKDATA();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 999;
     
          /* Call quick sort function */
          QKSORT();
     
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 1000;
     
          /* Call shell sort function */
          SHELL();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 999;
     
          /* Call bubble sort function */
          BUBBLE();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 999;
     
          /* Call bubble sort with swap flag function */
          FBUBBLE();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 999;
     
          /* Call insertion sort function */
          INSERTION();
     }

Here are the profiler results of the above test program run on 1000 and
5000 random items;

STRING SORT - 1000 RANDOM ITEMS

FBUBBLE        26.436 sec  41%
|********************************************
BUBBLE         26.315 sec  41%
|*******************************************
INSERTION      10.210 sec  15% |***************
SHELL               0.8050 sec   1%  |*
QKSORT         0.3252 sec  <1% |

STRING SORT - 5000 RANDOM ITEMS

FBUBBLE        563.70 sec  41%
|********************************************
BUBBLE         558.01 sec  41%
|********************************************
INSERTION      220.61 sec  16% |***************
SHELL               5.2531 sec  <1% |
QKSORT         0.8379 sec  <1% |

Here is the same test program amended for sorting tables of integers;

     /* Integer sort test program */
     
     #include <stdio.h>
     #include <stdlib.h>
     
     void INITDATA(void);
     void QKSORT(void);
     void SHELL(void);
     void BUBBLE(void);
     void FBUBBLE(void);
     void INSERTION(void);
     void MKDATA(void);
     
     int data[1000];
     int save[1000];
     
     int lastone;
     
     void QKSORT()
     {
          /* Implementation of QUICKSORT algorithm */
     
          int i;
          int j;
          int l;
          int p;
          int r;
          int s;
          int temp;
          static int sl[1000][2];
     
          l = 0;
          r = lastone;
          p = 0;
     
          do
          {
               while(l < r)
               {
                    i = l;
                    j = r;
                    s = -1;
     
                    while(i < j)
                    {
                         if (data[i] > data[j])
                         {
                              temp = data[i];
                              data[i] = data[j];
                              data[j] = temp;
                              s = 0 - s;
                         }
     
                         if (s == 1)
                              i++;
                         else
                              j--;
                    }
     
                    if (i + 1 < r)
                    {
                         p++;
                         sl[p][0] = i + 1;
                         sl[p][1] = r;
                    }
                    r = i - 1;
               }
               if (p != 0)
               {
                    l = sl[p][0];
                    r = sl[p][1];
                    p--;
               }
          }
          while(p > 0);
     }
     
     void SHELL()
     {
          /* SHELL Sort Courtesy of K & R */
     
          int gap;
          int i;
          int j;
          int temp;
     
          for(gap = lastone / 2; gap > 0; gap /= 2)
          for(i = gap; i < lastone; i++)
               for(j = i - gap; j >= 0 && data[j] > data[j + gap];
                      j -= gap)
               {
                    temp = data[j];
                    data[j] = data[j + gap];
                    data[j + gap] = temp;
               }
     }
     
     void BUBBLE()
     {
          int a;
          int b;
          int temp;
     
          for(a = lastone; a >= 0; a--)
          {
               for(b = 0; b < a; b++)
               {
                    if(data[b] > data[b + 1])
                    {
                         temp = data[b];
                         data[b] = data[b + 1];
                         data[b + 1] = temp;
                    }
               }
          }
     }
     
     void FBUBBLE()
     {
          /* bubble sort with swap flag */
     
          int a;
          int b;
          int s;
          int temp;
     
          s = 1;
     
          for(a = lastone; a >= 0 && s == 1; a--)
          {
               s = 0;
               for(b = 0; b < lastone - a; b++)
               {
                    if(data[b] > data[b + 1])
                    {
                         temp = data[b];
                         data[b] = data[b + 1];
                         data[b + 1] = temp;
                         s = 1;
                    }
               }
          }
     }
     
     void INSERTION()
     {
          int a;
          int b;
          int temp;
     
          for(a = 0; a < lastone; a++)
          {
               b = a;
               temp = data[a + 1];
               while(b >= 0)
               {
                    if (temp < data[b])
                    {
                         data[b+1] = data[b];
                         b--;
                    }
                    else
                         break;
               }
               data[b+1] = temp;
          }
     }
     
     void MKDATA()
     {
          int n;
     
          for(n = 0; n < 1000; n++)
               save[n] = random(1000);
     }
     
     void INITDATA()
     {
          int n;
     
          for(n = 0; n < 1000; n++)
          data[n] = save[n];
     }
     
     void main()
     {
          int n;
     
          /* Create 1000 random elements */
          MKDATA();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 999;
     
          /* Call quick sort function */
          QKSORT();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 1000;
     
          /* Call shell sort function */
          SHELL();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 999;
     
          /* Call bubble sort function */
          BUBBLE();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 999;
     
          /* Call bubble sort with swap flag function */
          FBUBBLE();
     
          /* Initialise arbitrary data */
          INITDATA();
     
          /* Set last element indicator */
          lastone = 999;
     
          /* Call insertion sort function */
          INSERTION();
     }

And here are the profiler results for this program;

INTEGER SORTS - 1000 RANDOM NUMBERS (0 - 999)

FBUBBLE        3.7197 sec  41%
|********************************************
BUBBLE         3.5981 sec  39%
|******************************************
INSERTION      1.4258 sec  15% |***************
SHELL               0.1207 sec   1%  |*
QKSORT         0.0081 sec  <1% |

INTEGER SORTS - 5000 RANDOM NUMBERS (0 - 999)

FBUBBLE        92.749 sec  42%
|********************************************
BUBBLE         89.731 sec  41%
|********************************************
INSERTION      35.201 sec  16% |***************
SHELL               0.9838 sec  <1% |
QKSORT         0.0420 sec  <1% |

INTEGER SORTS - 5000 RANDOM NUMBERS (0 - 99)

FBUBBLE        92.594 sec  42% |*****************************************
BUBBLE         89.595 sec  40% |****************************************
INSERTION      35.026 sec  16% |***************
SHELL               0.7563 sec  <1% |
QKSORT         0.6018 sec  <1% |

INTEGER SORTS - 5000 RANDOM NUMBERS (0 - 9)

FBUBBLE        89.003 sec  41%
|*******************************************
BUBBLE         86.921 sec  40%
|*******************************************
INSERTION      31.544 sec  14% |***************
QKSORT         6.0358 sec   2%  |**
SHELL               0.5424 sec  <1% |

INTEGER SORTS - 5000 DESCENDING ORDERED NUMBERS

FBUBBLE        122.99 sec  39%
|******************************************
BUBBLE         117.22 sec  37% |****************************************
INSERTION      70.595 sec  22% |**********************
SHELL               0.6438 sec  <1% |
QKSORT         0.0741 sec  <1% |

INTEGER SORTS - 5000 ORDERED NUMBERS

BUBBLE         62.908 sec  99%
|******************************************
SHELL               0.3971 sec  <1% |
INSERTION      0.0510 sec  <1% |
QKSORT         0.0382 sec  <1% |
FBUBBLE        0.0251 sec  <1% |

INTEGER SORTS - 10000 RANDOM NUMBERS (0 - 999)

FBUBBLE        371.18 sec  42% |****************************************
BUBBLE         359.06 sec  41% |***************************************
INSERTION      140.88 sec  16% |**************
SHELL               2.0423 sec  <1% |
QKSORT         0.6183 sec  <1% |

Theory has it that the performance of a sorting algorithm is dependant
upon;

     a) the number of items to be sorted and
     b) how unsorted the list is to start with.

With this in mind it is worth testing the various sorting routines
described here to determine which one will best suit your particular
application. If you examine the above profiler results you will see that:

     1) With an already sorted list FBUBBLE() executes fastest
     
     2) With a random list of small variations between the values SHELL()
         executes fastest
     
     3) With a random list of large variations between the values
     QKSORT()
          executes the fastest

What the profiler does not highlight is that when the comparison aspect
of a sort function takes a disproportionately long time to execute in
relation to the rest of the sort function, then the bubble sort with a
swap flag will execute faster than the bubble sort with out a swap flag.

When considering a sort routine take into consideration memory
constraints and the type of data to be sorted as well as the relative
performances of the sort functions. Generally, the faster a sort
operates, the more memory it requires. Compare the simple bubble sort
with the quick sort, and you will see that the quick sort requires far
more memory than the bubble sort.

                                    
                        DYNAMIC MEMORY ALLOCATION


If a program needs a table of data, but the size of the table is
variable, perhaps for a list of all file names in the current directory,
it is inefficient to waste memory by declaring a data table of the
maximum possible size. Rather it is better to dynamically allocate the
table as required.

Turbo C allocates RAM as being available for dynamic allocation into an
area called the "heap". The size of the heap varies with memory model.
The tiny memory model defaults to occupy 64K of RAM. The small memory
model allocates upto 64K for the program/code and heap with a far heap
being available within the remainder of conventional memory. The other
memory models make all conventional memory available to the heap. This is
significant when programming in the tiny memory model when you want to
reduce the memory overhead of your program to a minimum. The way to do
this is to reduce the heap to a minimum size. The smallest is 1 byte.

C provides a function malloc() which allocates a block of free memory of
a specified size and returns a pointer to the start of the block; it also
provides free() which deallocates a block of memory previously allocated
by malloc(). Notice, however, that the IBM PC doesnot properly free
blocks of memory, and contiuous use of malloc() and free() will
fragmentise memory, eventually causing no memory to be available untill
the program terminates.

This program searches a specified file for a specified string (with case
sensitivity). It uses malloc() to allocate just enough memory for the
file to be read into memory.

     #include <stdio.h>
     #include <stdlib.h>
     
     char *buffer;
     
     void main(int argc, char *argv[])
     {
          FILE *fp;
          long flen;
     
          /* Check number of parameters */
          if (argc != 3)
          {
               fputs("Usage is sgrep <text> <file spec>",stderr);
               exit(0);
          }
     
          /* Open stream fp to file */
          fp = fopen(argv[2],"r");
          if (!fp)
          {
               perror("Unable to open source file");
               exit(0);
          }
     
          /* Locate file end */
          if(fseek(fp,0L,SEEK_END))
          {
               fputs("Unable to determine file length",stderr);
               fclose(fp);
               exit(0);
          }
     
          /* Determine file length */
          flen = ftell(fp);
     
          /* Check for error */
          if (flen == -1L)
          {
               fputs("Unable to determine file length",stderr);
               fclose(fp);
               exit(0);
          }
     
          /* Set file pointer to start of file */
          rewind(fp);
     
          /* Allocate memory buffer */
          buffer = malloc(flen);
     
          if (!buffer)
          {
               fputs("Unable to allocate memory",stderr);
               fclose(fp);
               exit(0);
          }
     
          /* Read file into buffer */
          fread(buffer,flen,1,fp);
     
          /* Check for read error */
          if(ferror(fp))
          {
               fputs("Unable to read file",stderr);
     
               /* Deallocate memory block */
               free(buffer);
     
               fclose(fp);
               exit(0);
          }
     
          printf("%s %s in %s",argv[1],(strstr(buffer,argv[1])) ? "was
     found" : "was not found",argv[2]);
     
          /* Deallocate memory block before exiting */
          free(buffer);
          fclose(fp);
     }
                                    
                         VARIABLE ARGUMENT LISTS


Some functions, such as printf(), accept a variable number and type of
arguments. C provides a mechanism to write your own functions which can
accept a variable argument list. This mechanism is the va_ family defined
in the header file `stdarg.h'.

There are three macros which allow implementation of variable argument
lists; va_start(), va_arg() and va_end() and a variable type va_list
which defines an array which holds the information required by the
macros.

va_start() takes two parameters, the first is the va_list variable and
the second is the last fixed parameter sent to the function. va_start()
must be called before attempting to access the variable argument list as
it sets up pointers required by the other macros.

va_arg() returns the next variable from the argument list. It is called
with two parameters, the first is the va_list variable and the second is
the type of the argument to be extracted. So, if the next variable in the
argument list is an integer, it can be extracted with;


     <int> = va_arg(<va_list>,int);

va_end() is called after extracting all required variables from the
argument list, and simply tidies up the internal stack if appropriate.
va_end() accepts a single parameter, the va_list variable.

The following simple example program illustrates the basis for a printf()
type implementation where the types of the arguments is not known, but
can be determined from the fixed parameter. This example only caters for
integer, string and character types, but could easily by extended to
cater for other variable types as well by following the method
illustrated;

     #include <stdarg.h>
     
     char *ITOS(long x, char *ptr)
     {
          /* Convert a signed decimal integer to a string */
     
          long pt[9] = { 100000000, 10000000, 1000000, 100000, 10000,
     1000, 100, 10, 1 };
          int n;
     
          /* Check sign */
          if (x < 0)
          {
               *ptr++ = '-';
               /* Convert x to absolute */
               x = 0 - x;
          }
     
          for(n = 0; n < 9; n++)
          {
               if (x > pt[n])
               {
                    *ptr++ = 48 + x / pt[n];
                    x %= pt[n];
               }
          }
          return(ptr);
     }
     
     void varfunc(char *format, ...)
     {
          va_list arg_ptr;
          char output[1000];
          char *ptr;
          int bytes;
          int x;
          char *y;
          char z;
     
          /* Initialise pointer to argument list */
          va_start(arg_ptr, format);
     
          /* loop format string */
          ptr = output;
          bytes = 0;
          while(*format)
          {
               /* locate next argument */
               while(*format != '%')
               {
                    *ptr++ = *format++;
                    bytes++;
               }
               /* A % has been located */
               format++;
               switch(*format)
               {
                    case '%' :  *ptr++ = '%';
                             break;
     
                    case 'd' : /* integer expression follows */
                            x = va_arg(arg_ptr,int);
                            ptr = ITOS(x,ptr);
                            *ptr = 0;
                            format++;
                            bytes += strlen(output) - bytes;
                            break;
     
                    case 's' : /* String expression follows */
                            y = va_arg(arg_ptr,char *);
                            strcat(output,y);
                            x = strlen(y);
                            format++;
                            ptr += x;
                            bytes += x;
                            break;
     
                    case 'c' : /* Char expression follows */
                            z = va_arg(arg_ptr,char);
                            *ptr++ = z;
                            format++;
                            bytes++;
                            break;
               }
     
          }
     
          /* Clean stack just in case! */
          va_end(arg_ptr);
     
          /* Null terminate output string */
          *ptr = 0;
     
          /* Display what we got */
          printf("\nOUTPUT==%s",output);
     }
     
     void main()
     {
          varfunc("%d %s %c",5,"hello world",49);
     }
     

A simpler variation is to use vsprintf() rather than implementing our own
variable argument list access. However, it is beneficial to understand
how variable argument lists behave. The following is a simplification of
the same program, but leaving the dirty work to the compiler;

     #include <stdio.h>
     #include <stdarg.h>
     
     void varfunc(char *format, ...)
     {
          va_list arg_ptr;
          char output[1000];
     
          va_start(arg_ptr, format);
     
          vsprintf(output,format,arg_ptr);
     
          va_end(arg_ptr);
     
          /* Display what we got */
          printf("\nOUTPUT==%s",output);
     }
     
     void main()
     {
          varfunc("%d %s %c",5,"hello world",49);
     }
                                    
                         TRIGONOMETRY FUNCTIONS

The ANSI standard on C defines a number of trigonometry functions, all of
which accept an angle parameter in radians;


FUNCTION  PROTOTYPE                DESCRIPTION
acos      double acos(double x)              arc cosine of x
asin      double asin(double x)              arc sine of x
atan      double atan(double x)              arc tangent of x
atan2          double atan2(double x, double y)        arc tangent of y/x
cos       double cos(double x)               cosine of x
cosh      double cosh(double x)              hyperbolic cosine of x
sin       double sin(double x)               sine of x
sinh      double sinh(double x)              hyperbolic sine of x
tan       double tan(double x)               tangent of x
tanh      double tanh(double x)              hyperbolic tangent of x

There are 2PI radians in a circle, therefore 1 radian is equal to 360/2PI
degrees or approximately 57 degrees in 1 radian. The calculation of any
of the above functions requires large floating point numbers to be used
which is a very slow process. If you are going to use calls to a trig'
function, it is a good idea to use a lookup table of values rather than
keep on calling the function. This approach is used in the discussion on
circle drawing later in this book.

                                    
                                 ATEXIT


When ever a program terminates, it should close any open files (this is
done for you by the C compiler's startup/termination code which it
surrounds your program with), and restore the host computer to some
semblance of order.  Within a large program where exit may occur from a
number of locations it is a pain to have to keep on writing calls to the
cleanup routine. Fortunately we don't have to!

The ANSI standard on C describes a function, atexit(), which registers
the specified function, supplied as a parameter to atexit(), as a
function which is called immediately before terminating the program. This
function is called automatically, so the following program calls
`leave()' whether an error occurs or not;

     #include <stdio.h>
     
     void leave()
     {
          puts("\nBye Bye!");
     }
     
     void main()
     {
          FILE *fp;
          int a;
          int b;
          int c;
          int d;
          int e;
          char text[100];
     
          atexit(leave);
     
          fp = fopen("data.txt","w");
     
          if(!fp)
          {
               perror("Unable to create file");
               exit(0);
          }
     
          fprintf(fp,"1 2 3 4 5 \"A line of numbers\"");
     
          fflush(fp);
     
          if (ferror(fp))
          {
               fputs("Error flushing stream",stderr);
               exit(1);
          }
     
          rewind(fp);
          if (ferror(fp))
          {
               fputs("Error rewind stream",stderr);
               exit(1);
          }
     
          fscanf(fp,"%d %d %d %d %d %s",&a,&b,&c,&d,&e,text);
          if (ferror(fp))
          {
               /* Unless you noticed the deliberate bug earlier */
               /* The program terminates here */
               fputs("Error reading from stream",stderr);
               exit(1);
          }
     
          printf("\nfscanf() returned %d %d %d %d %d %s",a,b,c,d,e,text);
     }
                                    
                            INCREASING SPEED


In order to reduce the time your program spends executing it is essential
to know your host computer. Most computers are very slow at displaying
information on the screen. And the IBM PC is no exception to this. C
offers various functions for displaying data, printf() being one of the
most commonly used and also the slowest. Whenever possible try to use
puts(varname) in place of printf("%s\n",varname). Remembering that puts()
appends a newline to the string sent to the screen.

When multiplying a variable by a constant which is a factor of 2 many C
compilers will recognise that a left shift is all that is required in the
assembler code to carry out the multiplication rapidly. When multiplying
by other values it is often faster to do a multiple addition instead, so;


     'x * 3' becomes 'x + x + x'

Don't try this with variable multipliers in a loop because it becomes
very slow! But, where the multiplier is a constant it can be faster.
(Sometimes!) Another way to speed up multiplication and division is with
the shift commands, << and >>.

The instruction x /= 2 can equally well be written x >>= 1, shift the
bits of x right one place. Many compilers actually convert integer
divisions by 2 into a shift right instruction. You can use the shifts for
multiplying and dividing by 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 &c.
If you have difficulty understanding the shift commands consider the
binary form of a number;


     01001101  equal to   77

shifted right one place it becomes;

     00100110  equal to   38

Try to use integers rather than floating point numbers where ever
possible.  Sometimes you can use integers where you didn't think you
could! For example, to convert a fraction to a decimal one would normally
say;

     percentage = x / y * 100

This requires floating point variables. However, it can also be written
as;

     z = x * 100;
     percentage = z / y

Which works fine with integers, so long as you don't mind the percentage
being truncated. eg;

     5 / 7 * 100 is equal to 71.43 with floating point

but with integers;

     5 * 100 / 7 is equal to 71

(Assuming left to right expression evaluation. You may need to force the
multiplication to be done first as with `z = x * 100').

Here is a test program using this idea;

     float funca(double x, double y)
     {
          return(x / y * 100);
     }
     
     int funcb(int x,int y)
     {
          return(x * 100 / y);
     }
     
     void main()
     {
          int n;
          double x;
          int y;
     
          for(n = 0; n < 5000; n++)
          {
               x = funca(5,7);
               y = funcb(5,7);
          }
     }
     
And here is the results of the test program fed through a profiler;

funca            1.9169 sec  96%
|**********************************************
funcb            0.0753 sec   3%  |*

You can clearly see that the floating point function is 25 times slower
than the integer equivalent!

NB: Although it is normal practice for expressions to be evaluated left
to right, the ANSI standard on C does not specify an order of preference
for expression evaluation, and as such you should check your compiler
manual.

Another way of increasing speed is to use pointers rather than array
indexing. When you access an array through an index, for example with;

     x = data[i];

the compiler has to calculate the offset of data[i] from the beginning of
the array. A slow process. Using pointers can often improve things as the
following two bubble sorts, one with array indexing and one with pointers
illustrates;

     void BUBBLE()
     {
          /* Bubble sort using array indexing */
     
          int a;
          int b;
          int temp;
     
          for(a = lastone; a >= 0; a--)
          {
               for(b = 0; b < a; b++)
               {
                    if(data[b] > data[b + 1])
                    {
                         temp = data[b];
                         data[b] = data[b + 1];
                         data[b + 1] = temp;
                    }
               }
          }
     }
     
     void PTRBUBBLE()
     {
          /* Bubble sort using pointers */
     
          int temp;
          int *ptr;
          int *ptr2;
     
          for(ptr = &data[lastone]; ptr >= data; ptr--)
          {
               for(ptr2 = data; ptr2 < ptr; ptr2++)
               {
                    if(*ptr2 > *(ptr2 + 1))
                    {
                         temp = *ptr2;
                         *ptr2 = *(ptr2 + 1);
                         *(ptr2 + 1) = temp;
                    }
               }
          }
     }
     
Here are the profiler results for the two versions of the same bubble
sort operating on the same 1000 item, randomly sorted list;

BUBBLE       3.1307 sec  59% |******************************************
PTRBUBBLE    2.1686 sec  40% |***************************


Here is another example of how to initialise an array using first the
common indexing approach, and secondly the pointer approach;

     /* Index array initialisation */
     int n;
     
     for(n = 0; n < 1000; n++)
          data[n] = random(1000);
     
     
     /* Pointer array initialisation */
     int *n;
     
     for(n = data; n < &data[1000]; n++)
          *n = random(1000);
     

Needless to say, the pointer approach is faster than the index. The
pointer approach is only really of benefit when an array is going to be
traversed, as in the above examples. In the case of say a binary search
where a different and non-adjacent element is going to be tested each
pass then the pointer approach is no better than using array indexing.

The exception to this rule of using pointers rather than indexed access,
comes with pointer to pointers. Say your program has declared a table of
static data, such as:

static char *colours[] = { "Black", "Blue", "Green", "Yellow", "Red",
"White" };

It is faster to access the table with colours[n] than it is with a
pointer, since each element in the table colours[], is a pointer. If you
need to scan a string table for a value you can use this very fast
approach instead;

First the table is changed into a single string, with some delimiter
between the elements.

     static char *colours = "Black/Blue/Green/Yellow/Red/White";

Then to confirm that a value is held in the table you can use strstr();

     result = strstr(colours,"Cyan");

Using in-line assembler code can provide the greatest speed increase.
Care must be taken however not to interfere with the compiled C code. It
is usually safe to write a complete function with in-line assembler code,
but mixing in-line assembler with C code can be hazardous. As a rule of
thumb, get your program working without assembler code, and then if you
want to use in-line assembler, convert small portions of the code at a
time, testing the program at each stage. Video I/O is a very slow process
with C, and usually benefits from in-line assembler, and we have used
this principle quite widely in the example programs which follow later.

                                    
                               PC GRAPHICS

When programming graphics you should bear in mind that they are a machine
dependant subject. Code to produce graphics on an IBM PC will not port to
an Amiga, or VAX or any other type of computer.



Introduction To PC Graphics

The IBM PC and compatible range of computers display information on a
visual display unit (VDU). To enable the computer to send information to
the VDU a component is included within the computer called a "display
card". There are various display cards and VDUs which have been produced
for the IBM PC range of computers; monochrome display adapter (MDA),
colour graphics adapter (CGA), Hercules graphics card (HGC), Enhanced
graphics adapter (EGA), video graphics array (VGA), super video graphics
array (SVGA), memory controller gate array (MCGA), 8514/A and the Txas
Instruments Graphics Architecture (TIGA). For simplicity, this section
will concern itself only with the three more common types of display;

     CGA, EGA and VGA

Information about the VGA display is also relevant to the SVGA display
which in simple terms can do the same and more. This section will not
concern itself with monochrome displays since they are of limited use in
graphics work.


Display Modes

When an IBM PC computer is first switched on is set to display 80 columns
by 25 rows of writing. This measurement, 80 x 25 is called the
"resolution". A display mode which is intended for displaying writing is
called a "text" mode.  Where as a display mode which is intended for
displaying pictures is called a "graphics" mode.

If you look closely at the display you will see that each displayed
character is comprised of dots. In reality the entire display is
comprised of dots, called "pixels", which may be set to different
colours. In text display modes these pixels are not very relevant,
however, in graphics display modes each pixel may be selected and set by
a program. The size of the pixels varies with the display resolution. In
80 x 25 text mode the pixels are half the width they are in 40 x 25 text
display mode.

Depending upon the display card installed in the computer, there are a
number of display modes which may be used;


MODE   TYPE               RESOLUTION         COLOURS
                                             
 0       Text             40 x 25            4 (CGA), 16 (EGA,
                                             VGA) Shades of
                                             grey
 1       Text             40 x 25            4 (CGA), 16 (EGA,
                                             VGA)
 2       Text             80 x 25            4 (CGA), 16 (EGA,
                                             VGA) Shades of
                                             grey
 3       Text             80 x 25            4 (CGA), 16 (EGA,
                                             VGA)
 4       Graphics         320 x 200          4
 5       Graphics         320 x 200          4 (grey on CGA
                                             and EGA)
 6       Graphics         640 x 200          2
 7       Text             80 x 25            Mono (EGA, VGA)
13       Graphics         320 x 200          16 (EGA, VGA)
14       Graphics         640 x 200          16 (EGA, VGA)
15       Graphics         640 x 350          Mono (EGA, VGA)
16       Graphics         640 x 350          16 (EGA, VGA)
17       Graphics         640 x 480          2 (VGA)
18       Graphics         640 x 480          16 (VGA)
19       Graphics         320 x 200          256 (VGA)

The term resolution in graphics modes refers to the number of pixels
across and down the VDU. The larger the number of pixels, the smaller
each is and the sharper any displayed image appears. As you can see from
the table, the VGA display can produce a higher resolution than the other
display cards, resulting in sharper images being produced.


The CGA display card can produce a maximum resolution of 320 x 200
pixels, where as the VGA display card can produce a resolution of 640 x
480 pixels.  This is why writing on a VGA VDU looks so much sharper than
the writing displayed on a CGA VDU.



Accessing The Display

Inside the IBM PC computer is a silicon chip called the BIOS ROM, this
chip contains functions which may be called by an external computer
program to access the display card, which in turn passes the information
on to the VDU.  The BIOS display functions are all accessed by generating
interrupt 10 calls, with the number of the appropriate function stored in
the assembly language AH register.

A programmer interested in creating graphics displays must first be able
to switch the display card to an appropriate graphics mode. This is
achieved by calling the BIOS display function 0, with th number of the
desired display mode from the table stored in the assembly language AL
register thus the following assembly language code segment will switch
the display card to CGA graphics mode 4, assuming that is that the
display card is capable of this mode;

          mov   ah , 00
          mov   al , 04
          int   10h


A C function for selecting video display modes can be written;

     #include <dos.h>
     
     void setmode(unsigned char mode)
     {
          /* Sets the video display mode */
     
          union REGS inregs outreg;
     
          inreg.h.ah = 0;
          inreg.h.al = mode;
          int86(0x10,&inreg,&outregs);
     }
     
Any graphics are created by setting different pixels to different
colours, this is termed "plotting", and is achieved by calling BIOS
display function 12 with the pixel's horizontal coordinate in the
assembly language CX register and it's vertical coordinate in the
assembly language DX register and the required colour in the assembly
language AL register thus;


          mov   ah, 12
          mov   al, colour
          mov   bh, 0
          mov   cx, x_coord
          mov   dx, y_coord
          int   10h

The corresponding C function is;

     #include <dos.h>
     
     void plot(int x_coord, int y_coord, unsigned char colour)
     {
          /* Sets the colour of a pixel */
     
          union REGS regs;
     
          regs.h.ah = 12;
          regs.h.al = colour;
          regs.h.bh = 0;
          regs.x.cx = x_coord;
          regs.x.dx = y_coord;
          int86(0x10,&regs,&regs);
     }

The inverse function of plot is to read the existing colour setting of a
pixel. This is done by calling BIOS ROM display function 13, again with
the pixel's horizontal coordinate in the assembly language CX register
and it's vertical coordinate in the assembly language DX register. This
function then returns the pixel's colour in the assembly language AL
register;


     #include <dos.h>
     
     unsigned char get_pixel(int x_coord, int y_coord)
     {
          /* Reads the colour of a pixel */
     
          union REGS inreg, outreg;
     
          inreg.h.ah = 13;
          inreg.h.bh = 0;
          inreg.x.cx = x_coord;
          inreg.x.dx = y_coord;
          int86(0x10,&inreg,&outreg);
          return(outreg.h.al);
     }


Colour And The CGA

The CGA display card can display a maximum of 4 colours simultaneously at
any time. However, the display card can generate a total of 8 colours.
There are two sets of colours, called "palettes". The first palette
contains the colours;

     background, cyan, magenta and white.

the second palette contains the colours;

       background, green, red and yellow.

Colour 0 is always the same as the background colour.

The pixels displayed on the VDU take their colours from the currently
active palette, and are continuously being refreshed. So, if the active
palette changes, so to do the colours of the displayed pixels on the VDU.

Selection of the active CGA palette is achieved by calling the BIOS
display function 11 with the number of the desired palette (either 0 or
1) in the assembly language BH register;


          mov   ah, 11
          mov   bh, palette
          int   10h

The C function for selecting the CGA palette is;

     void palette(unsigned char palette)
     {
          union REGS inreg, outreg;
     
          inreg.h.ah = 11;
          inreg.h.bh = palette;
          int86(0x10,&inreg,&outreg);
     }
     
The background colour may be selected independantly from any of the eight
available colours by calling the same BIOS display function with a value
of 0 stored in the assembly language BH register and the desired
background colour in the assembly language BL register;


     mov   ah, 11
     mov   bh, 0
     mov   bl, colour
     int   10h

In C this function can be written;

     void background(unsigned char colour)
     {
          union REGS inreg, outreg;
     
          inreg.h.ah = 11;
          inreg.h.bh = 0;
          inreg.h.bl = colour;
          int86(0x10,&inreg,&outreg);
     }
     
The background colours available are;

     0       Black
     1       Blue
     2       Green
     3       Cyan
     4       Red
     5       Magenta
     6       Yellow
     7       White


Colour And The EGA

The EGA display card can display a maximum of 16 colours simultaneously
at any time. The colour of all pixels is continuously refreshed by the
display card by reading the colour from the EGA palette. Unlike the CGA
display card, the EGA display card allows you to redefine any or all of
the colours in the palette. Unfortunately only the first 8 colours may be
loaded into other palette colours.

The colours are;

     0    Black
     1    Blue
     2    Green
     3    Cyan
     4    Red
     5    Magenta
     6    Brown
     7    Light grey
     8    Dark grey
     9    Light blue
     10        Light green
     11   Light cyan
     12        Light red
     13        Light magenta
     14        Yellow
     15        White

Changing a palette colour is achieved by calling the BIOS display
function 16 with a value of 0 in the assembly language AL register, the
colour value (0 to 7) in the assembly language BH register and the number
of the palette colour (0 to 15) in the assembly language BL register
thus;


     mov   ah,16
     mov   al,0
     mov   bl,palette
     mov   bh,colour
     int   10h

In C this function may be written;

     void ega_palette(unsigned char colour, unsigned char palette)
     {
          union REGS inreg,outreg;
     
          inreg.h.ah = 16;
          inreg.h.al = 0;
          inreg.h.bl = palette;
          inreg.h.bh = colour;
     
          int86(0x10,&inreg,&outreg);
     }



Colour And The VGA

The VGA display card can display a maximum of 256 colours on the VDU at
any one time, these colours are defined by information held in 256
special registers called "DAC" registers. As with the CGA and EGA
displays, the colour of displayed pixels is continuously being refreshed,
and as such any change to a DAC register is immediately visible. Each DAC
register has three component data parts which record the amount of green,
blue and red colours which make up the displayed colour. Each of these
seperate data components can hold a value between 0 and 63 giving the VGA
display card the ability to display 262,144 colours! Although only a
small subset of them can be displayed at any one time.

Setting the value of a DAC register is achieved by calling the BIOS
display function 16 with a value of 16 stored in the assembly language AL
register, the green value stored in the assembly language CH register,
the blue value stored in the assembly language CL register and the red
value stored in the assembly language DH register and the number of the
DAC register to be set stored in the assembly language BX register;


          mov   ah,16
          mov   al,16
          mov   ch,green
          mov   cl,blue
          mov   dh,red
          mov   bx,dac
          int   10h


The C function to set a DAC register looks lik this;

     void set_dac(int dac, unsigned char green, unsigned char blue,
     unsigned char red)
     {
          union REGS regs;
     
          regs.h.ah = 16;
          regs.h.al = 16;
          regs.x.bx = dac;
          regs.h.ch = green;
          regs.h.cl = blue;
          regs.h.dh = red;
          int86(0x10,&regs,&regs);
     }
     


Displaying Text

The BIOS ROM provides three functions for displaying a single character.
The first function to consider is the one used extensively by DOS for
displaying messages, this is function 14 called "write text in teletype
mode". This function interprets some control characters; bell (ascii 7),
backspace (ascii 8), carriage return (ascii 10) and line feed (ascii 13)
but all other ascii codes are displayed, and the current cursor position
updated accordingly, moving down a row when a character is displayed in
the far right column. To call this function the assembly language
register AL holds the ascii code of the character to be displayed and
assembly language register BL holds the foreground colour for the
character to be displayed in if a graphics mode is active;


          mov   ah,14
          mov   al,character
          mov   bh,0
          mov   bl,foreground
          int   10h


A C function for accessing the write text in teletype mode may be written
like this;

     #include <dos.h>
     
     void teletype(unsigned char character, unsigned char foreground)
     {
          union REGS inreg, outreg;
     
          inreg.h.ah = 14;
          inreg.h.al = character;
          inreg.h.bh = 0;
          inreg.h.bl = foreground;
          int86(0x10,&inrg,&outreg);
     }
     
The second BIOS ROM display function for displaying a character allows
the foreground and background colours of the displayed character to be
defined. It also allows multiple copies of the character to be displayed
one after another automatically displaying subsequent characters at the
next display position, although the current cursor position is not
changed by this function.

This function is called "write character and attribute", and is BIOS ROM
display function number 9. It is called with the ascii code of the
character to be displayed in the assembly language AL register, the
display page in assembly language register BH, the foreground colour in
the first four bits of the assembly language register BL and the
background colour in the last four bits of the assembly language register
BL, the number of times the character is to be displayed is stored in the
assembly language CX register thus;


          mov   ah,9
          mov   al,character
          mov   bh,0
          mov   bl,foreground + 16 * background
          mov   cx,number
          int   10h

And in C;

     #include <dos.h>
     
     void char_attrib(unsigned char character, unsigned char foreground,
     unsigned char background, int number)
     {
          union REGS inreg,outreg;
     
          inreg.h.ah = 9;
          inreg.h.al = character;
          inreg.h.bh = 0;
          inreg.h.bl = (background << 4) + foreground;
          inreg.x.cx = number;
          int86(0x10,&inreg,&outreg);
     }

The last BIOS ROM display function for displaying a character retains the
foreground and background colours of the display position.

This function is called "write character", and is BIOS ROM display
function number 10. It is identical to BIOS ROM display function 9 except
that the colours of the displayed character are those which are prevalent
at the display position, except in graphics modes when the foreground
colour of the character is determined by the value in the assembly
language BL register.  Its use is as follows;


          mov   ah,10
          mov   al,character
          mov   bh,0
          mov   bl,foreground   ; For graphics modes ONLY
          mov   cx,number
          int   10h

And in C;

     #include <dos.h>
     
     void char_attrib(unsigned char character, unsigned char foreground,
     int number)
     {
          union REGS inreg,outreg;
     
          inreg.h.ah = 10;
          inreg.h.al = character;
          inreg.h.bh = 0;
          inreg.h.bl = foreground; /* For graphics modes ONLY */
          inreg.x.cx = number;
          int86(0x10,&inreg,&outreg);
     }
     

Positioning of the text cursor is provided for by the ROM BIOS display
function number 2. It is called with the row number in the assembly
language register DH and the column number in the assembly language
register DL;

          mov   ah,2
          mov   bh,0
          mov   dh,row
          mov   dl,column
          int   10h

The corresponding function in C looks like this;

     #include <dos.h>
     
     void at(unsigned char row, unsigned char column)
     {
          union REGS regs;
     
          regs.h.ah = 2;
          regs.h.bh = 0;
          regs.h.dh = row;
          regs.h.dl = column;
          int86(0x10,&regs,&regs);
     }

From these basic functions a more useful replacement for the C language's
"printf()" function can be written which allows data to be displayed at
the current cursor position, previously set by a call to "at()", with
prescribed attributes;


     #include <dos.h>
     #include <stdarg.h>
     
     void at(unsigned char row, unsigned char column)
     {
          union REGS regs;
     
          regs.h.ah = 2;
          regs.h.bh = 0;
          regs.h.dh = row;
          regs.h.dl = column;
          int86(0x10,&regs,&regs);
     }
     
     void xprintf(unsigned char foreground, unsigned char background,
     char *format,...)
     {
          union REGS inreg,outreg;
     
          va_list arg_ptr;
          static char output[1000];
          unsigned char col;
          unsigned char row;
          unsigned char n;
          unsigned char p;
          unsigned char text;
          unsigned char attr;
     
          /* Convert foreground and background colours into a single
     attribute */
          attr = (background << 4) + foreground;
     
          /* Copy data into a single string */
          va_start(arg_ptr, format);
          vsprintf(output, format, arg_ptr);
     
          /* Determine number of display columns */
          inreg.h.ah = 15;
          int86(0x10,&inreg,&outreg);
          n = outreg.h.ah;
     
          /* Determine current cursor position */
          inreg.h.ah = 3;
          inreg.h.bh = 0;
          int86(0x10,&inreg,&outreg);
          row = outreg.h.dh;
          col = outreg.h.dl;
     
          /* Now display data */
          p = 0;
          while (output[p])
          {
               /* Display this character */
               inreg.h.bh = 0;
               inreg.h.bl = attr;
               inreg.x.cx = 01;
               inreg.h.ah = 9;
               inreg.h.al = output[p++];
               int86(0x10,&inreg,&outreg);
     
               /* Update cursor position */
               /* moving down a row if required */
               col++;
               if (col < (n - 1))
                    at(row, col);
               else
               {
                    col = 0;
                    at(++row, col);
               }
          }
     }
     
This function, "xprintf()" illustrates two more functions of the BIOS
ROM. The first is the call to function 15 which returns the number of
text display columns for the currently active display mode.

The other function illustrated, but not yet discussed, is BIOS ROM
function 3 which returns information about the cursor. The cursor's row
is returned in the assembly language register DH, and it's column in the
assembly language register DL.


                                    
                     ADVANCED GRAPHICS ON THE IBM PC

This section aims to reveal more about the graphics facilities offered by
the IBM PC, in particular topics which are of a more complex nature than
those addressed in the previous section.



Display Pages

The information for display by the display card is stored in an area of
memory called the "video RAM". The size of the video RAM varies from one
display card to another, and the amount of video RAM required for a
display varies with the selected display mode. Display modes which do not
require all of the video RAM use the remaining video RAM for additional
display pages.


MODE               PAGES
 0                 8
 1                 8
 2                 4 (CGA) 8 (EGA, VGA)
 3                 4 (CGA) 8 (EGA, VGA)
 4                 1
 5                 1
 6                 1
 7                 8 (EGA, VGA)
13                 8 (EGA, VGA)
14                 4 (EGA, VGA)
15                 2 (EGA, VGA)
16                 2 (EGA, VGA)
17                 1 (VGA)
18                 1 (VGA)
19                 1 (VGA)

Many of the BIOS ROM display functions allow selection of the display
page to be written to, regardless of which page is currently being
displayed.

The display card continuously updates the VDU from the information in the
active display page. Changing the active display page instantaneously
changes the display.

This provides the graphics programmer with the means to build a display
on an undisplayed page, and to then change the active display page so
that the viewer does not see the display being drawn.

Selection of the active display page is achieved by calling BIOS ROM
display function 5 with the number of the required display page stored in
the assembly language register AL;

          mov   ah,5
          mov   al,page
          int   10h

Or, from C this function becomes;

     #include <dos.h>
     
     void set_page(unsigned char page)
     {
          union REGS inreg, outreg;
     
          inreg.h.ah = 5;
          inreg.h.al = page;
          int86(0x10,&inreg,&outreg);
     }
     
The display page to which BIOS ROM display functions write is decided by
the value stored in the assembly language BH register. The functions for
setting a pixel's colour may be amended thus;

          mov   ah, 12
          mov   al, colour
          mov   bh, page
          mov   cx, x_coord
          mov   dx, y_coord
          int   10h

And the corresponding C function becomes;

     #include <dos.h>
     
     void plot(int x_coord, int y_coord, unsigned char colour, unsigned
     char page)
     {
          /* Sets the colour of a pixel */
     
          union REGS inreg, outreg;
     
          inreg.h.ah = 12;
          inreg.h.al = colour;
          inreg.h.bh = page;
          inreg.x.cx = x_coord;
          inreg.x.dx = y_coord;
          int86(0x10,&inreg,&outreg);
     }

The currently active display page can be determined by calling BIOS ROM
display function 15. This function returns the active display page in the
assembly language register BH;

          mov   ah,15
          int   10h
                              ; BH now holds active page number

                                    
                         Advanced Text Routines

When the IBM PC display is in a text mode a blinking cursor is displayed
at the current cursor position. This cursor is formed of a rectangle
which is one complete character width, but it's top and bottom pixel
lines are definable within the limits of the character height by calling
BIOS ROM display function 1. A CGA display has a character height of 8
pixel lines, an EGA display has a character height of 14 lines and a VGA
display has a character height of 16 lines.

BIOS ROM function 1 is called with the top pixel line number of the
desired cursor shape in assembly language register CH and the bottom
pixel line number in assembly language register CL;


          mov   ah,1
          mov   ch,top
          mov   cl,bottom
          int   10h

A C function to set the cursor shape may be be written thus;

     #include <dos.h>
     
     void setcursor(unsigned char top, unsigned char bottom)
     {
          union REGS inreg, outreg;
     
          inreg.h.ch = start;
          inreg.h.cl = end;
          inreg.h.ah = 1;
          int86(0x10, &inreg, &outreg);
     }
     
If the top pixel line is defined as larger than the bottom line, then the
cursor will appear as a pair of parallel rectangles.

The cursor may be removed from view by calling BIOS ROM display function
1 with a value of 32 in the assembly language CH register.

The current shape of the cursor can be determined by calling BIOS ROM
function 3, which returns the top pixel line number of the cursor shape
in the assembly language CH register, and the bottom line number in the
assembly language register CL.

Two functions are provided by the BIOS ROM for scrolling of the currently
active display page. These are function 6, which scrolls the display up
and function 7 which scrolls the display down.

Both functions accept the same parameters, these being the number of
lines to scroll in the assembly language register AL, the colour
attribute for the resulting blank line in the assembly language BH
register, the top row of the area to be scrolled in the assembly language
CH register, the left column of the area to be scrolled in the assembly
language CL register, the bottom row to be scrolled in the assembly
language DH register and the right most column to be scrolled in the
assembly language DL register.

If the number of lines being scrolled is greater than the number of lines
in the specified area, then the result is to clear the specified area,
filling it with spaces in the attribute specified in the assembly
language BH register.


Scrolling

A C function to scroll the entire screen down one line can be written
thus;

     #include <dos.h>
     
     void scroll_down(unsigned char attr)
     {
          union REGS inreg, outreg;
     
          inreg.h.al = 1;
          inreg.h.cl = 0;
          inreg.h.ch = 0;
          inreg.h.dl = 79;
          inreg.h.dh = 24;    /* Assuming a 25 line display */
          inreg.h.bh = attr;
          inreg.h.ah = 7;
          int86(0x10, &inreg, &outreg);
     }


Clear Screen
A simple clear screen function can be written in C based upon the
"scroll_down()" function simply by changing the value assigned to
inreg.h.al to 0;

     #include <dos.h>
     
     void cls(unsigned char attr)
     {
          union REGS inreg, outreg;
     
          inreg.h.al = 0;
          inreg.h.cl = 0;
          inreg.h.ch = 0;
          inreg.h.dl = 79;
          inreg.h.dh = 24;    /* Assuming a 25 line display */
          inreg.h.bh = attr;
          inreg.h.ah = 7;
          int86(0x10, &inreg, &outreg);
     }



Windowing
Windowing functions need to preserve the display they overwrite, and
restore it when the window is removed from display. The BIOS ROM provides
a display function which enables this to be done.

Function 8 requires the appropriate display page number to be stored in
assembly language register BH, and then when called it returns the ascii
code of the character at the current cursor position of that display page
in the assembly language AL register, and the display attribute of the
character in the assembly language AH register.

The following C functions allow an area of the display to be preserved,
and later restored;

     #include <dos.h>
     
     void at(unsigned char row, unsigned char column, unsigned char page)
     {
          /* Position the cursor */
     
          union REGS inreg,outreg;
     
          inreg.h.ah = 2;
          inreg.h.bh = page;
          inreg.h.dh = row;
          inreg.h.dl = column;
          int86(0x10,&inreg,&outreg);
     }
     
     void get_win(unsigned char left, unsigned char top, unsigned char
     right,unsigned char bottom, unsigned char page,        char *buffer)
     {
          /* Read a text window into a variable */
     
          union REGS inreg,outreg;
     
          unsigned char old_left;
          unsigned char old_row;
          unsigned char old_col;
     
          /* save current cursor position */
          inreg.h.ah = 3;
          inreg.h.bh = page;
          int86(0x10,&inreg,&outreg);
          old_row = outreg.h.dh;
          old_col = outreg.h.dl;
     
          while(top <= bottom)
          {
               old_left = left;
               while(left <= right)
               {
                    at(top,left,page);
                    inreg.h.bh = page;
                    inreg.h.ah = 8;
                    int86(0x10,&inreg,&outreg);
                    *buffer++ = outreg.h.al;
                    *buffer++ = outreg.h.ah;
                    left++;
               }
     
               left = old_left;
               top++;
          }
     
          /* Restore cursor to original location */
          at(old_row,old_col,page);
     }
     
     void put_win(unsigned char left, unsigned char top, unsigned char
     right,  unsigned char bottom, unsigned char            page, char
     *buffer)
     {
          /* Display a text window from a variable */
     
          union REGS inreg,outreg;
     
          unsigned char old_left;
          unsigned char chr;
          unsigned char attr;
          unsigned char old_row;
          unsigned char old_col;
     
          /* save current cursor position */
          inreg.h.ah = 3;
          inreg.h.bh = page;
          int86(0x10,&inreg,&outreg);
          old_row = outreg.h.dh;
          old_col = outreg.h.dl;
     
          while(top <= bottom)
          {
               old_left = left;
               while(left <= right)
               {
                    at(top,left,page);
                    chr = *buffer++;
                    attr = *buffer++;
                    inreg.h.bh = page;
                    inreg.h.ah = 9;
                    inreg.h.al = chr;
                    inreg.h.bl = attr;
                    inreg.x.cx = 1;
                    int86(0x10,&inreg,&outreg);
                    left++;
               }
               left = old_left;
               top++;
          }
     
          /* Restore cursor to original location */
          at(old_row,old_col,page);
     }
                                    
                   DIRECT VIDEO ACCESS WITH THE IBM PC

Accessing video RAM directly is much faster than using the BIOS ROM
display functions. There are problems however. Different video modes
arrange their use of video RAM in different ways so a number of functions
are required for plotting using direct video access, where as only one
function is required if use is made of the BIOS ROM display function.

The following C function will set a pixel in CGA display modes 4 and 5
directly;


     void dplot4(int y, int x, int colour)
     {
          /* Direct plotting in modes 4 & 5 ONLY! */
     
          union mask
          {
               char c[2];
               int i;
          }bit_mask;
     
          int index;
          int bit_position;
     
          unsigned char t;
          char xor;
     
          char far *ptr = (char far *) 0xB8000000;
     
          bit_mask.i = 0xFF3F;
     
          if ( y < 0 || y > 319 || x < 0 || x > 199)
               return;
     
          xor = colour & 128;
     
          colour = colour & 127;
     
          bit_position = y % 4;
     
          colour <<= 2 * (3 - bit_position);
     
          bit_mask.i >>= 2 * bit_position;
     
          index = x * 40 + (y / 4);
     
          if (x % 2)
               index += 8152;
     
     
     
     
          if (!xor)
          {
               t = *(ptr + index) & bit_mask.c[0];
               *(ptr + index) = t | colour;
          }
          else
          {
               t = *(ptr + index) | (char)0;
               *(ptr + index) = t ^ colour;
          }
     }
     

Direct plotting in VGA mode 19 is very much simpler;

     void dplot19(int x, int y, unsigned char colour)
     {
          /* Direct plot in mode 19 ONLY */
          char far *video;
     
          video = MK_FP(0xA000,0);
          video[x + y * 320] = colour;
}
                                    
              ADVANCED GRAPHICS TECHNIQUES WITH THE IBM PC




Increasing Colours

The EGA display is limited displaying a maximum of 16 colours, however in
high resolution graphics modes (such as mode 16) the small physical size
of the pixels allows blending of adjacent colours to produce additional
shades.

If a line is drawn straight across a black background display in blue,
and then a subsequent line is drawn beneath it also in blue but only
plotting alternate pixels, the second line will appear in a darker shade
of the same colour.

The following C program illustrates this idea;


     #include <dos.h>
     
     union REGS inreg, outreg;
     
     void setvideo(unsigned char mode)
     {
          /* Sets the video display mode */
     
          inreg.h.al = mode;
          inreg.h.ah = 0;
          int86(0x10, &inreg, &outreg);
     }
     
     void plot(int x, int y, unsigned char colour)
     {
          /* Sets a pixel at the specified coordinates */
     
          inreg.h.al = colour;
          inreg.h.bh = 0;
          inreg.x.cx = x;
          inreg.x.dx = y;
          inreg.h.ah = 0x0C;
          int86(0x10, &inreg, &outreg);
     }
     
     void line(int a, int b, int c, int d, unsigned char colour)
     {
          /* Draws a straight line from (a,b) to (c,d) */
     
          int u;
          int v;
          int d1x;
          int d1y;
          int d2x;
          int d2y;
          int m;
          int n;
          int s;
          int i;
     
          u = c - a;
          v = d - b;
          if (u == 0)
          {
               d1x = 0;
               m = 0;
          }
          else
          {
               m = abs(u);
               if (u < 0)
                    d1x = -1;
               else
               if (u > 0)
                    d1x = 1;
          }
          if ( v == 0)
          {
               d1y = 0;
               n = 0;
          }
          else
          {
               n = abs(v);
               if (v < 0)
                    d1y = -1;
               else
               if (v > 0)
                    d1y = 1;
          }
          if (m > n)
          {
               d2x = d1x;
               d2y = 0;
          }
          else
          {
               d2x = 0;
               d2y = d1y;
               m = n;
               n = abs(u);
          }
          s = m / 2;
     
          for (i = 0; i <= m; i++)
          {
               plot(a,b,colour);
               s += n;
               if (s >= m)
               {
                    s -= m;
                    a += d1x;
                    b += d1y;
               }
               else
               {
                    a += d2x;
                    b += d2y;
               }
          }
     }
     
     void dot_line(int a, int b, int c, int d, int colour)
     {
          /* Draws a dotted straight line from (a,b) to (c,d) */
     
          int u;
          int v;
          int d1x;
          int d1y;
          int d2x;
          int d2y;
          int m;
          int n;
          int s;
          int i;
     
          u = c - a;
          v = d - b;
          if (u == 0)
          {
               d1x = 0;
               m = 0;
          }
          else
          {
               m = abs(u);
               if (u < 0)
                    d1x = -2;
               else
               if (u > 0)
                    d1x = 2;
          }
          if (v == 0)
          {
               d1y = 0;
               n = 0;
          }
          else
          {
               n = abs(v);
               if (v < 0)
                    d1y = -2;
               else
               if (v > 0)
                    d1y = 2;
          }
          if (m > n)
          {
               d2x = d1x;
               d2y = 0;
          }
          else
          {
               d2x = 0;
               d2y = d1y;
               m = n;
               n = abs(u);
          }
          s = m / 2;
     
          for (i = 0; i <= m; i+=2)
          {
               plot(a,b,colour);
               s += n;
               if (s >= m)
               {
                    s -= m;
                    a += d1x;
                    b += d1y;
               }
               else
               {
                    a += d2x;
                    b += d2y;
               }
          }
     }
     
     
     void main(void)
     {
          int n;
     
          /* Display different colour bands */
     
          setvideo(16);
     
          for(n = 1; n < 16; n++)
          {
               line(0,n * 20,639,n * 20,n);
               line(0,1 + n * 20,639,1 + n * 20,n);
               line(0,2 + n * 20,639,2 + n * 20,n);
     
               dot_line(0,4 + n * 20,639,4 + n * 20,n);
               dot_line(1,5 + n * 20,639,5 + n * 20,n);
               dot_line(0,6 + n * 20,639,6 + n * 20,n);
     
               dot_line(0,8 + n * 20,639,8 + n * 20,n);
               dot_line(1,9 + n * 20,639,9 + n * 20,n);
               dot_line(0,10 + n * 20,639,10 + n * 20,n);
     
               dot_line(1,8 + n * 20,639,8 + n * 20,7);
               dot_line(0,9 + n * 20,639,9 + n * 20,7);
               dot_line(1,10 + n * 20,639,10 + n * 20,7);
     
               dot_line(1,12 + n * 20,639,12 + n * 20,n);
               dot_line(0,13 + n * 20,639,13 + n * 20,n);
               dot_line(1,14 + n * 20,639,14 + n * 20,n);
     
               dot_line(0,12 + n * 20,639,12 + n * 20,14);
               dot_line(1,13 + n * 20,639,13 + n * 20,14);
               dot_line(0,14 + n * 20,639,14 + n * 20,14);
          }
     }

This technique can be put to good use for drawing three dimensional
boxes;

     void box3d(int xa,int ya, int xb, int yb, int col)
     {
          /* Draws a box for use in 3d histogram graphs etc */
     
          int xc;
          int xd;
     
          int n;
     
          xd = (xb - xa) / 2;
          xc = xa + xd;
     
          /* First draw the solid face */
          for(n = xa; n < xb; n++)
               line(n,ya,n,yb,col);
     
          /* Now "shaded" top and side */
          for(n = 0; n < xd; n++)
          {
               dotline(xa + n,yb - n ,xc + n,yb - n,col);
               dotline(xa + xd + n,yb - n ,xc + xd + n,yb - n,col);
               dotline(xb +n ,ya - n ,xb + n,yb - n,col);
          }
     }
     


Displaying Text At Pixel Coordinates

When using graphics display modes it is useful to be able to display text
not at the fixed character boundaries, but at pixel coordinates. This can
be achieved by implementing a print function which reads the character
definition data from the BIOS ROM and uses it to plot pixels to create
the shapes of the desired text. The following C function, "gr_print()"
illustrates this idea using the ROM CGA (8x8) character set;

     void gr_print(char *output, int x, int y, unsigned char colour)
     {
          unsigned char far *ptr;
          unsigned char chr;
          unsigned char bmask;
          int i;
          int k;
          int oldy;
          int p;
          int height;
     
          /* The height of the characters in the font being accessed */
          height = 8;
     
          /* Set pointer to start of font definition in the ROM */
          ptr = getfontptr(3);
     
          oldy = y;
          p = 0;
     
          /* Loop output string */
          while(output[p])
          {
               /* Get first character to be displayed */
               chr = output[p];
     
               /* Loop pixel lines in character definition */
               for(i = 0; i < height; i++)
               {
                    /* Get pixel line definition from the ROM */
                    bmask = *(ptr + (chr * height) + i);
     
                    /* Loop pixel columns */
                    for (k = 0; k < 8; ++k, bmask <<= 1)
                    {
                         /* Test for a set bit */
                         if(bmask & 128)
                              /* Plot a pixel if appropriate */
                              plot(x,y,colour);
                         else
                              plot(x,y,0);
                         x++;
                    }
                    /* Down to next row and left to start of character */
                    y++;
                    x -= 8;
               }
               /* Next character to be displayed */
               p++;
     
               /* Back to top row of the display position */
               y = oldy;
     
               /* Right to next character display position */
               x += 8;
          }
     }

The following assembly language support function is required to retrieve
the address of the ROM font by calling the BIOS ROM display function
which will return the address;


     ; GET FONT POINTER
     ; Small memory model
     ; compile with tasm /mx
     
     _TEXT     segment   byte public 'CODE'
                     assume   cs:_TEXT,ds:NOTHING
     
     _getfontptr    proc near
               push bp
               mov   bp,sp
               mov   ax,1130h
               mov   bh, [bp+4]         ; Number for font to be retrieved
               int   10h
               mov   dx,es
               mov   ax,bp
               pop   bp
               ret
     _getfontptr    endp
     
     _TEXT     ends
     
          public    _getfontptr
          end
     
The font number supplied to "getfontptr()" can be one of;

     2    ROM EGA 8 x 14 font
     3    ROM CGA 8 x 8 font
     6    ROM VGA 8 x 16 font

A Graphics Function Library For Turbo C

     /* Graphics library for 'Turbo C'  (V2.01) */
     
     /* (C)1992 Copyright Servile Software */
     
     #include <stdio.h>
     #include <dos.h>
     #include <stdlib.h>
     #include <stdarg.h>
     #include <string.h>
     #include <math.h>
     
     /* Global variables */
     static unsigned char attribute;
     int _dmode;
     int _lastx;
     int _lasty;
     
     /* Maximum coordinates for graphics screen */
     static int maxx;
     static int maxy;
     
     /* Sprite structure */
     struct SP
     {
          int x;
          int y;
          char data[256];
          char save[256];
     };
     typedef struct SP SPRITE;
     
     
     /* Sine and cosine tables for trig' operations */
     
     static double sintable[360] =
     {0.000000000000000001,0.017452406437283512,
                    0.034899496702500969,0.052335956242943835,
                    0.069756473744125302,0.087155742747658166,
                    0.104528463267653457,0.121869343405147462,
                    0.139173100960065438,0.156434465040230869,
                    0.173648177666930331,0.190808995376544804,
                    0.207911690817759315,0.224951054343864976,
                    0.241921895599667702,0.258819045102520739,
                    0.275637355816999163,0.292371704722736769,
                    0.309016994374947451,0.325568154457156755,
                    0.342020143325668824,0.358367949545300379,
                    0.374606593415912181,0.390731128489273882,
                    0.406736643075800375,0.422618261740699608,
                    0.438371146789077626,0.453990499739547027,
                    0.469471562785891028,0.484809620246337225,
                    0.500000000000000222,0.515038074910054489,
                    0.529919264233205234,0.544639035015027417,
                    0.559192903470747127,0.573576436351046381,
                    0.587785252292473470,0.601815023152048600,
                    0.615661475325658625,0.629320391049837835,
                    0.642787609686539696,0.656059028990507720,
                    0.669130606358858682,0.681998360062498921,
                    0.694658370458997698,0.707106781186548017,
                    0.719339800338651636,0.731353701619170904,
                    0.743144825477394688,0.754709580222772458,
                    0.766044443118978569,0.777145961456971346,
                    0.788010753606722458,0.798635510047293384,
                    0.809016994374947895,0.819152044288992243,
                    0.829037572555042179,0.838670567945424494,
                    0.848048096156426512,0.857167300702112778,
                    0.866025403784439152,0.874619707139396296,
                    0.882947592858927432,0.891006524188368343,
                    0.898794046299167482,0.906307787036650381,
                    0.913545457642601311,0.920504853452440819,
                    0.927183854566787868,0.933580426497202187,
                    0.939692620785908761,0.945518575599317179,
                    0.951056516295153975,0.956304755963035880,
                    0.961261695938319227,0.965925826289068645,
                    0.970295726275996806,0.974370064785235579,
                    0.978147600733805911,0.981627183447664198,
                    0.984807753012208353,0.987688340595137992,
                    0.990268068741570473,0.992546151641322205,
                    0.994521895368273512,0.996194698091745656,
                    0.997564050259824309,0.998629534754573944,
                    0.999390827019095762,0.999847695156391270,
                    1.000000000000000000,0.999847695156391159,
                    0.999390827019095651,0.998629534754573833,
                    0.997564050259824087,0.996194698091745323,
                    0.994521895368273179,0.992546151641321761,
                    0.990268068741570029,0.987688340595137437,
                    0.984807753012207687,0.981627183447663532,
                    0.978147600733805245,0.974370064785234802,
                    0.970295726275996029,0.965925826289067757,
                    0.961261695938318228,0.956304755963034880,
                    0.951056516295152865,0.945518575599316069,
                    0.939692620785907651,0.933580426497200966,
                    0.927183854566786536,0.920504853452439486,
                    0.913545457642599978,0.906307787036649048,
                    0.898794046299166149,0.891006524188367122,
                    0.882947592858926211,0.874619707139395186,
                    0.866025403784438042,0.857167300702111778,
                    0.848048096156425624,0.838670567945423717,
                    0.829037572555041513,0.819152044288991688,
                    0.809016994374947451,0.798635510047293051,
                    0.788010753606722236,0.777145961456971346,
                    0.766044443118978569,0.754709580222772680,
                    0.743144825477395132,0.731353701619171459,
                    0.719339800338652302,0.707106781186548794,
                    0.694658370458998808,0.681998360062500142,
                    0.669130606358860014,0.656059028990509274,
                    0.642787609686541472,0.629320391049839833,
                    0.615661475325660845,0.601815023152051043,
                    0.587785252292476135,0.573576436351049268,
                    0.559192903470750236,0.544639035015030637,
                    0.529919264233208676,0.515038074910058152,
                    0.500000000000004219,0.484809620246341444,
                    0.469471562785895413,0.453990499739551634,
                    0.438371146789082455,0.422618261740704715,
                    0.406736643075805704,0.390731128489279489,
                    0.374606593415918010,0.358367949545306430,
                    0.342020143325675152,0.325568154457163306,
                    0.309016994374954279,0.292371704722743819,
                    0.275637355817006491,0.258819045102528289,
                    0.241921895599675502,0.224951054343872997,
                    0.207911690817767558,0.190808995376553270,
                    0.173648177666939019,0.156434465040239751,
                    0.139173100960074542,0.121869343405156802,
                    0.104528463267663005,0.087155742747667922,
                    0.069756473744135267,0.052335956242954007,
                    0.034899496702511350,0.017452406437294093,
                    0.000000000000010781,-0.017452406437272534,
                    -0.034899496702489805,-0.052335956242932476,
                    -0.069756473744113756,-0.087155742747646453,
                    -0.104528463267641564,-0.121869343405135402,
                    -0.139173100960053198,-0.156434465040218462,
                    -0.173648177666917786,-0.190808995376532092,
                    -0.207911690817746464,-0.224951054343851986,
                    -0.241921895599654574,-0.258819045102507472,
                    -0.275637355816985785,-0.292371704722723225,
                    -0.309016994374933796,-0.325568154457142933,
                    -0.342020143325654891,-0.358367949545286335,
                    -0.374606593415898026,-0.390731128489259616,
                    -0.406736643075785997,-0.422618261740685175,
                    -0.438371146789063082,-0.453990499739532427,
                    -0.469471562785876373,-0.484809620246322570,
                    -0.499999999999985512,-0.515038074910039723,
                    -0.529919264233190468,-0.544639035015012540,
                    -0.559192903470732361,-0.573576436351031616,
                    -0.587785252292458593,-0.601815023152033834,
                    -0.615661475325643859,-0.629320391049823069,
                    -0.642787609686525041,-0.656059028990493065,
                    -0.669130606358844027,-0.681998360062484377,
                    -0.694658370458983265,-0.707106781186533584,
                    -0.719339800338637314,-0.731353701619156804,
                    -0.743144825477380699,-0.754709580222758580,
                    -0.766044443118964691,-0.777145961456957690,
                    -0.788010753606709025,-0.798635510047280062,
                    -0.809016994374934795,-0.819152044288979364,
                    -0.829037572555029412,-0.838670567945412060,
                    -0.848048096156414188,-0.857167300702100676,
                    -0.866025403784427272,-0.874619707139384750,
                    -0.882947592858916108,-0.891006524188357352,
                    -0.898794046299156713,-0.906307787036639945,
                    -0.913545457642591208,-0.920504853452430938,
                    -0.927183854566778320,-0.933580426497192972,
                    -0.939692620785899990,-0.945518575599308742,
                    -0.951056516295145871,-0.956304755963028108,
                    -0.961261695938311900,-0.965925826289061651,
                    -0.970295726275990256,-0.974370064785229362,
                    -0.978147600733800138,-0.981627183447658869,
                    -0.984807753012203468,-0.987688340595133552,
                    -0.990268068741566587,-0.992546151641318764,
                    -0.994521895368270514,-0.996194698091743103,
                    -0.997564050259822310,-0.998629534754572390,
                    -0.999390827019094763,-0.999847695156390714,
                    -1.000000000000000000,-0.999847695156391714,
                    -0.999390827019096761,-0.998629534754575388,
                    -0.997564050259826307,-0.996194698091748099,
                    -0.994521895368276398,-0.992546151641325647,
                    -0.990268068741574470,-0.987688340595142433,
                    -0.984807753012213349,-0.981627183447669860,
                    -0.978147600733812128,-0.974370064785242240,
                    -0.970295726276004022,-0.965925826289076417,
                    -0.961261695938327665,-0.956304755963044872,
                    -0.951056516295163523,-0.945518575599327393,
                    -0.939692620785919530,-0.933580426497213511,
                    -0.927183854566799748,-0.920504853452453253,
                    -0.913545457642614411,-0.906307787036664148,
                    -0.898794046299181804,-0.891006524188383331,
                    -0.882947592858942976,-0.874619707139412506,
                    -0.866025403784455916,-0.857167300702130208,
                    -0.848048096156444498,-0.838670567945443146,
                    -0.829037572555061497,-0.819152044289012227,
                    -0.809016994374968434,-0.798635510047314479,
                    -0.788010753606744219,-0.777145961456993772,
                    -0.766044443119001550,-0.754709580222796106,
                    -0.743144825477418891,-0.731353701619195773,
                    -0.719339800338677060,-0.707106781186574107,
                    -0.694658370459024455,-0.681998360062526232,
                    -0.669130606358886548,-0.656059028990536253,
                    -0.642787609686568784,-0.629320391049867478,
                    -0.615661475325688934,-0.601815023152079465,
                    -0.587785252292504890,-0.573576436351078467,
                    -0.559192903470779767,-0.544639035015060502,
                    -0.529919264233238985,-0.515038074910088794,
                    -0.500000000000035083,-0.484809620246372641,
                    -0.469471562785926888,-0.453990499739583386,
                    -0.438371146789114541,-0.422618261740737022,
                    -0.406736643075838289,-0.390731128489312296,
                    -0.374606593415951039,-0.358367949545339737,
                    -0.342020143325708625,-0.325568154457197001,
                    -0.309016994374988196,-0.292371704722777903,
                    -0.275637355817040797,-0.258819045102562761,
                    -0.241921895599710085,-0.224951054343907719,
                    -0.207911690817802419,-0.190808995376588242,
                    -0.173648177666974129,-0.156434465040274973,
                    -0.139173100960109847,-0.121869343405192190,
                    -0.104528463267698463,-0.087155742747703435,
                    -0.069756473744170822,-0.052335956242989604,
                    -0.034899496702546981,-0.017452406437329739
          };

static double costable[360] = {
1.000000000000000000,0.999847695156391270,
                    0.999390827019095762,0.998629534754573833,
                    0.997564050259824198,0.996194698091745545,
                    0.994521895368273290,0.992546151641321983,
                    0.990268068741570362,0.987688340595137770,
                    0.984807753012208020,0.981627183447663976,
                    0.978147600733805689,0.974370064785235246,
                    0.970295726275996473,0.965925826289068312,
                    0.961261695938318894,0.956304755963035436,
                    0.951056516295153531,0.945518575599316735,
                    0.939692620785908317,0.933580426497201743,
                    0.927183854566787313,0.920504853452440264,
                    0.913545457642600867,0.906307787036649826,
                    0.898794046299166927,0.891006524188367788,
                    0.882947592858926766,0.874619707139395630,
                    0.866025403784438486,0.857167300702112112,
                    0.848048096156425846,0.838670567945423828,
                    0.829037572555041513,0.819152044288991576,
                    0.809016994374947229,0.798635510047292607,
                    0.788010753606721681,0.777145961456970569,
                    0.766044443118977680,0.754709580222771681,
                    0.743144825477393911,0.731353701619170127,
                    0.719339800338650748,0.707106781186547129,
                    0.694658370458996810,0.681998360062498032,
                    0.669130606358857682,0.656059028990506721,
                    0.642787609686538697,0.629320391049836836,
                    0.615661475325657626,0.601815023152047601,
                    0.587785252292472471,0.573576436351045382,
                    0.559192903470746128,0.544639035015026307,
                    0.529919264233204124,0.515038074910053378,
                    0.499999999999999112,0.484809620246336115,
                    0.469471562785889862,0.453990499739545861,
                    0.438371146789076460,0.422618261740698442,
                    0.406736643075799154,0.390731128489272661,
                    0.374606593415910960,0.358367949545299158,
                    0.342020143325667547,0.325568154457155479,
                    0.309016994374946230,0.292371704722735493,
                    0.275637355816997887,0.258819045102519463,
                    0.241921895599666398,0.224951054343863643,
                    0.207911690817757955,0.190808995376543389,
                    0.173648177666928888,0.156434465040229398,
                    0.139173100960063939,0.121869343405145950,
                    0.104528463267651903,0.087155742747656584,
                    0.069756473744123679,0.052335956242942190,
                    0.034899496702499304,0.017452406437281822,
                    -0.000000000000001715,-0.017452406437285253,
                    -0.034899496702502732,-0.052335956242945618,
                    -0.069756473744127107,-0.087155742747659998,
                    -0.104528463267655317,-0.121869343405149350,
                    -0.139173100960067325,-0.156434465040232784,
                    -0.173648177666932274,-0.190808995376546747,
                    -0.207911690817761285,-0.224951054343866974,
                    -0.241921895599669701,-0.258819045102522793,
                    -0.275637355817001217,-0.292371704722738768,
                    -0.309016994374949450,-0.325568154457158698,
                    -0.342020143325670822,-0.358367949545302322,
                    -0.374606593415914124,-0.390731128489275825,
                    -0.406736643075802318,-0.422618261740701329,
                    -0.438371146789079125,-0.453990499739548303,
                    -0.469471562785892083,-0.484809620246338169,
                    -0.500000000000000999,-0.515038074910054933,
                    -0.529919264233205567,-0.544639035015027528,
                    -0.559192903470747127,-0.573576436351046159,
                    -0.587785252292473026,-0.601815023152048045,
                    -0.615661475325657848,-0.629320391049836947,
                    -0.642787609686538697,-0.656059028990506499,
                    -0.669130606358857238,-0.681998360062497477,
                    -0.694658370458996033,-0.707106781186546240,
                    -0.719339800338649749,-0.731353701619168906,
                    -0.743144825477392579,-0.754709580222770238,
                    -0.766044443118976237,-0.777145961456968903,
                    -0.788010753606719905,-0.798635510047290720,
                    -0.809016994374945231,-0.819152044288989578,
                    -0.829037572555039404,-0.838670567945421719,
                    -0.848048096156423625,-0.857167300702109891,
                    -0.866025403784436265,-0.874619707139393410,
                    -0.882947592858924435,-0.891006524188365345,
                    -0.898794046299164484,-0.906307787036647494,
                    -0.913545457642598424,-0.920504853452437932,
                    -0.927183854566784982,-0.933580426497199412,
                    -0.939692620785906096,-0.945518575599314515,
                    -0.951056516295151311,-0.956304755963033326,
                    -0.961261695938316785,-0.965925826289066314,
                    -0.970295726275994586,-0.974370064785233358,
                    -0.978147600733803912,-0.981627183447662310,
                    -0.984807753012206577,-0.987688340595136327,
                    -0.990268068741569030,-0.992546151641320873,
                    -0.994521895368272291,-0.996194698091744657,
                    -0.997564050259823532,-0.998629534754573389,
                    -0.999390827019095318,-0.999847695156391048,
                    -1.000000000000000000,-0.999847695156391381,
                    -0.999390827019096095,-0.998629534754574499,
                    -0.997564050259825086,-0.996194698091746544,
                    -0.994521895368274622,-0.992546151641323537,
                    -0.990268068741572027,-0.987688340595139658,
                    -0.984807753012210241,-0.981627183447666418,
                    -0.978147600733808353,-0.974370064785238244,
                    -0.970295726275999804,-0.965925826289071865,
                    -0.961261695938322669,-0.956304755963039654,
                    -0.951056516295157972,-0.945518575599321509,
                    -0.939692620785913424,-0.933580426497207072,
                    -0.927183854566793086,-0.920504853452446370,
                    -0.913545457642607195,-0.906307787036656598,
                    -0.898794046299173921,-0.891006524188375226,
                    -0.882947592858934649,-0.874619707139403846,
                    -0.866025403784447034,-0.857167300702120993,
                    -0.848048096156435061,-0.838670567945433487,
                    -0.829037572555051505,-0.819152044289001902,
                    -0.809016994374957998,-0.798635510047303709,
                    -0.788010753606733227,-0.777145961456982559,
                    -0.766044443118990004,-0.754709580222784449,
                    -0.743144825477407012,-0.731353701619183672,
                    -0.719339800338664737,-0.707106781186561450,
                    -0.694658370459011576,-0.681998360062513242,
                    -0.669130606358873337,-0.656059028990522708,
                    -0.642787609686555128,-0.629320391049853711,
                    -0.615661475325674945,-0.601815023152065254,
                    -0.587785252292490457,-0.573576436351063812,
                    -0.559192903470765002,-0.544639035015045625,
                    -0.529919264233223886,-0.515038074910073473,
                    -0.500000000000019651,-0.484809620246357043,
                    -0.469471562785911123,-0.453990499739567510,
                    -0.438371146789098498,-0.422618261740720869,
                    -0.406736643075822024,-0.390731128489295920,
                    -0.374606593415934497,-0.358367949545323083,
                    -0.342020143325691917,-0.325568154457180181,
                    -0.309016994374971266,-0.292371704722760861,
                    -0.275637355817023644,-0.258819045102545497,
                    -0.241921895599692793,-0.224951054343890372,
                    -0.207911690817784989,-0.190808995376570756,
                    -0.173648177666956560,-0.156434465040257376,
                    -0.139173100960092194,-0.121869343405174496,
                    -0.104528463267680741,-0.087155742747685686,
                    -0.069756473744153044,-0.052335956242971805,
                    -0.034899496702529162,-0.017452406437311916,
                    -0.000000000000028605,0.017452406437254715,
                    0.034899496702471985,0.052335956242914670,
                    0.069756473744095979,0.087155742747628689,
                    0.104528463267623842,0.121869343405117708,
                    0.139173100960035545,0.156434465040200865,
                    0.173648177666900216,0.190808995376514606,
                    0.207911690817729033,0.224951054343834611,
                    0.241921895599637282,0.258819045102490264,
                    0.275637355816968632,0.292371704722706127,
                    0.309016994374916809,0.325568154457126058,
                    0.342020143325638126,0.358367949545269682,
                    0.374606593415881484,0.390731128489243240,
                    0.406736643075769733,0.422618261740669021,
                    0.438371146789047095,0.453990499739516551,
                    0.469471562785860608,0.484809620246306972,
                    0.499999999999970080,0.515038074910024402,
                    0.529919264233175369,0.544639035014997663,
                    0.559192903470717484,0.573576436351016961,
                    0.587785252292444271,0.601815023152019624,
                    0.615661475325629759,0.629320391049809191,
                    0.642787609686511385,0.656059028990479520,
                    0.669130606358830815,0.681998360062471387,
                    0.694658370458970387,0.707106781186521038,
                    0.719339800338624991,0.731353701619144592,
                    0.743144825477368709,0.754709580222746812,
                    0.766044443118953255,0.777145961456946477,
                    0.788010753606698033,0.798635510047269292,
                    0.809016994374924359,0.819152044288969150,
                    0.829037572555019531,0.838670567945402290,
                    0.848048096156404752,0.857167300702091572,
                    0.866025403784418391,0.874619707139376090,
                    0.882947592858907782,0.891006524188349247,
                    0.898794046299148941,0.906307787036632395,
                    0.913545457642583991,0.920504853452423943,
                    0.927183854566771659,0.933580426497186644,
                    0.939692620785893884,0.945518575599302968,
                    0.951056516295140320,0.956304755963022890,
                    0.961261695938306904,0.965925826289056988,
                    0.970295726275985926,0.974370064785225365,
                    0.978147600733796474,0.981627183447655538,
                    0.984807753012200360,0.987688340595130776,
                    0.990268068741564034,0.992546151641316543,
                    0.994521895368268627,0.996194698091741548,
                    0.997564050259821089,0.998629534754571502,
                    0.999390827019094097,0.999847695156390381
     
     int dtoi(double x)
     {
          /* Rounds a floating point number to an integer */
     
          int y;
     
          y = x;
          if (y < (x + 0.5))
               y++;
          return(y);
     }
     
     int getmode(int *ncols)
     {
          /* Returns current video mode and number of columns in ncols */
     
          union REGS inreg,outreg;
     
          inreg.h.ah = 0x0F;
          int86(0x10, &inreg, &outreg);
          *ncols = outreg.h.ah;
          return(outreg.h.al);
     }
     
     void setvideo(unsigned char mode)
     {
          /* Sets the video display mode and clears the screen */
          /* (modes above 127 on VGA monitors do not clear the screen) */
     
          asm mov ax , mode;
          asm int 10h;
     
          /* Set global variables */
          switch(mode)
          {
               case 0:
               case 1:
               case 2:
               case 3: break;
               case 4:
               case 9:
               case 13:
               case 19:
               case 5: maxx = 320;
                    maxy = 200;
                    break;
               case 14:
               case 10:
               case 6: maxx = 640;
                    maxy = 200;
                    break;
               case 7: maxx = 720;
                    maxy = 400;
                    break;
               case 8: maxx = 160;
                    maxy = 200;
                    break;
               case 15:
               case 16: maxx = 640;
                     maxy = 350;
                     break;
               case 17:
               case 18: maxx = 640;
                     maxy = 480;
                     break;
     
          }
          _dmode = mode;
     }
     
     void getcursor(int *row, int *col)
     {
          /* Returns the cursor position and size for the currently
     active
             video display page */
     
          union REGS inreg,outreg;
     
          inreg.h.bh = 0;
          inreg.h.ah = 0x03;
          int86(0x10, &inreg, &outreg);
          *row = outreg.h.dh;
          *col = outreg.h.dl;
     }
     
     void setcursor(char status)
     {
          /* Set cursor size, if status is 0x20 the cursor is not
     displayed */
     
          asm mov ah,1
          asm mov ch,status
          asm mov cl,7
          asm int 10h
     }
     
     void at(int row, int col)
     {
          /* Position text cursor on current screen */
     
          asm mov bh , 0;
          asm mov dh , row;
          asm mov dl , col;
          asm mov ah , 02h;
          asm int 10h;
     }
     
     void clsc(unsigned char attrib)
     {
          /* Clear display and fill with new attribute */
     
          asm mov ax , 073dh;
          asm mov bh , attrib;
          asm mov cx , 0;
          asm mov dx , 3c4fh;
          asm int 10h;
     
          /* Now move text cursor to origin (0,0) */
          asm mov bh , 0;
          asm mov dx , 0;
          asm mov ah , 02h;
          asm int 10h;
     }
     
     void clrwindow(int x1, int y1,int x2, int y2, unsigned char attrib)
     {
          /* Clear a text window */
     
          union REGS inreg,outreg;
     
          inreg.h.al = (unsigned char)(y2 - y1 + 1);
          inreg.h.bh = attrib;
          inreg.h.cl = (unsigned char)x1;
          inreg.h.ch = (unsigned char)y1;
          inreg.h.dl = (unsigned char)x2;
          inreg.h.dh = (unsigned char)y2;
          inreg.h.ah = 0x06;
          int86(0x10, &inreg, &outreg);
     }
     
     void scrollsc(void)
     {
          /* Scroll the text screen */
     
          asm mov al,01h;
          asm mov cx,0
          asm mov dx,3c4fh;
          asm mov bh,attribute;
          asm mov ah, 06h;
          asm int 10h;
     }
     
     void winscr(unsigned char left,unsigned char top, unsigned char
     right,unsigned char bottom,
               unsigned char direction, unsigned char numlines)
     {
          /* Scroll a text window */
     
          asm mov al , numlines;
          asm mov cl , left;
          asm mov ch , top;
          asm mov dl , right;
          asm mov dh , bottom;
          asm mov bh , attribute;
          asm mov ah , direction;
          asm int 10h;
     }
     
     unsigned char attr(int foregrnd, int backgrnd)
     {
          /* Convert a colour pair into a single attribute */
     
          return((char)((backgrnd << 4) + foregrnd));
     }
     
     void shadewin(unsigned char left, unsigned char top, unsigned char
     right,   unsigned char bottom)
     {
          /* Shade a text window */
     
          int row;
          int col;
          int oldrow;
          int oldcol;
     
          /* Preserve existing coords */
          getcursor(&oldrow,&oldcol);
     
          col = right + 1;
          for (row = top + 1; row <= bottom + 1; row++)
          {
               /* Move to row,col */
               asm mov bh , 0;
               asm mov dh , row;
               asm mov dl , col;
               asm mov ah , 02h;
               asm int 10h;
               /* Get character */
               asm mov ah , 0Fh;
               asm int 10h;
               asm mov ah , 08h;
               asm int 10h;
     
               /* Write in attribute 7 */
               asm mov ah , 09h;
               asm mov bl, 07h;
               asm mov cx, 01h;
               asm int 10h;
          }
          bottom++;
          for (col = left + 1; col <= right + 1; col++)
          {
               /* Move to row,col */
               asm mov bh , 0;
               asm mov dh , bottom;
               asm mov dl , col;
               asm mov ah , 02h;
               asm int 10h;
     
               /* Get character */
               asm mov ah , 0Fh;
               asm int 10h;
               asm mov ah , 08h;
               asm int 10h;
     
               /* Write in attribute 7 */
               asm mov ah , 09h;
               asm mov bl, 07h;
               asm mov cx, 01h;
               asm int 10h;
          }
     
          /* Return to original position */
          /* Move to row,col */
          asm mov bh , 0;
          asm mov dh , oldrow;
          asm mov dl , oldcol;
          asm mov ah , 02h;
          asm int 10h;
     }
     
     void bprintf(char *format, ...)
     {
          /* print text to graphics screen correctly */
     
          va_list arg_ptr;
          char output[1000];
          int c, r, n, p = 0;
          unsigned char text;
          unsigned char page;
     
          va_start(arg_ptr, format);
          vsprintf(output, format, arg_ptr);
     
          if (strcmp(output,"\r\n") == 0)
               fputs(output,stdout);
     
          else
          {
               asm mov ah , 0Fh;
               asm int 10h;
               asm mov page, bh;
     
               getmode(&n);
               getcursor(&r,&c);
     
               while (output[p])
               {
                    text = output[p++];
                    asm mov bh , page;
                    asm mov bl , attribute;
                    asm mov cx , 01h;
                    asm mov ah , 09h;
                    asm mov al , text;
                    asm int 10h;
     
                    c++;
                    if (c < (n-1))
                         at( r, c);
                    else
                    {
                         c = 0;
                         at(++r,0);
                    }
               }
          }
     }
     
     void wrtstr(char *output)
     {
          /* TTY text output. The original screen attributes remain
     unaffected */
     
          int p = 0;
          unsigned char page;
          unsigned char text;
     
          asm mov ah , 0Fh;
          asm int 10h;
          asm mov page, bh;
     
          while (output[p])
          {
               text = output[p++];
               asm mov bh , page;
               asm mov ah , 0Eh;
               asm mov al , text;
               asm int 10h;
          }
     }
     
     void getwin(int left, int top, int right, int bottom, char *buffer)
     {
          /* Read a text window into a variable */
     
          int oldleft;
          unsigned char page;
     
          asm mov ah , 0Fh;
          asm int 10h;
          asm mov page, bh;
     
          while(top <= bottom)
          {
               oldleft = left;
               while(left <= right)
               {
                    at(top,left);
                    asm mov bh , page;
                    asm mov ah , 08h;
                    asm int 10h;
                    *buffer++ = _AL;
                    *buffer++ = _AH;
                    left++;
               }
               left = oldleft;
               top++;
          }
     }
     
     void putwin(int left, int top, int right, int bottom,char *buffer)
     {
          /* Display a text window from a variable */
     
          int oldleft;
          unsigned char chr;
          unsigned char attr;
          unsigned char page;
     
          asm mov ah , 0Fh;
          asm int 10h;
          asm mov page, bh;
     
          while(top <= bottom)
          {
               oldleft = left;
               while(left <= right)
               {
                    at(top,left);
                    chr = *buffer++;
                    attr = *buffer++;
                    asm mov bh , page;
                    asm mov ah , 09h;
                    asm mov al, chr;
                    asm mov bl, attr;
                    asm mov cx,1;
                    asm int 10h;
                    left++;
               }
               left = oldleft;
               top++;
          }
     }
     
     void setpalette(unsigned char palno)
     {
          /* Sets the video palette */
     
          asm mov bh,01h;
          asm mov bl,palno;
          asm mov ah,0Bh;
          asm int 10h;
     }
     
     void setborder(unsigned char x)
     {
          /* Set border colour */
     
          asm mov bh, x;
          asm mov ax ,1001h;
          asm int 10h;
     }
     
     void setlines(unsigned char x)
     {
          /* Set text display number of lines */
     
          asm mov ah,11h;
          asm mov al,x;
          asm mov bl,0;
          asm int 10h;
     }
     
     void graphbackground(unsigned char colour)
     {
          /* Selects the background colour for a graphics mode */
     
          asm mov bh,0;
          asm mov bl,colour;
          asm mov ah, 0Bh;
          asm int 10h;
     }
     
     void plot(int x, int y, unsigned char colour)
     {
          /* Sets a pixel at the specified coordinates to the specified
     colour.
             The variables _lastx and _lasty are updated. */
     
          unsigned char far *video;
     
          _lastx = x;
          _lasty = y;
     
          if (_dmode == 19)
          {
               video = MK_FP(0xA000,0);
               video[x+y*320] = colour;
          }
          else
          {
               asm mov al , colour;
               asm mov bh , 00;
               asm mov cx , x;
               asm mov dx , y;
               asm mov ah , 0Ch;
               asm int 10h;
          }
     }
     
     int pixset(int x, int y)
     {
          /* Returns the colour of the specified pixel */
     
          asm mov cx ,x;
          asm mov dx ,y;
          asm mov ah ,0Dh;
          asm int 10h;
          return(_AL);
     }
     
     void move(int x, int y)
     {
          /* Sets the value of the variables _lastx and _lasty */
     
          _lastx = x;
          _lasty = y;
     }
     
     void line(int a, int b, int c, int d, int col)
     {
          /* Draws a straight line from (a,b) to (c,d) in colour col */
     
          int u;
          int v;
          int d1x;
          int d1y;
          int d2x;
          int d2y;
          int m;
          int n;
          int s;
          int i;
     
          if (a < 0)
               a = 0;
          else
          if (a > maxx)
               a = maxx;
     
          if (c < 0)
               c = 0;
          else
          if (c > maxx)
               c = maxx;
     
          if (b < 0)
               b = 0;
          else
          if (b > maxy)
               b = maxy;
     
          if (d < 0)
               d = 0;
          else
          if (d > maxy)
               d = maxy;
     
          u = c - a;
          v = d - b;
     
          if (u == 0)
          {
               d1x = 0;
               m = 0;
          }
          else
          {
               m = abs(u);
               if (u < 0)
                    d1x = -1;
               else
               if (u > 0)
                    d1x = 1;
          }
          if ( v == 0)
          {
               d1y = 0;
               n = 0;
          }
          else
          {
               n = abs(v);
               if (v < 0)
                    d1y = -1;
               else
               if (v > 0)
                    d1y = 1;
          }
          if (m > n)
          {
               d2x = d1x;
               d2y = 0;
          }
          else
          {
               d2x = 0;
               d2y = d1y;
               m = n;
               n = abs(u);
          }
          s = m / 2;
     
          for (i = 0; i <= m; i++)
          {
               asm mov al , col;
               asm mov bh , 0;
               asm mov ah ,0Ch;
               asm mov cx ,a;
               asm mov dx ,b;
               asm int 10h;
     
               s += n;
               if (s >= m)
               {
                    s -= m;
                    a += d1x;
                    b += d1y;
               }
               else
               {
                    a += d2x;
                    b += d2y;
               }
          }
          _lastx = a;
          _lasty = b;
     }
     
     void ellipse(int x, int y, int xrad, int yrad,double incline,int
     col)
     {
          /* Draws an ellipse */
     
          int f;
          float a;
          float b;
          float c;
          float d;
          int cols;
          double div;
     
          incline = 1 / sintable[(int)incline];
     
          if (getmode(&cols) == 6)
               div = 2.2;
          else
               div = 1.3;
     
               /* Compensate for pixel shape */
     
          a = x + xrad;
          b = y + sintable[0] * yrad + xrad/incline / div;
     
          for(f = 5; f < 360; f += 5)
          {
               c = x + costable[f] * xrad;
               d = y + sintable[f] * yrad + (costable[f] *
     xrad)/incline/div;
     
               line(a,b,c,d,col);
     
               a = c;
               b = d;
          }
          /* Ensure join */
          line(a,b,x + xrad,y + sintable[0] * yrad + xrad/incline /
     div,col);
     }
     
     void polygon(int x, int y, int rad, int col, int sides, int start)
     {
          /* Draws a regular polygon */
     
          double f;
          double div;
          double a;
          double b;
          double c;
          double d;
          double aa;
          double bb;
          int cols;
          double step;
     
          step = 360 / sides;
     
          if (getmode(&cols) == 6)
               div = 2.2;
          else
               div = 1.3;
          aa = a = x + costable[start] * rad;
          bb = b = y + sintable[start] * rad / div;
     
          for(f = start + step; f < start + 360; f += step)
          {
               c = x + costable[(int)f] * rad;
               d = y + sintable[(int)f] * rad / div;
               line(a,b,c,d,col);
               a = c;
               b = d;
          }
          line(a,b,aa,bb,col);
     }
     
     void arc(int x, int y, int rad, int start, int end,int col)
     {
          /* Draw an arc */
     
          int f;
          float a;
          float b;
          float c;
          float d;
          int cols;
          float div;
     
          if (getmode(&cols) == 6)
               div = 2.2;
          else
               div = 1.3;
          a = x + costable[start] * rad;
          b = y + sintable[start] * rad / div;
     
          for(f = start; f <= end; f ++)
          {
               c = x + costable[f] * rad;
               d = y + sintable[f] * rad / div;
               line(a,b,c,d,col);
               a = c;
               b = d;
          }
     }
     
     void segm(int x, int y, int rad, int start, int end,int col)
     {
          /* Draw a segment of a circle */
     
          int f;
          float a;
          float b;
          float c;
          float d;
          int cols;
          double div;
     
          if (getmode(&cols) == 6)
               div = 2.2;
          else
               div = 1.3;
          a = x + costable[start] * rad;
          b = y + sintable[start] * rad / div;
     
          line(x,y,a,b,col);
     
          for(f = start; f <= end ; f ++)
          {
               c = x + costable[f] * rad;
               d = y + sintable[f] * rad / div;
               line(a,b,c,d,col);
               a = c;
               b = d;
          }
          line(x,y,a,b,col);
     }
     
     void box(int xa,int ya, int xb, int yb, int col)
     {
          /* Draws a box for use in 2d histogram graphs etc */
     
          line(xa,ya,xa,yb,col);
          line(xa,yb,xb,yb,col);
          line(xb,yb,xb,ya,col);
          line(xb,ya,xa,ya,col);
     }
     
     void tri(int xa,int ya, int xb, int yb, int xc, int yc,int col)
     {
          /* Draw a triangle */
     
          line(xa,ya,xb,yb,col);
          line(xb,yb,xc,yc,col);
          line(xc,yc,xa,ya,col);
     }
     
     void fill(int x, int y, int col,int pattern)
     {
          /* Fill a boundered shape using a hatch pattern */
     
          int xa;
          int ya;
          int bn;
          int byn;
     
          int hatch[10][8] = {     255,255,255,255,255,255,255,255,
                         128,64,32,16,8,4,2,1,
                         1,2,4,8,16,32,64,128,
                         1,2,4,8,8,4,2,1,
                         238,238,238,238,238,238,238,238,
                         170,85,170,85,170,85,170,85,
                         192,96,48,24,12,6,3,1,
                         62,62,62,0,227,227,227,0,
                         129,66,36,24,24,36,66,129,
                         146,36,146,36,146,36,146,36
                    };
     
          /* Patterns for fill, each integer describes a row of dots */
     
          xa = x;
          ya = y;  /* Save Origin */
     
          if(pixset(x,y))
               return;
     
          bn = 1;
          byn = 0;
     
          do
          {
               if (hatch[pattern][byn] != 0)
               {
                    /* If blank ignore */
                    do
                    {
                         if ((bn & hatch[pattern][byn]) == bn)
                         {
                              asm mov al , col;
                              asm mov bh , 00;
                              asm mov cx , x;
                              asm mov dx , y;
                              asm mov ah , 0Ch;
                              asm int 10h;
                         }
                         x--;
                         bn <<= 1;
                         if (bn > 128)
                              bn = 1;
                    }
                    while(!pixset(x,y) && (x > -1));
     
                    x = xa + 1;
                    bn = 128;
     
                    do
                    {
                         if ((bn & hatch[pattern][byn]) == bn)
                         {
                              asm mov al , col;
                              asm mov bh , 00;
                              asm mov cx , x;
                              asm mov dx , y;
                              asm mov ah , 0Ch;
                              asm int 10h;
                         }
                         x++;
                         bn >>=1;
                         if (bn <1)
                              bn = 128;
                    }
                    while((!pixset(x,y)) && (x <= maxx));
               }
               x = xa;
               y--;
               bn = 1;
               byn++;
               if (byn > 7)
                    byn = 0;
     
          }
          while(!pixset(x,y) && ( y > -1));
     
          /* Now travel downwards */
     
          y = ya + 1;
     
          byn = 7;
          bn = 1;
          do
          {
               /* Travel left */
               if (hatch[pattern][byn] !=0)
               {
                    do
                    {
                         if ( (bn & hatch[pattern][byn]) == bn)
                         {
                              asm mov al , col;
                              asm mov bh , 00;
                              asm mov cx , x;
                              asm mov dx , y;
                              asm mov ah , 0Ch;
                              asm int 10h;
                         }
     
                         x--;
                         bn <<= 1;
                         if (bn > 128)
                              bn = 1;
                    }
                    while(!pixset(x,y) && (x > -1));
     
                    /* Back to x origin */
                    x = xa + 1 ;
                    bn = 128;
     
                    /* Travel right */
                    do
                    {
                         if ((bn & hatch[pattern][byn]) == bn)
                         {
                              asm mov al , col;
                              asm mov bh , 00;
                              asm mov cx , x;
                              asm mov dx , y;
                              asm mov ah , 0Ch;
                              asm int 10h;
                         }
                         x++;
                         bn >>=1;
                         if (bn <1)
                              bn = 128;
                    }
                    while((!pixset(x,y)) && (x <= maxx));
               }
               x = xa;
               bn = 1;
               y++;
               byn--;
               if (byn < 0)
                    byn = 7;
          }
          while((!pixset(x,y)) && (y <= maxy));
     }
     
     void invert(int xa,int ya, int xb, int yb, int col)
     {
          /* Invert a pixel window */
     
          union REGS inreg,outreg;
     
          inreg.h.al = (unsigned char)(128 | col);
          inreg.h.ah = 0x0C;
          for(inreg.x.cx = (unsigned int)xa;      inreg.x.cx <= (unsigned
     int)xb; inreg.x.cx++)
               for(inreg.x.dx = (unsigned)ya; inreg.x.dx <= (unsigned)yb;
     inreg.x.dx++)
                    int86(0x10, &inreg, &outreg);
     }
     
     void circle(int x_centre , int y_centre, int radius, int colour)
     {
          int x,y,delta;
          int startx,endx,x1,starty,endy,y1;
          int asp_ratio;
     
          if (_dmode == 6)
               asp_ratio = 22;
          else
               asp_ratio = 13;
     
          y = radius;
          delta = 3 - 2 * radius;
          for(x = 0; x < y; )
          {
               starty = y * asp_ratio / 10;
               endy = (y + 1) * asp_ratio / 10;
               startx = x * asp_ratio / 10;
               endx = (x + 1) * asp_ratio / 10;
               for(x1 = startx; x1 < endx; ++x1)
               {
                    plot(x1+x_centre,y+y_centre,colour);
                    plot(x1+x_centre,y_centre - y,colour);
                    plot(x_centre - x1,y_centre - y,colour);
                    plot(x_centre - x1,y + y_centre,colour);
               }
     
               for(y1 = starty; y1 < endy; ++y1)
               {
                    plot(y1+x_centre,x+y_centre,colour);
                    plot(y1+x_centre,y_centre - x,colour);
                    plot(x_centre - y1,y_centre - x,colour);
                    plot(x_centre - y1,x + y_centre,colour);
               }
     
               if (delta < 0)
                    delta += 4 * x + 6;
               else
               {
                    delta += 4*(x-y)+10;
                    y--;
               }
               x++;
          }
     
          if(y)
          {
               starty = y * asp_ratio / 10;
               endy = (y + 1) * asp_ratio / 10;
               startx = x * asp_ratio / 10;
               endx = (x + 1) * asp_ratio / 10;
               for(x1 = startx; x1 < endx; ++x1)
               {
                    plot(x1+x_centre,y+y_centre,colour);
                    plot(x1+x_centre,y_centre - y,colour);
                    plot(x_centre - x1,y_centre - y,colour);
                    plot(x_centre - x1,y + y_centre,colour);
               }
     
               for(y1 = starty; y1 < endy; ++y1)
               {
                    plot(y1+x_centre,x+y_centre,colour);
                    plot(y1+x_centre,y_centre - x,colour);
                    plot(x_centre - y1,y_centre - x,colour);
                    plot(x_centre - y1,x + y_centre,colour);
               }
          }
     }
     
     void draw(int x, int y, int colour)
     {
          /* Draws a line from _lastx,_lasty to x,y */
     
          line(_lastx,_lasty,x,y,colour);
     }
     
     void psprite(SPRITE *sprite,int x,int y)
     {
          int origx;
          int origy;
          int z;
          int count;
          int col;
          int pos;
          unsigned char far *video;
     
          if (_dmode == 19)
          {
               /* Super fast direct video write in mode 19 for sprites */
     
               video = MK_FP(0xA000,0);
     
               origx = x;
               origy = y;
     
               if (sprite->x != -1)
               {
                    /* This sprite has been displayed before */
                    /* replace background */
                    /* This must be done first in case the sprite
     overlaps itself */
                    x = sprite->x;
                    y = sprite->y;
                    col = 0;
                    pos = x + y * 320;
                    for(count = 0; count < 256; count++)
                    {
                         video[pos] = sprite->save[count];
                         col++;
                         if (col == 16)
                         {
                              pos += 305;
                              col = 0;
                         }
                         else
                              pos++;
                    }
               }
     
               x = origx;
               y = origy;
               col = 0;
     
               pos = x + y * 320;
     
               for(count = 0; count < 256; count++)
               {
                    sprite->save[count] = video[pos];
                    z = sprite->data[count];
                    if (z != 255)
                         video[pos] = z;
     
                    col++;
                    if (col == 16)
                    {
                         pos += 305;
                         col = 0;
                    }
                    else
                         pos++;
               }
               sprite->x = origx;
               sprite->y = origy;
     
               return;
          }
     
          origx = x;
          origy = y;
     
          if (sprite->x != -1)
          {
               /* This sprite has been displayed before */
               /* replace background */
               /* This must be done first in case the sprite overlaps
     itself */
               x = sprite->x;
               y = sprite->y;
               col = 0;
               for(count = 0; count < 256; count++)
               {
                    if ((x >= 0) && (y >= 0) && (x < maxx) && (y < maxy))
                    {
                         z = sprite->save[count];
                         asm mov al , z;
                         asm mov bh , 00;
                         asm mov cx , x;
                         asm mov dx , y;
                         asm mov ah , 0Ch;
                         asm int 10h;
                    }
                    col++;
                    if (col == 16)
                    {
                         y++;
                         x = sprite->x;
                         col = 0;
                    }
                    else
                         x++;
               }
          }
     
          x = origx;
          y = origy;
          col = 0;
     
          for(count = 0; count < 256; count++)
          {
               if ((x >= 0) && (y >= 0) && (x < maxx) && (y < maxy))
               {
                    asm mov cx , x;
                    asm mov dx , y;
                    asm mov ah , 0Dh;
                    asm int 10h;
                    asm mov z ,al;
                    sprite->save[count] = z;
                    z = sprite->data[count];
     
                    if (z != 255)
                    {
                         asm mov al , z;
                         asm mov bh , 0;
                         asm mov cx , x;
                         asm mov dx , y;
                         asm mov ah , 0Ch;
                         asm int 10h;
                    }
               }
               col++;
               if (col == 16)
               {
                    y++;
                    x = origx;
                    col = 0;
               }
               else
                    x++;
          }
          sprite->x = origx;
          sprite->y = origy;
          return;
     }
     
     

Displaying A PCX File

The following program is offered as a practical example of graphics with
the IBM PC. It reads a file of the `PCX' format and displays the image on
the screen.

     
     /* Read a PCX file and display image */
     
     #include <dos.h>
     #include <io.h>
     #include <fcntl.h>
     
     typedef struct
     {
          unsigned char man;
          unsigned char version;
          unsigned char encoding;
          unsigned char bpp;
          int xmin;
          int ymin;
          int xmax;
          int ymax;
          int hdpi;
          int vdpi;
          int colormap[24];
          char reserved;
          unsigned char planes;
          int bpl;
          int palette;
          int hss;
          int vsize;
          char pad[54];
     }
     PCX_HEADER;
     
     PCX_HEADER header;
     
     int x;
     int y;
     
     union REGS inreg,outreg;
     
     void setvideo(unsigned char mode)
     {
          /* Sets the video display mode     and clears the screen */
     
          inreg.h.al = mode;
          inreg.h.ah = 0x00;
          int86(0x10, &inreg, &outreg);
     }
     
     void plot(int x, int y, unsigned char colour)
     {
     
             if (x < header.xmax && y < header.ymax)
             {
     
             /* Direct video plot in modes 16 & 18 only! */
             asm mov   ax,y;
             asm mov   dx,80;
             asm mul   dx;
             asm mov   bx,x;
             asm mov   cl,bl;
     
             asm shr   bx,1;
             asm shr   bx,1;
             asm shr   bx,1;
             asm add   bx,ax;
     
             asm and   cl,7;
             asm xor   cl,7;
             asm mov   ah,1;
             asm shl   ah,cl;
     
             asm mov   dx,3ceh;
             asm mov   al,8;
             asm out   dx,ax;
     
             asm mov   ax,(02h shl 8) + 5;
             asm out   dx,ax;
     
             asm mov   ax,0A000h;
             asm mov   es,ax;
     
             asm mov   al,es:[bx];
             asm mov   al,byte ptr colour;
             asm mov   es:[bx],al;
     
             asm mov   ax,(0FFh shl 8 ) + 8;
             asm out   dx,ax;
     
             asm mov   ax,(00h shl 8) + 5;
             asm out   dx,ax;
          }
     }
     
     void DISPLAY(unsigned char data)
     {
          int n;
          int bit;
     
          bit = 32;
     
          for (n = 0; n < 6; n++)
          {
               if (data & bit)
                    plot(x,y,1);
               else
                    plot(x,y,0);
               bit >>= 1;
               x++;
          }
     }
     
     main(int argc, char *argv[])
     {
          int fp;
          int total_bytes;
          int n;
          unsigned char data;
          int count;
          int scan;
     
          if (argc != 2)
          {
               puts("USAGE IS getpcx <filename>");
               exit(0);
          }
     
          setvideo(16);
     
          x = 0;
          y = 0;
     
          fp = open(argv[1],O_RDONLY|O_BINARY);
     
          _read(fp,&header,128);
     
          total_bytes = header.planes * header.bpl;
     
          for(scan = 0; scan <= header.ymax; scan++)
          {
               x = 0;
     
               /* First scan line */
     
               for(n = 0; n < total_bytes; n++)
               {
                    /* Read byte */
                    _read(fp,&data,1);
     
                    count = data & 192;
     
                    if (count == 192)
                    {
                         count = data & 63;
                         n += count - 1;
                         _read(fp,&data,1);
                         while(count)
                         {
                              DISPLAY(data);
                              count--;
                         }
                    }
                    else
                         DISPLAY(data);
     
               }
               x = 0;
               y++;
          }
     }
          

Drawing Circles


What has drawing circles got to do with advanced C programming? Well
quite a lot, it is a task which is often desired by modern programmers,
and it is a task which can be attacked from a number of angles. This
example illustrates some of the ideas already discussed for replacing
floating point numbers with integers, and using lookup tables rather than
repeat calls to maths functions.

A circle may be drawn by plotting each point on its circumference. The
location of any point is given by;


     Xp = Xo + Sine(Angle) * Radius
     Yp = Yo + Cosine(Angle) * Radius

Where Xp,Yp is the point to be plotted, and Xo,Yo is the centre of the
circle.

Thus, the simplest way to draw a circle is to calculate Xp and Yp for
each angle between 1 and 360 degrees and to plot these points. There is
however one fundamental error with this. As the radius of the circle
increases, so also does the length of the arc between each angular point.
Thus a circle of sufficient radius will be drawn with a dotted line
rather than a solid line.

The problem of the distance between the angular points may be tackled in
two ways;

  1)The number of points to be plotted can be increased, to say every 30
     minutes.
  2)A straight line may be drawn between each angular point.

The simplest circle drawing pseudo-code may then look like this;


     FOR angle = 1 TO 360
          PLOT Xo + SINE(angle) * radius, Yo + COSINE(angle) * radius
     NEXT angle

This code has two major disadvantages;

     1) It uses REAL numbers for the sine and cosine figures
     2) It makes numerous calculations of sine and cosine values

Both of these disadvantages result in a very slow piece of code. Since a
circle is a regular figure with two axis of symmetry, one in both the X
and Y axis, one only needs to calculate the relative offsets of points in
one quadrant of the circle and then these offsets may be applied to the
other three quadrants to produce a faster piece of code. Faster because
the slow sine and cosine calculations are only done 90 times instead of
360 times;


     FOR angle = 1 TO 90
          Xa = SINE(angle) * radius
          Ya = COSINE(angle) * radius
          PLOT Xo + Xa, Yo + Ya
          PLOT Xo + Xa, Yo - Ya
          PLOT Xo - Xa, Yo + Ya
          PLOT Xo - Xa, Yo - Ya
     NEXT angle

A further enhancement may be made by making use of sine and cosine lookup
tables instead of calculating them. This means calculating the sine and
cosine values for each required angle and storing them in a table. Then,
instead of calculating the values for each angle the circle drawing code
need only retrieve the values from a table;


     FOR angle = 1 TO 90
          Xa = SINE[angle] * radius
          Ya = COSINE[angle] * radius
          PLOT Xo + Xa, Yo + Ya
          PLOT Xo + Xa, Yo - Ya
          PLOT Xo - Xa, Yo + Ya
          PLOT Xo - Xa, Yo - Ya
     NEXT angle


Most computer languages work in RADIANS rather than DEGREES. There being
approximately 57 degrees in one radian, 2 * PI radians in one circle.
This implies that to calculate sine and cosine values of sufficient
points to draw a reasonable circle using radians one must again use real
numbers, that is numbers which have figures following a decimal point.
Real number arithmetic, also known as floating point arithmetic, is
horrendously slow to calculate. Integer arithmetic on the other hand is
very quick to calculate, but less precise.

To use integer arithmetic in circle drawing code requires ingenuity. If
one agrees to use sine and cosine lookup tables for degrees, rather than
radians. Then the sine value of an angle of 1 degree is;

       0.0175

Which, truncated to an integer becomes zero! To overcome this the sine
and cosine values held in the table should be multiplied by some factor,
say 10000. Then, the integer value of the sine of an angle of 1 degree
becomes;

       175

If the sine and cosine values have been multiplied by a factor, then when
the calculation of the point's offset is carried out one must remember to
divide the result by the same factor. Thus the calculation becomes;

          Xa = SINE[angle] * radius / factor
          Ya = COSINE[angle] * radius / factor

The final obstacle to drawing circles on a computer is the relationship
between the width of the display screen and its height. This ratio
between width and height is known as the "aspect ratio" and varies upon
video display mode. The IBM VGA 256 colour mode for example can display
320 pixels across and 200 up the screen. This equates to an aspect ratio
of 1:1.3. If the circle drawing code ignores the aspect ratio, then the
shape displayed will often be ovalar to a greater or lesser degree due to
the rectangular shape of the display pixels. Thus in order to display a
true circle, the formulae to calculate each point on the circumference
must be amended to calculate a slight ellipse in compensation of the
distorting factor of the display.

The offset formulae then become;

          Xa = SINE[angle] * radius / factor
          Ya = COSINE[angle] * radius / (factor * aspect ratio)

The following short C program illustrates a practical circle drawing code
segment, in a demonstrable  form;


     /* Circles.c   A demonstration circle drawing program for the IBM PC
     */
     
     
     #include <stdlib.h>
     
     int sintable[91] = {0,175,349,523,698,
               872,1045,1219,1392,
               1564,1736,1908,2079,
               2250,2419,2588,2756,
               2924,3090,3256,3420,
               3584,3746,3907,4067,
               4226,4384,4540,4695,
               4848,5000,5150,5299,
               5446,5592,5736,5878,
               6018,6157,6293,6428,
               6561,6691,6820,6947,
               7071,7193,7314,7431,
               7547,7660,7771,7880,
               7986,8090,8192,8290,
               8387,8480,8572,8660,
               8746,8829,8910,8988,
               9063,9135,9205,9272,
               9336,9397,9455,9511,
               9563,9613,9659,9703,
               9744,9781,9816,9848,
               9877,9903,9925,9945,
               9962,9976,9986,9994,
               9998,10000
     };
     
     int costable[91] = { 10000,9998,9994,9986,9976,
               9962,9945,9925,9903,
               9877,9848,9816,9781,
               9744,9703,9659,9613,
               9563,9511,9455,9397,
               9336,9272,9205,9135,
               9063,8988,8910,8829,
               8746,8660,8572,8480,
               8387,8290,8192,8090,
               7986,7880,7771,7660,
               7547,7431,7314,7193,
               7071,6947,6820,6691,
               6561,6428,6293,6157,
               6018,5878,5736,5592,
               5446,5299,5150,5000,
               4848,4695,4540,4384,
               4226,4067,3907,3746,
               3584,3420,3256,3090,
               2924,2756,2588,2419,
               2250,2079,1908,1736,
               1564,1392,1219,1045,
               872,698,523,349,
               175,0
     };
     
     void setvideo(unsigned char mode)
     {
          /* Sets the video display mode for an IBM PC */
     
          asm mov al , mode;
          asm mov ah , 00;
          asm int 10h;
     }
     
     void plot(int x, int y, unsigned char colour)
     {
          /* Code for IBM PC BIOS ROM */
          /* Sets a pixel at the specified coordinates to a specified
     colour */
     
          /* Return if out of range */
          if (x < 0 || y < 0 || x > 320 || y > 200)
               return;
     
          asm mov al , colour;
          asm mov bh , 0;
          asm mov cx , x;
          asm mov dx , y;
          asm mov ah, 0Ch;
          asm int 10h;
     }
     
     void Line(int a, int b, int c, int d, int col)
     {
          /* Draws a straight line from point a,b to point c,d in colour
     col */
     
          int u;
          int v;
          int d1x;
          int d1y;
          int d2x;
          int d2y;
          int m;
          int n;
          double s; /* The only real number variable, but it's essential
     */
          int i;
     
          u = c - a;
          v = d - b;
          if (u == 0)
          {
               d1x = 0;
               m = 0;
          }
          else
          {
               m = abs(u);
               if (u < 0)
                    d1x = -1;
               else
               if (u > 0)
                    d1x = 1;
          }
     
          if ( v == 0)
          {
               d1y = 0;
               n = 0;
          }
          else
          {
               n = abs(v);
               if (v < 0)
                    d1y = -1;
               else
               if (v > 0)
                    d1y = 1;
          }
          if (m > n)
          {
               d2x = d1x;
               d2y = 0;
          }
          else
          {
               d2x = 0;
               d2y = d1y;
               m = n;
               n = abs(u);
          }
          s = m / 2;
     
          for (i = 0; i <= m; i++)
          {
               plot(a,b,col);
               s += n;
               if (s >= m)
               {
                    s -= m;
                    a += d1x;
                    b += d1y;
               }
               else
               {
                    a += d2x;
                    b += d2y;
               }
          }
     }
     
     
     void Circle(int x, int y, int rad, int col)
     {
          /* Draws a circle about origin x,y */
          /* With a radius of rad */
          /* The col parameter defines the colour for plotting */
     
          int f;
          long xa;
          long ya;
          int a1;
          int b1;
          int a2;
          int b2;
          int a3;
          int b3;
          int a4;
          int b4;
     
     
          /* Calculate first point in each segment */
     
          a1 = x + ((long)(costable[0]) * (long)(rad) + 5000) / 10000;
          b1 = y + ((long)(sintable[0]) * (long)(rad) + 5000) / 13000;
     
          a2 = x - ((long)(costable[0]) * (long)(rad) + 5000) / 10000;
          b2 = y + ((long)(sintable[0]) * (long)(rad) + 5000) / 13000;
     
          a3 = x - ((long)(costable[0]) * (long)(rad) + 5000) / 10000;
          b3 = y - ((long)(sintable[0]) * (long)(rad) + 5000) / 13000;
     
          a4 = x + ((long)(costable[0]) * (long)(rad) + 5000) / 10000;
          b4 = y - ((long)(sintable[0]) * (long)(rad) + 5000) / 13000;
     
          /* Start loop at second point */
          for(f = 1; f <= 90; f++)
          {
               /* Calculate offset to new point */
               xa = ((long)(costable[f]) * (long)(rad) + 5000) / 10000;
               ya = ((long)(sintable[f]) * (long)(rad) + 5000) / 13000;
     
               /* Draw a line from the previous point to the new point in
                  each segment */
               Line(a1,b1,x + xa, y + ya,col);
               Line(a2,b2,x - xa, y + ya,col);
               Line(a3,b3,x - xa, y - ya,col);
               Line(a4,b4,x + xa, y - ya,col);
     
               /* Update the previous point in each segment */
               a1 = x + xa;
               b1 = y + ya;
               a2 = x - xa;
               b2 = y + ya;
               a3 = x - xa;
               b3 = y - ya;
               a4 = x + xa;
               b4 = y - ya;
          }
     }
     
     main()
     {
          int n;
     
          /* Select VGA 256 colour 320 x 200 video mode */
          setvideo(19);
     
          /* Draw some circles */
          for(n = 0; n < 100; n++)
               Circle(160,100,n,n + 20);
     }
     
     

Vesa Mode

The VESA BIOS provides a number of new, and exciting video modes not
supported by the standard BIOS. These modes vary from one video card to
another, but most support the following modes:

Mode               Display
                   
0x54               Text 16 colours 132 x 43
0x55               Text 16 colours 132 x 25
0x58               Graphics 16 colours 800 x 600
0x5C               Graphics 256 colours 800 x 600
0x5D               Graphics 16 colours 1024 x 768
0x5F               Graphics 256 colours 640 x 480
0x60               Graphics 256 colours 1024 x 768
0x64               Graphics 64k colours 640 x 480
0x65               Graphics 64k colours 800 x 600
0x6A               Graphics 16 colours 800 x 600
0x6C               Graphics 16 colours 1280 x 1024
0x70               Graphics 16m colours 320 x 200
0x71               Graphics 16m colours 640 x 480

These modes are in addition to the standard BIOS video modes described
earlier.

Setting a VESA video mode requires a call to a different BIOS function
than the standard BIOS, as illustrated in the following example which
enables any VESA or standard display mode to be selected from the DOS
command line.

     #include <dos.h>
     #include <ctype.h>
     
     void setvideo(int mode)
     {
          /* Sets the video display to a VESA or normal mode and clears
     the screen */
     
          union REGS inreg,outreg;
     
          inreg.h.ah = 0x4f;
          inreg.h.al = 0x02;
          inreg.x.bx = mode;
          int86(0x10, &inreg, &outreg);
     }
     
     main(int argc, char *argv[])
     {
          setvideo(atoi(argv[1]));
     }


Plotting pixels in a VESA mode graphics display can be acgieved with the
standard BIOS plot functiona call, as illustrated here;

     
     void plot(int x, int y, unsigned char colour)
     {
          asm mov al , colour;
          asm mov bh , 00;
          asm mov cx , x;
          asm mov dx , y;
          asm mov ah , 0Ch;
          asm int 10h;
     }


Or, in a 800 x 600 resolution mode you can use this fast direct video
access plot function;

     void plot( int x, int y, unsigned char pcolor)
     {
          /*
               Fast 800 x 600 mode (0x58 or 0x102) plotting
          */
     
          asm mov   ax,y;
          asm mov   dx,800/8;
          asm mul   dx;
          asm mov   bx,x;
          asm mov   cl,bl;
     
          asm shr   bx,1;
          asm shr   bx,1;
          asm shr   bx,1;
          asm add   bx,ax;
     
          asm and   cl,7;
          asm xor   cl,7;
          asm mov   ah,1;
          asm shl   ah,cl;
     
          asm mov   dx,03CEh;
          asm mov   al,8;
          asm out   dx,ax;
     
          asm mov   ax,(02h shl 8) + 5;
          asm out   dx,ax;
     
          asm mov   ax,0A000h;
          asm mov   es,ax;
     
          asm mov   al,es:[bx];
          asm mov   al,byte ptr pcolor;
          asm mov   es:[bx],al;
     
          asm mov   ax,(0FFh shl 8 ) + 8;
          asm out   dx,ax;
     
          asm mov   ax,(00h shl 8) + 5;
          asm out   dx,ax;
     }

There are lots more functions supported by the VESA BIOS, but this will
get you going with the basic operations. Remember though that when using
VESA display modes, that the direct console I/O functions in the C
compiler library may not function correctly.
                                    
               DIRECTORY SEARCHING WITH THE IBM PC AND DOS

Amongst the many functions provided by DOS for programmers, are a pair of
functions; "Find first" and "Find next" which are used to search a DOS
directory for a specified file name or names. The first function, "Find
first" is accessed via DOS interrupt 21, function 4E. It takes an ascii
string file specification, which can include wildcards, and the required
attribute for files to match. Upon return the function fills the disk
transfer area (DTA) with details of the located file and returns with the
carry flag clear. If an error occurs, such as no matching files are
located, the function returns with the carry flag set.

Following a successful call to "Find first", a program can call "Find
next", DOS interrupt 21, function 4F, to locate the next file matching
the specifications provided by the initial call to "Find first". If this
function succeeds, then the DTA is filled in with details of the next
matching file, and the function returns with the carry flag clear.
Otherwise a return is made with the carry flag set.

Most C compilers for the IBM PC provide non standard library functions
for accessing these two functions. Turbo C provides "findfirst()" and
"findnext()". Making use of the supplied library functions shields the
programmer from the messy task of worrying about the DTA. Microsoft C
programmers should substitue findfirst() with _dos_findfirst() and
findnext() with _dos_findnext().

The following Turbo C example imitates the DOS directory command, in a
basic form;

     #include <stdio.h>
     #include <dir.h>
     #include <dos.h>
     
     void main(void)
     {
          /* Display directory listing of current directory */
     
          int done;
          int day;
          int month;
          int year;
          int hour;
          int min;
          char amflag;
          struct ffblk ffblk;
          struct fcb fcb;
     
          /* First display sub directory entries */
          done = findfirst("*.",&ffblk,16);
     
          while (!done)
          {
               year = (ffblk.ff_fdate >> 9) + 80;
               month = (ffblk.ff_fdate >> 5) & 0x0f;
               day = ffblk.ff_fdate & 0x1f;
               hour = (ffblk.ff_ftime >> 11);
               min = (ffblk.ff_ftime >> 5) & 63;
     
               amflag = 'a';
     
               if (hour > 12)
               {
                    hour -= 12;
                    amflag = 'p';
               }
     
               printf("%-11.11s  <DIR>   %02d-%02d-%02d  %2d:%02d%c\n",
                       ffblk.ff_name,day,month,year,hour,min,amflag);
               done = findnext(&ffblk);
          }
     
          /* Now all files except directories */
          done = findfirst("*.*",&ffblk,231);
     
          while (!done)
          {
               year = (ffblk.ff_fdate >> 9) + 80;
               month = (ffblk.ff_fdate >> 5) & 0x0f;
               day = ffblk.ff_fdate & 0x1f;
               hour = (ffblk.ff_ftime >> 11);
               min = (ffblk.ff_ftime >> 5) & 63;
     
               amflag = 'a';
     
               if (hour > 12)
               {
                    hour -= 12;
                    amflag = 'p';
               }
     
               parsfnm(ffblk.ff_name,&fcb,1);
     
               printf("%-8.8s %-3.3s %8ld  %02d-%02d-%02d  %2d:%02d%c\n",
                         fcb.fcb_name,fcb.fcb_ext,ffblk.ff_fsize,
                         day,month,year,hour,min,amflag);
               done = findnext(&ffblk);
          }
     }
     

The function "parsfnm()" is a Turbo C library command which makes use of
the DOS function for parsing an ascii string containing a file name, into
its component parts. These component parts are then put into a DOS file
control block (fcb), from where they may be easily retrieved for
displaying by printf().


The DOS DTA is comprised as follows;

Offset                   Length                   Contents
                                                  
00                       15                       Reserved
15                       Byte                     Attribute of matched
                                                  file
16                       Word                     File time
18                       Word                     File date
1A                       04                       File size
1E                       0D                       File name and extension
                                                  as ascii string

The file time word contains the time at which the file was last written
to disc and is comprised as follows;

Bits      Contents

 0 -  4             Seconds divided by 2
 5 - 10        Minutes
11 - 15   Hours

The file date word holds the date on which the file was last written to
disc and is comprised of;

Bits      Contents

 0 -  4             Day
 5 -  8             Month
 9 - 15        Years since 1980

To extract these details from the DTA requires a little manipulation, as
illustrated in the above example.

The DTA attribute flag is comprised of the following bits being set or
not;

Bit       Attribute

0         Read only
1         Hidden
2         System
3         Volume label
4         Directory
5         Archive
                                    
                        ACCESSING EXPANDED MEMORY

Memory (RAM) in an IBM PC comes in three flavours; Conventional, Expanded
and Extended. Conventional memory is the 640K of RAM which the operating
system DOS can access. This memory is normally used. However, it is often
insufficient for todays RAM hungry systems. Expanded memory is RAM which
is addressed outside of the area of conventional RAM not by DOS but by a
second program called a LIM EMS driver. Access to this device driver is
made through interrupt 67h.

The main problem with accessing expanded memory is that no matter how
much expanded memory is fitted to the computer, it can only be accessed
through 16K blocks refered to as pages. So for example. If you have 2mB
of expanded RAM fitted to your PC then that is comprised of 128 pages
(128 * 16K = 2mB).

A program can determine whether a LIM EMS driver is installed by
attempting to open the file `EMMXXXX0' which is guarranteed by the LIM
standard to be present as an IOCTL device when the device driver is
active.

The following source code illustrates some basic functions for testing
for and accessing expanded memory.

     /*
     Various functions for using Expanded memory
     */
     
     #include <dos.h>
     #define   EMM  0x67
     
     char far *emmbase;
     emmtest()
     {
          /*
          Tests for the presence of expnaded memory by attempting to
          open the file EMMXXXX0.
          */
     
          union REGS regs;
          struct SREGS sregs;
          int error;
          long handle;
     
          /* Attempt to open the file device EMMXXXX0 */
          regs.x.ax = 0x3d00;
          regs.x.dx = (int)"EMMXXXX0";
          sregs.ds = _DS;
          intdosx(&regs,&regs,&sregs);
          handle = regs.x.ax;
          error = regs.x.cflag;
     
          if (!error)
          {
               regs.h.ah = 0x3e;
               regs.x.bx = handle;
               intdos(&regs,&regs);
          }
          return error;
     }
     
     emmok()
     {
          /*
          Checks whether the expanded memory manager responds correctly
          */
     
          union REGS regs;
     
          regs.h.ah = 0x40;
          int86(EMM,&regs,&regs);
     
          if (regs.h.ah)
               return 0;
     
          regs.h.ah = 0x41;
          int86(EMM,&regs,&regs);
     
          if (regs.h.ah)
               return 0;
     
          emmbase = MK_FP(regs.x.bx,0);
          return 1;
     }
     
     long emmavail()
     {
        /*
        Returns the number of available (free) 16K pages of expanded
     memory
        or -1 if an error occurs.
        */
     
             union REGS regs;
     
          regs.h.ah = 0x42;
          int86(EMM,&regs,&regs);
          if (!regs.h.ah)
               return regs.x.bx;
          return -1;
     }
     
     long emmalloc(int n)
     {
          /*
          Requests 'n' pages of expanded memory and returns the file
     handle
          assigned to the pages or -1 if there is an error
          */
     
          union REGS regs;
     
          regs.h.ah = 0x43;
          regs.x.bx = n;
          int86(EMM,&regs,&regs);
          if (regs.h.ah)
               return -1;
          return regs.x.dx;
     }
     
     emmmap(long handle, int phys, int page)
     {
          /*
          Maps a physical page from expanded memory into the page frame
     in the
          conventional memory 16K window so that data can be transfered
     between
          the expanded memory and conventional memory.
          */
     
          union REGS regs;
     
          regs.h.ah = 0x44;
          regs.h.al = page;
          regs.x.bx = phys;
          regs.x.dx = handle;
          int86(EMM,&regs,&regs);
          return (regs.h.ah == 0);
     }
     
     void emmmove(int page, char *str, int n)
     {
          /*
          Move 'n' bytes from conventional memory to the specified
     expanded memory
          page
          */
     
          char far *ptr;
     
          ptr = emmbase + page * 16384;
          while(n-- > 0)
               *ptr++ = *str++;
     }
     
     void emmget(int page, char *str, int n)
     {
          /*
          Move 'n' bytes from the specified expanded memory page into
     conventional
          memory
          */
     
          char far *ptr;
     
          ptr = emmbase + page * 16384;
          while(n-- > 0)
               *str++ = *ptr++;
     }
     
     emmclose(long handle)
     {
          /*
          Release control of the expanded memory pages allocated to
     'handle'
          */
     
          union REGS regs;
     
          regs.h.ah = 0x45;
          regs.x.dx = handle;
          int86(EMM,&regs,&regs);
          return (regs.h.ah == 0);
     }
     
     /*
     Test function for the EMM routines
     */
     
     void main()
     {
          long emmhandle;
          long avail;
          char teststr[80];
          int i;
     
          if(!emmtest())
          {
               printf("Expanded memory is not present\n");
               exit(0);
          }
     
          if(!emmok())
          {
               printf("Expanded memory manager is not present\n");
               exit(0);
          }
     
          avail = emmavail();
          if (avail == -1)
          {
               printf("Expanded memory manager error\n");
               exit(0);
          }
          printf("There are %ld pages available\n",avail);
     
          /* Request 10 pages of expanded memory */
          if((emmhandle = emmalloc(10)) < 0)
          {
               printf("Insufficient pages available\n");
               exit(0);
          }
     
          for (i = 0; i < 10; i++)
          {
               sprintf(teststr,"%02d This is a test string\n",i);
               emmmap(emmhandle,i,0);
               emmmove(0,teststr,strlen(teststr) + 1);
          }
     
          for (i = 0; i < 10; i++)
          {
               emmmap(emmhandle,i,0);
               emmget(0,teststr,strlen(teststr) + 1);
               printf("READING BLOCK %d: %s\n",i,teststr);
          }
     
          emmclose(emmhandle);
     }
                                    
                        ACCESSING EXTENDED MEMORY


Extended memory has all but taken over from Expanded Memory now (1996).
It is faster and more useable than expanded memory. As with Expanded
memory, Extended memory cannot be directly accessed through the standard
DOS mode, and so a transfer buffer in conventional or "real-mode" memory
needs to be used. The process to write data to Extended memory then
involves copying the data to the transfer buffer in conventional memory,
and from there copying it to Extended memory.

Before any use may be made of Extended memory, a program should test to
see if Extended memory is available. The following function, XMS_init(),
tests for the presence of Extended memory, and if available calls another
function, GetXMSEntry()  to initialise the program for using Extended
Memory. The function also allocates a conventional memory transfer
buffer.


     
     /*
          BLOCKSIZE will be the size of our real-memory buffer that
          we'll swap XMS through (must be a multiple of 1024, since
          XMS is allocated in 1K chunks.)
     */
     
     #ifdef __SMALL__
     #define BLOCKSIZE (16L * 1024L)
     #endif
     
     
     #ifdef __MEDIUM__
     #define BLOCKSIZE (16L * 1024L)
     #endif
     
     
     #ifdef __COMPACT__
     #define BLOCKSIZE (64L * 1024L)
     #endif
     
     #ifdef __LARGE__
     #define BLOCKSIZE (64L * 1024L)
     #endif
     
     
     char XMS_init()
     {
          /*
               returns 0 if XMS present,
                    1 if XMS absent
                    2 if unable to allocate conventional memory transfer
     buffer
          */
          unsigned char status;
          _AX=0x4300;
          geninterrupt(0x2F);
          status = _AL;
          if(status==0x80)
          {
               GetXMSEntry();
               XMSBuf = (char far *) farmalloc(BLOCKSIZE);
               if (XMSBuf == NULL)
                    return 2;
               return 0;
          }
          return 1;
     }
     
     void GetXMSEntry(void)
     {
          /*
               GetXMSEntry sets XMSFunc to the XMS Manager entry point
               so we can call it later
          */
     
          _AX=0x4310;
          geninterrupt(0x2F);
          XMSFunc= (void (far *)(void)) MK_FP(_ES,_BX);
     }


Once the presence of Extended memory has been confirmed, a program can
find out how much Extended memory is available;

     void XMSSize(int *kbAvail, int *largestAvail)
     {
          /*
               XMSSize returns the total kilobytes available, and the
     size
               in kilobytes of the largest available block
          */
     
          _AH=8;
          (*XMSFunc)();
          *largestAvail=_DX;
          *kbAvail=_AX;
     }


The following function may be called to allocate a block of Extended
memory, like you would allocate a block of conventional memory.

     char AllocXMS(unsigned long numberBytes)
     {
          /*
               Allocate a block of XMS memory numberBytes long
               Returns 1 on success
                    0 on failure
          */
     
          _DX = (int)(numberBytes / 1024);
          _AH = 9;
          (*XMSFunc)();
          if (_AX==0)
          {
               return 0;
          }
          XMSHandle=_DX;
          return 1;
     }
     

Allocated Extended memory is not freed by DOS. A program using Extended
memory must release it before terminating. This function frees a block of
extended memory previously allocated by AllocXMS. Note, XMSHandle is a
global variable of type int.

     void XMS_free(void)
     {
          /*
               Free used XMS
          */
          _DX=XMSHandle;
          _AH=0x0A;
          (*XMSFunc)();
     }


Two functions are now given. One for writing data to Extended memory, and
one for reading data from Extended memory into conventional memory.

     /*
          XMSParms is a structure for copying information to and from
          real-mode memory to XMS memory
     */
     
     struct parmstruct
     {
          /*
               blocklength is the size in bytes of block to copy
          */
          unsigned long blockLength;
     
          /*
               sourceHandle is the XMS handle of source; 0 means that
               sourcePtr will be a 16:16 real-mode pointer, otherwise
               sourcePtr is a 32-bit offset from the beginning of the
               XMS area that sourceHandle points to
          */
     
          unsigned int sourceHandle;
          far void *sourcePtr;
     
          /*
               destHandle is the XMS handle of destination; 0 means that
               destPtr will be a 16:16 real-mode pointer, otherwise
               destPtr is a 32-bit offset from the beginning of the XMS
               area that destHandle points to
          */
     
          unsigned int destHandle;
          far void *destPtr;
     }
     XMSParms;
     
     
     char XMS_write(unsigned long loc, char far *val, unsigned length)
     {
          /*
               Round length up to next even value
          */
          length += length % 2;
     
          XMSParms.sourceHandle=0;
          XMSParms.sourcePtr=val;
          XMSParms.destHandle=XMSHandle;
          XMSParms.destPtr=(void far *) (loc);
          XMSParms.blockLength=length;  /* Must be an even number! */
          _SI = FP_OFF(&XMSParms);
          _AH=0x0B;
          (*XMSFunc)();
          if (_AX==0)
          {
               return 0;
          }
          return 1;
     }
     
     
     void *XMS_read(unsigned long loc,unsigned length)
     {
          /*
               Returns pointer to data
               or NULL on error
          */
     
          /*
               Round length up to next even value
          */
          length += length % 2;
     
          XMSParms.sourceHandle=XMSHandle;
          XMSParms.sourcePtr=(void far *) (loc);
          XMSParms.destHandle=0;
          XMSParms.destPtr=XMSBuf;
          XMSParms.blockLength=length;       /* Must be an even number */
          _SI=FP_OFF(&XMSParms);
          _AH=0x0B;
          (*XMSFunc)();
          if (_AX==0)
          {
               return NULL;
          }
          return XMSBuf;
     }
     
     
And now putting it all together is a demonstration program.

     /* A sequential table of variable length records in XMS */
     
     #include <dos.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <alloc.h>
     #include <string.h>
     
     #define TRUE 1
     #define FALSE 0
     
     /*
          BLOCKSIZE will be the size of our real-memory buffer that
          we'll swap XMS through (must be a multiple of 1024, since
          XMS is allocated in 1K chunks.)
     */
     
     #ifdef __SMALL__
     #define BLOCKSIZE (16L * 1024L)
     #endif
     
     
     #ifdef __MEDIUM__
     #define BLOCKSIZE (16L * 1024L)
     #endif
     
     
     #ifdef __COMPACT__
     #define BLOCKSIZE (64L * 1024L)
     #endif
     
     #ifdef __LARGE__
     #define BLOCKSIZE (64L * 1024L)
     #endif
     
     
     /*
          XMSParms is a structure for copying information to and from
          real-mode memory to XMS memory
     */
     
     struct parmstruct
     {
          /*
               blocklength is the size in bytes of block to copy
          */
          unsigned long blockLength;
     
          /*
               sourceHandle is the XMS handle of source; 0 means that
               sourcePtr will be a 16:16 real-mode pointer, otherwise
               sourcePtr is a 32-bit offset from the beginning of the
               XMS area that sourceHandle points to
          */
     
          unsigned int sourceHandle;
          far void *sourcePtr;
     
          /*
               destHandle is the XMS handle of destination; 0 means that
               destPtr will be a 16:16 real-mode pointer, otherwise
               destPtr is a 32-bit offset from the beginning of the XMS
               area that destHandle points to
          */
     
          unsigned int destHandle;
          far void *destPtr;
     }
     XMSParms;
     
     void far (*XMSFunc) (void);   /* Used to call XMS manager
     (himem.sys) */
     char GetBuf(void);
     void GetXMSEntry(void);
     
     char *XMSBuf;  /* Conventional memory buffer for transfers */
     
     unsigned int XMSHandle;  /* handle to allocated XMS block */
     
     
     char XMS_init()
     {
          /*
               returns 0 if XMS present,
                    1 if XMS absent
                    2 if unable to allocate transfer buffer
          */
          unsigned char status;
          _AX=0x4300;
          geninterrupt(0x2F);
          status = _AL;
          if(status==0x80)
          {
               GetXMSEntry();
               XMSBuf = (char far *) farmalloc(BLOCKSIZE);
               if (XMSBuf == NULL)
                    return 2;
               return 0;
          }
          return 1;
     }
     
     void GetXMSEntry(void)
     {
          /*
               GetXMSEntry sets XMSFunc to the XMS Manager entry point
               so we can call it later
          */
     
          _AX=0x4310;
          geninterrupt(0x2F);
          XMSFunc= (void (far *)(void)) MK_FP(_ES,_BX);
     }
     
     
     void XMSSize(int *kbAvail, int *largestAvail)
     {
          /*
               XMSSize returns the total kilobytes available, and the
     size
               in kilobytes of the largest available block
          */
     
          _AH=8;
          (*XMSFunc)();
          *largestAvail=_DX;
          *kbAvail=_AX;
     }
     
     char AllocXMS(unsigned long numberBytes)
     {
          /*
               Allocate a block of XMS memory numberBytes long
          */
     
          _DX = (int)(numberBytes / 1024);
          _AH = 9;
          (*XMSFunc)();
          if (_AX==0)
          {
               return FALSE;
          }
          XMSHandle=_DX;
          return TRUE;
     }
     
     void XMS_free(void)
     {
          /*
               Free used XMS
          */
          _DX=XMSHandle;
          _AH=0x0A;
          (*XMSFunc)();
     }
     
     char XMS_write(unsigned long loc, char far *val, unsigned length)
     {
          /*
               Round length up to next even value
          */
          length += length % 2;
     
          XMSParms.sourceHandle=0;
          XMSParms.sourcePtr=val;
          XMSParms.destHandle=XMSHandle;
          XMSParms.destPtr=(void far *) (loc);
          XMSParms.blockLength=length;  /* Must be an even number! */
          _SI = FP_OFF(&XMSParms);
          _AH=0x0B;
          (*XMSFunc)();
          if (_AX==0)
          {
               return FALSE;
          }
          return TRUE;
     }
     
     
     void *XMS_read(unsigned long loc,unsigned length)
     {
          /*
               Returns pointer to data
               or NULL on error
          */
     
          /*
               Round length up to next even value
          */
          length += length % 2;
     
          XMSParms.sourceHandle=XMSHandle;
          XMSParms.sourcePtr=(void far *) (loc);
          XMSParms.destHandle=0;
          XMSParms.destPtr=XMSBuf;
          XMSParms.blockLength=length;  /* Must be an even number */
          _SI=FP_OFF(&XMSParms);
          _AH=0x0B;
          (*XMSFunc)();
          if (_AX==0)
          {
               return NULL;
          }
          return XMSBuf;
     }
     
     
     /*
          Demonstration code
          Read various length strings into a single XMS block (EMB)
          and write them out again
     */
     
     int main()
     {
          int kbAvail,largestAvail;
          char buffer[80];
          char *p;
          long pos;
          long end;
     
          if (XMS_init() == 0)
               printf("XMS Available ...\n");
          else
          {
               printf("XMS Not Available\n");
               return(1);
          }
     
          XMSSize(&kbAvail,&largestAvail);
          printf("Kilobytes Available: %d; Largest block:
     %dK\n",kbAvail,largestAvail);
     
          if (!AllocXMS(2000 * 1024L))
               return(1);
     
     
          pos = 0;
     
          do
          {
               p = fgets(buffer,1000,stdin);
               if (p != NULL)
               {
                    XMS_write(pos,buffer,strlen(buffer) + 1);
                    pos += strlen(buffer) + 1;
               }
          }
          while(p != NULL);
     
          end = pos;
     
          pos = 0;
     
          do
          {
               memcpy(buffer,XMS_read(pos,100),70);
               printf("%s",buffer);
               pos += strlen(buffer) + 1;
          }
          while(pos < end);
     
          /*
               It is VERY important to free any XMS before exiting!
          */
          XMS_free();
          return 0;
     }

                                    
                                    
                                    
                             TSR PROGRAMMING


Programs which remain running and resident in memory while other programs
are running are the most exciting line of programming for many PC
developers. This type of program is known as a "Terminate and Stay
Resident" or "TSR" program and they are very difficult to program
sucessfuly.

The difficulties in programming TSRs comes from the limitations of DOS
which is not a multi-tasking operating system, and does not react well to
re-enterant code. That is it's own functions (interrupts) calling
themselves.

In theory a TSR is quite simple. It is an ordinary program which
terminates not through the usual DOS terminate function, but through the
DOS "keep" function - interrupt 27h. This function reserves an area of
memory, used by the program so that no other programs will overwrite it.
This in itself is not a very difficult task, excepting that the program
needs to tell DOS how much memory to leave it!

The problems stem mainly from not being able to use DOS function calls
within the TSR program once it has "gone resident".

There are a few basic rules which help to clarify the problems
encountered in programming TSRs:

  1.Avoid DOS function calls
  2.Monitor the DOS busy flag, when this flag is nonzero, DOS is
     executing an interrupt 21h function and MUST NOT be disturbed!
  3.Monitor interrupt 28h. This reveals when DOS is busy waiting for
     console input. At this time you can disturb DOS regardless of the
     DOS busy flag setting.
  4.Provide some way of checking whether the TSR is already loaded to
     prevent multiple copies occuring in memory.
  5.Remember that other TSR programs may be chained to interrupts, and
     so you must chain any interrupt vectors that your program needs.
  6.Your TSR program must use its own stack, and NOT that of the running
     process.
  7.TSR programs must be compiled in a small memory model with stack
     checking turned off.
  8.When control passes to your TSR program, it must tell DOS that the
     active process has changed.


The following three source code modules describe a complete TSR program.
This is a useful pop-up address book database which can be activated
while any other program is running by pressing the key combination `Alt'
and `.'.  If the address book does not respond to the key press, it is
probably because DOS cannot be disturbed, and you should try to pop-it-up
again.


     /*
        A practical TSR program (a pop-up address book database)
        Compile in small memory model with stack checking OFF
     */
     
     #include <dos.h>
     #include <stdio.h>
     #include <string.h>
     #include <dir.h>
     
     static union REGS rg;
     
     /*
        Size of the program to remain resident
        experimentation is required to make this as small as possible
     */
     unsigned sizeprogram = 28000/16;
     
     /* Activate with Alt . */
     unsigned scancode = 52;  /* . */
     unsigned keymask = 8;         /* ALT */
     
     char signature[]= "POPADDR";
     char fpath[40];
     
     /*
          Function prototypes
     */
     
     void curr_cursor(int *x, int *y);
     int resident(char *, void interrupt(*)());
     void resinit(void);
     void terminate(void);
     void restart(void);
     void wait(void);
     void resident_psp(void);
     void exec(void);
     
     /*
        Entry point from DOS
     */
     
     main(int argc, char *argv[])
     {
          void interrupt ifunc();
          int ivec;
     
          /*
             For simplicity, assume the data file is in the root
     directory
             of drive C:
          */
          strcpy(fpath,"C:\\ADDRESS.DAT");
     
          if ((ivec = resident(signature,ifunc)) != 0)
          {
               /* TSR is resident */
               if (argc > 1)
               {
                    rg.x.ax = 0;
                    if (strcmp(argv[1],"quit") == 0)
                         rg.x.ax = 1;
                    else if (strcmp(argv[1],"restart") == 0)
                         rg.x.ax = 2;
                    else if (strcmp(argv[1],"wait") == 0)
                         rg.x.ax = 3;
                    if (rg.x.ax)
                    {
                         int86(ivec,&rg,&rg);
                         return;
                    }
               }
               printf("\nPopup Address Book is already resident");
          }
          else
          {
               /* Initial load of TSR program */
               printf("Popup Address Book Resident.\nPress Alt . To
     Activate....\n");
               resinit();
          }
     }
     
     void interrupt ifunc(bp,di,si,ds,es,dx,cx,bx,ax)
     {
          if(ax == 1)
               terminate();
          else if(ax == 2)
               restart();
          else if(ax == 3)
               wait();
     }
     
     popup()
     {
          int x,y;
     
          curr_cursor(&x,&y);
     
          /* Call the TSR C program here */
          exec();
          cursor(x,y);
     }
     
     /*
          Second source module
     */
     
     #include <dos.h>
     #include <stdio.h>
     
     static union REGS rg;
     static struct SREGS seg;
     static unsigned mcbseg;
     static unsigned dosseg;
     static unsigned dosbusy;
     static unsigned enddos;
     char far *intdta;
     static unsigned intsp;
     static unsigned intss;
     static char far *mydta;
     static unsigned myss;
     static unsigned stack;
     static unsigned ctrl_break;
     static unsigned mypsp;
     static unsigned intpsp;
     static unsigned pids[2];
     static int pidctr = 0;
     static int pp;
     static void interrupt (*oldtimer)();
     static void interrupt (*old28)();
     static void interrupt (*oldkb)();
     static void interrupt (*olddisk)();
     static void interrupt (*oldcrit)();
     
     void interrupt newtimer();
     void interrupt new28();
     void interrupt newkb();
     void interrupt newdisk();
     void interrupt newcrit();
     
     extern unsigned sizeprogram;
     extern unsigned scancode;
     extern unsigned keymask;
     
     static int resoff = 0;
     static int running = 0;
     static int popflg = 0;
     static int diskflag = 0;
     static int kbval;
     static int cflag;
     
     void dores(void);
     void pidaddr(void);
     
     void resinit()
     {
          segread(&seg);
          myss = seg.ss;
     
          rg.h.ah = 0x34;
          intdos(&rg,&rg);
          dosseg = _ES;
          dosbusy = rg.x.bx;
     
          mydta = getdta();
          pidaddr();
          oldtimer = getvect(0x1c);
          old28 = getvect(0x28);
          oldkb = getvect(9);
          olddisk = getvect(0x13);
     
          setvect(0x1c,newtimer);
          setvect(9,newkb);
          setvect(0x28,new28);
          setvect(0x13,newdisk);
     
          stack = (sizeprogram - (seg.ds - seg.cs)) * 16 - 300;
          rg.x.ax = 0x3100;
          rg.x.dx = sizeprogram;
          intdos(&rg,&rg);
     }
     
     void interrupt newdisk(bp,di,si,ds,es,dx,cx,bx,ax,ip,cs,flgs)
     {
          diskflag++;
          (*olddisk)();
          ax = _AX;
          newcrit();
          flgs = cflag;
          --diskflag;
     }
     
     void interrupt newcrit(bp,di,si,ds,es,dx,cx,bx,ax,ip,cs,flgs)
     {
          ax = 0;
          cflag = flgs;
     }
     
     void interrupt newkb()
     {
          if (inportb(0x60) == scancode)
          {
               kbval = peekb(0,0x417);
               if (!resoff && ((kbval & keymask) ^ keymask) == 0)
               {
                    kbval = inportb(0x61);
                    outportb(0x61,kbval | 0x80);
                    outportb(0x61,kbval);
                    disable();
                    outportb(0x20,0x20);
                    enable();
                    if (!running)
                         popflg = 1;
                    return;
               }
          }
          (*oldkb)();
     }
     
     void interrupt newtimer()
     {
          (*oldtimer)();
          if (popflg && peekb(dosseg,dosbusy) == 0)
               if(diskflag == 0)
               {
                    outportb(0x20,0x20);
                    popflg = 0;
                    dores();
               }
     }
     
     void interrupt new28()
     {
          (*old28)();
          if (popflg && peekb(dosseg,dosbusy) != 0)
          {
               popflg = 0;
               dores();
          }
     }
     
     resident_psp()
     {
          intpsp = peek(dosseg,*pids);
          for(pp = 0; pp < pidctr; pp++)
               poke(dosseg,pids[pp],mypsp);
     }
     
     interrupted_psp()
     {
          for(pp = 0; pp < pidctr; pp++)
               poke(dosseg,pids[pp],intpsp);
     }
     
     void dores()
     {
          running = 1;
          disable();
          intsp = _SP;
          intss = _SS;
          _SP = stack;
          _SS = myss;
          enable();
          oldcrit = getvect(0x24);
          setvect(0x24,newcrit);
          rg.x.ax = 0x3300;
          intdos(&rg,&rg);
          ctrl_break = rg.h.dl;
          rg.x.ax = 0x3301;
          rg.h.dl = 0;
          intdos(&rg,&rg);
          intdta = getdta();
          setdta(mydta);
          resident_psp();
          popup();
          interrupted_psp();
          setdta(intdta);
          setvect(0x24,oldcrit);
          rg.x.ax = 0x3301;
          rg.h.dl = ctrl_break;
          intdos(&rg,&rg);
          disable();
          _SP = intsp;
          _SS = intss;
          enable();
          running = 0;
     }
     
     static int avec = 0;
     
     unsigned resident(char *signature,void interrupt(*ifunc)())
     {
          char *sg;
          unsigned df;
          int vec;
     
          segread(&seg);
          df = seg.ds-seg.cs;
          for(vec = 0x60; vec < 0x68; vec++)
          {
               if (getvect(vec) == NULL)
               {
                    if (!avec)
                         avec = vec;
                    continue;
               }
               for(sg = signature; *sg; sg++)
               if (*sg != peekb(peek(0,2+vec*4)+df,(unsigned)sg))
                    break;
               if (!*sg)
                    return vec;
          }
          if (avec)
               setvect(avec,ifunc);
          return 0;
     }
     
     static void pidaddr()
     {
          unsigned adr = 0;
     
          rg.h.ah = 0x51;
          intdos(&rg,&rg);
          mypsp = rg.x.bx;
          rg.h.ah = 0x52;
          intdos(&rg,&rg);
          enddos = _ES;
          enddos = peek(enddos,rg.x.bx-2);
          while(pidctr < 2 && (unsigned)((dosseg<<4) + adr) < (enddos
     <<4))
          {
               if (peek(dosseg,adr) == mypsp)
               {
                    rg.h.ah = 0x50;
                    rg.x.bx = mypsp + 1;
                    intdos(&rg,&rg);
                    if (peek(dosseg,adr) == mypsp + 1)
                         pids[pidctr++] = adr;
                    rg.h.ah = 0x50;
                    rg.x.bx = mypsp;
                    intdos(&rg,&rg);
               }
               adr++;
          }
     }
     
     static resterm()
     {
          setvect(0x1c,oldtimer);
          setvect(9,oldkb);
          setvect(0x28,old28);
          setvect(0x13,olddisk);
          setvect(avec,(void interrupt (*)()) 0);
          rg.h.ah = 0x52;
          intdos(&rg,&rg);
          mcbseg = _ES;
          mcbseg = peek(mcbseg,rg.x.bx-2);
          segread(&seg);
          while(peekb(mcbseg,0) == 0x4d)
          {
               if(peek(mcbseg,1) == mypsp)
               {
                    rg.h.ah = 0x49;
                    seg.es = mcbseg+1;
                    intdosx(&rg,&rg,&seg);
               }
               mcbseg += peek(mcbseg,3) + 1;
          }
     }
     
     terminate()
     {
          if (getvect(0x13) == (void interrupt (*)()) newdisk)
               if (getvect(9) == newkb)
                    if(getvect(0x28) == new28)
                         if(getvect(0x1c) == newtimer)
                         {
                              resterm();
                              return;
                         }
          resoff = 1;
     }
     
     restart()
     {
          resoff = 0;
     }
     
     wait()
     {
          resoff = 1;
     }
     
     void cursor(int y, int x)
     {
          rg.x.ax = 0x0200;
          rg.x.bx = 0;
          rg.x.dx = ((y << 8) & 0xff00) + x;
          int86(16,&rg,&rg);
     }
     
     void curr_cursor(int *y, int *x)
     {
          rg.x.ax = 0x0300;
          rg.x.bx = 0;
          int86(16,&rg,&rg);
          *x = rg.h.dl;
          *y = rg.h.dh;
     }
     
     /*
        Third module, the simple pop-up address book
        with mouse support
     */
     
     #include <stdio.h>
     #include <stdlib.h>
     #include <io.h>
     #include <string.h>
     #include <fcntl.h>
     #include <sys\stat.h>
     #include <dos.h>
     #include <conio.h>
     #include <graphics.h>
     #include <bios.h>
     
     /* left cannot be less than 3 */
     #define left   4
     
     /* Data structure for records */
     typedef struct
     {
          char name[31];
          char company[31];
          char address[31];
          char area[31];
          char town[31];
          char county[31];
          char post[13];
          char telephone[16];
          char fax[16];
     }
     data;
     
     extern char fpath[];
     
     static char scr[4000];
     
     static char sbuff[2000];
     char stext[30];
     data rec;
     int handle;
     int recsize;
     union REGS inreg,outreg;
     
     /*
          Function prototypes
     */
     void FATAL(char *);
     void OPENDATA(void);
     void CONTINUE(void);
     void EXPORT_MULTI(void);
     void GETDATA(int);
     int GETOPT(void);
     void DISPDATA(void);
     void ADD_REC(void);
     void PRINT_MULTI(void);
     void SEARCH(void);
     void MENU(void);
     
     int GET_MOUSE(int *buttons)
     {
          inreg.x.ax = 0;
          int86(0x33,&inreg,&outreg);
          *buttons = outreg.x.bx;
          return outreg.x.ax;
     }
     
     void MOUSE_CURSOR(int status)
     {
          /* Status = 0 cursor off */
          /*          1 cursor on */
     
          inreg.x.ax = 2 - status;
          int86(0x33,&inreg,&outreg);
     }
     
     int MOUSE_LOCATION(int *x, int *y)
     {
          inreg.x.ax = 3;
          int86(0x33,&inreg,&outreg);
     
          *x = outreg.x.cx / 8;
          *y = outreg.x.dx / 8;
     
          return outreg.x.bx;
     }
     
     int GETOPT()
     {
          int result;
          int x;
          int y;
     
          do
          {
               do
               {
                    result = MOUSE_LOCATION(&x,&y);
                    if (result & 1)
                    {
                         if (x >= 52 && x <= 53 && y >= 7 && y <= 15)
                              return y - 7;
                         if (x >= 4 && x <= 40 && y >= 7 && y <= 14)
                              return y + 10;
     
                         if (x >= 4 && x <= 40 && y == 15)
                              return y + 10;
                    }
               }
               while(!bioskey(1));
     
               result = bioskey(0);
               x = result & 0xff;
               if (x == 0)
               {
                    result = result >> 8;
                    result -= 60;
               }
          }
          while(result < 0 || result > 8);
          return result;
     }
     
     void setvideo(unsigned char mode)
     {
          /* Sets the video display mode     and clears the screen */
     
          inreg.h.al = mode;
          inreg.h.ah = 0x00;
          int86(0x10, &inreg, &outreg);
     }
     
     
     int activepage(void)
     {
          /* Returns the currently selected video display page */
     
          union REGS inreg,outreg;
     
          inreg.h.ah = 0x0F;
          int86(0x10, &inreg, &outreg);
          return(outreg.h.bh);
     }
     
     void print(char *str)
     {
          /*
             Prints characters only directly to the current display page
             starting at the current cursor position. The cursor is not
             advanced.
             This function assumes a colour display card. For use with a
             monochrome display card change 0xB800 to read 0xB000
          */
     
          int page;
          int offset;
          unsigned row;
          unsigned col;
          char far *ptr;
     
          page = activepage();
          curr_cursor(&row,&col);
     
          offset = page * 4000 + row * 160 + col * 2;
     
          ptr = MK_FP(0xB800,offset);
     
          while(*str)
          {
               *ptr++= *str++;
               ptr++;
          }
     }
     
     
     void TRUESHADE(int lef, int top, int right, int bottom)
     {
          int n;
     
          /* True Shading of a screen block */
     
          gettext(lef,top,right,bottom,sbuff);
          for(n = 1; n < 2000; n+= 2)
               sbuff[n] = 7;
          puttext(lef,top,right,bottom,sbuff);
     }
     
     void DBOX(int l, int t, int r, int b)
     {
          /* Draws a double line box around the described area */
     
          int n;
     
          cursor(t,l);
          print("�");
          for(n = 1; n < r - l; n++)
          {
               cursor(t,l + n);
               print("I");
          }
          cursor(t,r);
          print("¯");
     
          for (n = t + 1; n < b; n++)
          {
               cursor(n,l);
               print("§");
               cursor(n,r);
               print("§");
          }
          cursor(b,l);
          print("E");
          for(n = 1; n < r - l; n++)
          {
               cursor(b,l+n);
               print("I");
          }
          cursor(b,r);
          print("¬");
     }
     
     int INPUT(char *text,unsigned length)
     {
          /* Receive a string from the operator */
     
          unsigned key_pos;
          int key;
          unsigned start_row;
          unsigned start_col;
          unsigned end;
          char temp[80];
          char *p;
     
          curr_cursor(&start_row,&start_col);
     
          key_pos = 0;
          end = strlen(text);
          for(;;)
          {
               key = bioskey(0);
               if ((key & 0xFF) == 0)
               {
                    key = key >> 8;
                    if (key == 79)
                    {
                         while(key_pos < end)
                              key_pos++;
                         cursor(start_row,start_col + key_pos);
                    }
                    else
                    if (key == 71)
                    {
                         key_pos = 0;
                         cursor(start_row,start_col);
                    }
                    else
                    if ((key == 75) && (key_pos > 0))
                    {
                         key_pos--;
                         cursor(start_row,start_col + key_pos);
                    }
                    else
                    if ((key == 77) && (key_pos < end))
                    {
                         key_pos++;
                         cursor(start_row,start_col + key_pos);
                    }
                    else
                    if (key == 83)
                    {
                         p = text + key_pos;
                         while(*(p+1))
                         {
                              *p = *(p+1);
                              p++;
                         }
                         *p = 32;
                         if (end > 0)
                              end--;
                         cursor(start_row,start_col);
                         cprintf(text);
                         cprintf(" ");
                         if ((key_pos > 0) && (key_pos == end))
                              key_pos--;
                         cursor(start_row,start_col + key_pos);
                    }
               }
               else
               {
                    key = key & 0xFF;
                    if (key == 13 || key == 27)
                         break;
                    else
                    if ((key == 8) && (key_pos > 0))
                    {
                         end--;
                         key_pos--;
                         text[key_pos--] = '\0';
                         strcpy(temp,text);
                         p = text + key_pos + 2;
                         strcat(temp,p);
                         strcpy(text,temp);
                         cursor(start_row,start_col);
                         cprintf("%-*.*s",length,length,text);
                         key_pos++;
                         cursor(start_row,start_col + key_pos);
                    }
                    else
                    if ((key > 31) && (key_pos < length)  &&
                       (start_col + key_pos < 80))
                    {
                         if (key_pos <= end)
                         {
                              p = text + key_pos;
                              memmove(p+1,p,end - key_pos);
                              if (end < length)
                                   end++;
                              text[end] = '\0';
                         }
                         text[key_pos++] = (char)key;
                         if (key_pos > end)
                         {
                              end++;
                              text[end] = '\0';
                         }
                         cursor(start_row,start_col);
                         cprintf("%-*.*s",length,length,text);
                         cursor(start_row,start_col + key_pos);
                    }
               }
          }
          text[end] = '\0';
          return key;
     }
     
     void FATAL(char *error)
     {
          /* A fatal error has occured */
     
          printf("\nFATAL ERROR: %s",error);
          exit(0);
     }
     
     void OPENDATA()
     {
          /* Check for existence of data file and if not create it */
          /* otherwise open it for reading/writing at end of file */
     
          handle = open(fpath,O_RDWR,S_IWRITE);
     
          if (handle == -1)
          {
               handle = open(fpath,O_RDWR|O_CREAT,S_IWRITE);
               if (handle == -1)
                    FATAL("Unable to create data file");
          }
          /* Read in first rec */
          read(handle,&rec,recsize);
     }
     
     void CLOSEDATA()
     {
          close(handle);
     }
     
     void GETDATA(int start)
     {
          /* Get address data from operator */
     
          textcolor(BLACK);
          textbackground(GREEN);
          gotoxy(left,8);
          print("Name ");
          gotoxy(left,9);
          print("Company ");
          gotoxy(left,10);
          print("Address ");
          gotoxy(left,11);
          print("Area ");
          gotoxy(left,12);
          print("Town ");
          gotoxy(left,13);
          print("County ");
          gotoxy(left,14);
          print("Post Code ");
          gotoxy(left,15);
          print("Telephone ");
          gotoxy(left,16);
          print("Fax ");
     
          switch(start)
          {
               case 0: gotoxy(left + 10,8);
                    if(INPUT(rec.name,30) == 27)
                         break;
               case 1: gotoxy(left + 10,9);
                    if(INPUT(rec.company,30) == 27)
                         break;
               case 2: gotoxy(left + 10,10);
                    if(INPUT(rec.address,30) == 27)
                         break;
               case 3: gotoxy(left + 10,11);
                    if(INPUT(rec.area,30) == 27)
                         break;
               case 4: gotoxy(left + 10,12);
                    if(INPUT(rec.town,30) == 27)
                         break;
               case 5: gotoxy(left + 10,13);
                    if(INPUT(rec.county,30) == 27)
                         break;
               case 6: gotoxy(left + 10,14);
                    if(INPUT(rec.post,12) == 27)
                         break;
               case 7: gotoxy(left + 10,15);
                    if(INPUT(rec.telephone,15) == 27)
                         break;
               case 8: gotoxy(left + 10,16);
                    INPUT(rec.fax,15);
                    break;
          }
          textcolor(WHITE);
          textbackground(RED);
          gotoxy(left + 23,21);
          print("                                                 ");
     }
     
     void DISPDATA()
     {
          /* Display address data */
          textcolor(BLACK);
          textbackground(GREEN);
          cursor(7,3);
          cprintf("Name    %-30.30s",rec.name);
          cursor(8,3);
          cprintf("Company   %-30.30s",rec.company);
          cursor(9,3);
          cprintf("Address   %-30.30s",rec.address);
          cursor(10,3);
          cprintf("Area    %-30.30s",rec.area);
          cursor(11,3);
          cprintf("Town    %-30.30s",rec.town);
          cursor(12,3);
          cprintf("County     %-30.30s",rec.county);
          cursor(13,3);
          cprintf("Post Code %-30.30s",rec.post);
          cursor(14,3);
          cprintf("Telephone %-30.30s",rec.telephone);
          cursor(15,3);
          cprintf("Fax      %-30.30s",rec.fax);
     }
     
     int LOCATE(char *text)
     {
          int result;
     
          do
          {
               /* Read rec into memory */
               result = read(handle,&rec,recsize);
               if (result > 0)
               {
                    /* Scan rec for matching data */
                    if (strstr(strupr(rec.name),text) != NULL)
                         return(1);
                    if (strstr(strupr(rec.company),text) != NULL)
                         return(1);
                    if (strstr(strupr(rec.address),text) != NULL)
                         return(1);
                    if (strstr(strupr(rec.area),text) != NULL)
                         return(1);
                    if (strstr(strupr(rec.town),text) != NULL)
                         return(1);
                    if (strstr(strupr(rec.county),text) != NULL)
                         return(1);
                    if (strstr(strupr(rec.post),text) != NULL)
                         return(1);
                    if (strstr(strupr(rec.telephone),text) != NULL)
                         return(1);
                    if (strstr(strupr(rec.fax),text) != NULL)
                         return(1);
               }
          }
          while(result > 0);
          return(0);
     }
     
     void SEARCH()
     {
          int result;
     
          gotoxy(left,21);
          textcolor(WHITE);
          textbackground(RED);
          cprintf("Enter data to search for ");
          strcpy(stext,"");
          INPUT(stext,30);
          if (*stext == 0)
          {
               gotoxy(left,21);
               cprintf("%70c",32);
               return;
          }
          gotoxy(left,21);
          textcolor(WHITE);
          textbackground(RED);
          cprintf("Searching for %s Please Wait....",stext);
          strupr(stext);
          /* Locate start of file */
          lseek(handle,0,SEEK_SET);
          result = LOCATE(stext);
          if (result == 0)
          {
               gotoxy(left,21);
               cprintf("%70c",32);
               gotoxy(left + 27,21);
               cprintf("NO MATCHING RECORDS");
               gotoxy(left + 24,22);
               cprintf("Press RETURN to Continue");
               bioskey(0);
               gotoxy(left,21);
               cprintf("%70c",32);
               gotoxy(left,22);
               cprintf("%70c",32);
          }
          else
          {
               lseek(handle,0 - recsize,SEEK_CUR);
               read(handle,&rec,recsize);
               DISPDATA();
          }
          textcolor(WHITE);
          textbackground(RED);
          gotoxy(left,21);
          cprintf("%70c",32);
          textcolor(BLACK);
          textbackground(GREEN);
     }
     
     void CONTINUE()
     {
          int result;
          long curpos;
     
          curpos = tell(handle) - recsize;
     
          result = LOCATE(stext);
          textcolor(WHITE);
          textbackground(RED);
          if (result == 0)
          {
               gotoxy(left + 24,21);
               cprintf("NO MORE MATCHING RECORDS");
               gotoxy(left + 24,22);
               cprintf("Press RETURN to Continue");
               bioskey(0);
               gotoxy(left,21);
               cprintf("%70c",32);
               gotoxy(left,22);
               cprintf("%70c",32);
               lseek(handle,curpos,SEEK_SET);
               read(handle,&rec,recsize);
               DISPDATA();
          }
          else
          {
               lseek(handle,0 - recsize,SEEK_CUR);
               read(handle,&rec,recsize);
               DISPDATA();
          }
          textcolor(WHITE);
          textbackground(RED);
          gotoxy(left,21);
          cprintf("%70c",32);
          gotoxy(left,22);
          cprintf("                                           ");
          textcolor(BLACK);
          textbackground(GREEN);
     }
     
     void PRINT_MULTI()
     {
          data buffer;
          char destination[60];
          char text[5];
          int result;
          int ok;
          int ok2;
          int blanks;
          int total_lines;
          char *p;
          FILE *fp;
     
          textcolor(WHITE);
          textbackground(RED);
          gotoxy(left + 23,21);
          cprintf("Enter selection criteria");
     
          /* Clear existing rec details */
          memset(&rec,0,recsize);
     
          DISPDATA();
          GETDATA(0);
     
          textcolor(WHITE);
          textbackground(RED);
          gotoxy(left,21);
          cprintf("Enter report destination PRN");
          strcpy(destination,"PRN");
          gotoxy(left,22);
          cprintf("Enter Address length in lines 18");
          strcpy(text,"18");
          gotoxy(left + 25,21);
          INPUT(destination,40);
          gotoxy(left +30,22);
          INPUT(text,2);
          gotoxy(left,21);
          cprintf("%72c",32);
          gotoxy(left,22);
          cprintf("%72c",32);
     
          total_lines = atoi(text) - 6;
          if (total_lines < 0)
               total_lines = 0;
     
          fp = fopen(destination,"w+");
          if (fp == NULL)
          {
               gotoxy(left,21);
               cprintf("Unable to print to %s",destination);
               gotoxy(left,22);
               cprintf("Press RETURN to Continue");
               bioskey(0);
               gotoxy(left,21);
               cprintf("%78c",32);
               gotoxy(left,22);
               cprintf("                          ");
          }
     
          /* Locate start of file */
          lseek(handle,0,SEEK_SET);
     
          do
          {
               /* Read rec into memory */
               result = read(handle,&buffer,recsize);
               if (result > 0)
               {
                    ok = 1;
                    /* Scan rec for matching data */
                    if (*rec.name)
                         if (stricmp(buffer.name,rec.name))
                              ok = 0;
                    if (*rec.company)
                         if (stricmp(buffer.company,rec.company))
                              ok = 0;
                    if (*rec.address)
                         if (stricmp(buffer.address,rec.address))
                              ok = 0;
                    if (*rec.area)
                         if (stricmp(buffer.area,rec.area))
                              ok = 0;
                    if (*rec.town)
                         if (stricmp(buffer.town,rec.town))
                              ok = 0;
                    if (*rec.county)
                         if (stricmp(buffer.county,rec.county))
                              ok = 0;
                    if (*rec.post)
                         if (stricmp(buffer.post,rec.post))
                         ok = 0;
                    if (*rec.telephone)
                         if (stricmp(buffer.telephone,rec.telephone))
                              ok = 0;
                    if (*rec.fax)
                         if (stricmp(buffer.fax,rec.fax))
                              ok = 0;
                    if (ok)
                    {
                         blanks = total_lines;
                         p = buffer.name;
                         ok2 = 0;
                         while(*p)
                         {
                              if (*p != 32)
                              {
                                   ok2 = 1;
                                   break;
                              }
                              p++;
                         }
                         if (!ok2)
                              blanks++;
                         else
                              fprintf(fp,"%s\n",buffer.name);
                         p = buffer.company;
                         ok2 = 0;
                         while(*p)
                         {
                              if (*p != 32)
                              {
                                   ok2 = 1;
                                   break;
                              }
                              p++;
                         }
                         if (!ok2)
                              blanks++;
                         else
                              fprintf(fp,"%s\n",buffer.company);
                         p = buffer.address;
                         ok2 = 0;
     
                         while(*p)
                         {
                              if (*p != 32)
                              {
                                   ok2 = 1;
                                   break;
                              }
                              p++;
                         }
                         if (!ok2)
                              blanks++;
                         else
                              fprintf(fp,"%s\n",buffer.address);
                         p = buffer.area;
                         ok2 = 0;
                         while(*p)
                         {
                              if (*p != 32)
                              {
                                   ok2 = 1;
                                   break;
                              }
                              p++;
                         }
                         if (!ok2)
                              blanks++;
                         else
                              fprintf(fp,"%s\n",buffer.area);
                         p = buffer.town;
                         ok2 = 0;
                         while(*p)
                         {
                              if (*p != 32)
                              {
                                   ok2 = 1;
                                   break;
                              }
                              p++;
                         }
                         if (!ok2)
                              blanks++;
                         else
                              fprintf(fp,"%s\n",buffer.town);
                         p = buffer.county;
                         ok2 = 0;
     
                         while(*p)
                         {
                              if (*p != 32)
                              {
                                   ok2 = 1;
                                   break;
                              }
                              p++;
                         }
                         if (!ok2)
                              blanks++;
                         else
                              fprintf(fp,"%s\n",buffer.county);
                         p = buffer.post;
                         ok2 = 0;
                         while(*p)
                         {
                              if (*p != 32)
                              {
                                   ok2 = 1;
                                   break;
                              }
                              p++;
                         }
                         if (!ok2)
                              blanks++;
                         else
                              fprintf(fp,"%s\n",buffer.post);
                         while(blanks)
                         {
                              fprintf(fp,"\n");
                              blanks--;
                         }
                    }
               }
          }
          while(result > 0);
          fclose(fp);
          lseek(handle,0,SEEK_SET);
          read(handle,&rec,recsize);
          DISPDATA();
     }
     
     void EXPORT_MULTI()
     {
          data buffer;
          char destination[60];
          int result;
          int ok;
          FILE *fp;
     
          textcolor(WHITE);
          textbackground(RED);
          gotoxy(left + 23,21);
          cprintf("Enter selection criteria");
     
          /* Clear existing rec details */
          memset(&rec,0,recsize);
     
          DISPDATA();
          GETDATA(0);
     
          textcolor(WHITE);
          textbackground(RED);
          gotoxy(left,21);
          cprintf("Enter export file address.txt");
          strcpy(destination,"address.txt");
          gotoxy(left + 18,21);
          INPUT(destination,59);
          gotoxy(left,21);
          cprintf("%70c",32);
     
          fp = fopen(destination,"w+");
          if (fp == NULL)
          {
               gotoxy(left,21);
               cprintf("Unable to print to %s",destination);
               gotoxy(left,22);
               cprintf("Press RETURN to Continue");
               bioskey(0);
               gotoxy(left,21);
               cprintf("%78c",32);
               gotoxy(left,22);
               cprintf("                          ");
          }
          /* Locate start of file */
          lseek(handle,0,SEEK_SET);
     
          do
          {
               /* Read rec into memory */
               result = read(handle,&buffer,recsize);
               if (result > 0)
               {
                    ok = 1;
                    /* Scan rec for matching data */
                    if (*rec.name)
                         if (stricmp(buffer.name,rec.name))
                              ok = 0;
                    if (*rec.company)
                         if (stricmp(buffer.company,rec.company))
                              ok = 0;
                    if (*rec.address)
                         if (stricmp(buffer.address,rec.address))
                              ok = 0;
                    if (*rec.area)
                         if (stricmp(buffer.area,rec.area))
                              ok = 0;
                    if (*rec.town)
                         if (stricmp(buffer.town,rec.town))
                              ok = 0;
                    if (*rec.county)
                         if (stricmp(buffer.county,rec.county))
                              ok = 0;
                    if (*rec.post)
                         if (stricmp(buffer.post,rec.post))
                         ok = 0;
                    if (*rec.telephone)
                         if (stricmp(buffer.telephone,rec.telephone))
                              ok = 0;
                    if (*rec.fax)
                         if (stricmp(buffer.fax,rec.fax))
                              ok = 0;
                    if (ok)
                    {
                         fprintf(fp,"\"%s\",",buffer.name);
                         fprintf(fp,"\"%s\",",buffer.company);
                         fprintf(fp,"\"%s\",",buffer.address);
                         fprintf(fp,"\"%s\",",buffer.area);
                         fprintf(fp,"\"%s\",",buffer.town);
                         fprintf(fp,"\"%s\",",buffer.county);
                         fprintf(fp,"\"%s\",",buffer.post);
                         fprintf(fp,"\"%s\",",buffer.telephone);
                         fprintf(fp,"\"%s\"\n",buffer.fax);
     
                    }
               }
          }
     
          while(result > 0);
          fclose(fp);
          lseek(handle,0,SEEK_SET);
          read(handle,&rec,recsize);
          DISPDATA();
     }
     
     void MENU()
     {
          int option;
          long result;
          long end;
          int new;
     
          do
          {
               cursor(21,26);
               print("Select option (F2 - F10)");
               cursor(7,52);
               print("F2 Next record");
               cursor(8,52);
               print("F3 Previous record");
               cursor(9,52);
               print("F4 Amend record");
               cursor(10,52);
               print("F5 Add new record");
               cursor(11,52);
               print("F6 Search");
               cursor(12,52);
               print("F7 Continue search");
               cursor(13,52);
               print("F8 Print address labels");
               cursor(14,52);
               print("F9 Export records");
               cursor(15,52);
               print("F10 Exit");
               MOUSE_CURSOR(1);
               option = GETOPT();
               MOUSE_CURSOR(0);
     
               switch(option)
               {
                    case 0 : /* Next rec */
                          result = read(handle,&rec,recsize);
                          if (!result)
                          {
                              lseek(handle,0,SEEK_SET);
                               result = read(handle,&rec,recsize);
                          }
                          DISPDATA();
                          break;
     
                    case 1 : /* Previous rec */
                         result = lseek(handle,0 - recsize * 2,SEEK_CUR);
                         if (result <= -1)
                              lseek(handle,0 - recsize,SEEK_END);
                         result = read(handle,&rec,recsize);
                         DISPDATA();
                         break;

                    case 3 : /* Add rec */
                          lseek(handle,0,SEEK_END);
                          memset(&rec,0,recsize);
                          DISPDATA();
     
                    case 2 : /* Amend current rec */
                          new = 1;
                          if (*rec.name)
                              new = 0;
                          else
                          if (*rec.company)
                              new = 0;
                          else
                          if (*rec.address)
                              new = 0;
                          else
                          if (*rec.area)
                              new = 0;
                          else
                          if (*rec.town)
                              new = 0;
                          else
                          if (*rec.county)
                              new = 0;
                          else
                          if (*rec.post)
                              new = 0;
                          else
                          if (*rec.telephone)
                              new = 0;
                          else
                          if (*rec.fax)
                              new = 0;
                          result = tell(handle);
                          lseek(handle,0,SEEK_END);
                          end = tell(handle);
     
                          /* Back to original position */
                          lseek(handle,result,SEEK_SET);
     
                          /* If not at end of file, && !new rewind one
rec */
                          if (result != end || ! new)
                              result = lseek(handle,0 -
recsize,SEEK_CUR);
                          result = tell(handle);
                          gotoxy(left + 22,21);
                          print(" Enter address details  ");
                          GETDATA(0);
                          if (*rec.name || *rec.company)
                              result =  write(handle,&rec,recsize);
                          break;

                    case 4 : /* Search */
                          gotoxy(left + 22,21);
                          print("                           ");
                          SEARCH();
                          break;
     
                    case 5 : /* Continue */
                          gotoxy(left + 22,21);
                          print("                           ");
                          CONTINUE();
                          break;
     
                    case 6 : /* Print */
                          gotoxy(left + 22,21);
                          print("                           ");
                          PRINT_MULTI();
                          break;

                    case 7 : /* Export */
                          gotoxy(left + 22,21);
                          print("                           ");
                          EXPORT_MULTI();
                          break;
     
                    case 8 : /* Exit */
                          break;
     
                    default: /* Amend current rec */
                          new = 1;
                          if (*rec.name)
                              new = 0;
                          else
                          if (*rec.company)
                              new = 0;
                          else
                          if (*rec.address)
                              new = 0;
                          else
                          if (*rec.area)
                              new = 0;
                          else
                          if (*rec.town)
                              new = 0;
                          else
                          if (*rec.county)
                              new = 0;
                          else
                          if (*rec.post)
                              new = 0;
                          else
                          if (*rec.telephone)
                              new = 0;
                          else
                          if (*rec.fax)
                              new = 0;
                          result = tell(handle);
                          lseek(handle,0,SEEK_END);
                          end = tell(handle);

                          /* Back to original position */
                          lseek(handle,result,SEEK_SET);

                          /* If not at end of file, && !new rewind one
rec */
                          if (result != end || ! new)
                              result = lseek(handle,0 -
recsize,SEEK_CUR);
                          result = tell(handle);
                          gotoxy(left + 22,21);
                          print(" Enter address details  ");
                          GETDATA(option - 17);
                          if (*rec.name || *rec.company)
                              result = write(handle,&rec,recsize);
                          option = -1;
                          break;

               }
          }
     
          while(option != 8);
     }
     
     void exec()
     {
          gettext(1,1,80,25,scr);
          setvideo(3);
          textbackground(WHITE);
          textcolor(BLACK);
          clrscr();
          recsize = sizeof(data);
     
          OPENDATA();
     
          TRUESHADE(left,3,79,5);
          window(left - 2,2 ,78, 4);
          textcolor(YELLOW);
          textbackground(MAGENTA);
          clrscr();
          DBOX(left - 3, 1, 77, 3);
          gotoxy(3,2);
          print("Servile Software             PC ADDRESS BOOK 5.2
     (c) 1994");
     
          TRUESHADE(left,8,left + 43,18);
          window(left - 2,7 , left + 42, 17);
          textcolor(BLACK);
          textbackground(GREEN);
          clrscr();
          DBOX(left - 3, 6, left + 41, 16);
     
          TRUESHADE(left + 48,8,79,18);
          window(left + 46, 7 , 78, 17);
          textbackground(BLUE);
          textcolor(YELLOW);
          clrscr();
          DBOX(left + 45,6,77,16);
     
          TRUESHADE(left ,21,79,24);
          window(left - 2, 20 , 78, 23);
          textbackground(RED);
          textcolor(WHITE);
          clrscr();
          DBOX(left - 3,19,77,22);
     
          window(1,1,80,25);
          textcolor(BLACK);
          textbackground(GREEN);
          DISPDATA();
     
          MENU();
     
          CLOSEDATA();
          puttext(1,1,80,25,scr);
          return;
     }
     
                                    
                       INTERFACING C WITH CLIPPER

The Clipper programming language is a popular xBase environment for the
PC.  However, it lacks many of the facilities available to programmers of
other languages, and it is quite slow compared to C. Because of this
there are a large number of third party add-on libraries available for
Clipper which provide the facilities lacked.

As a programmer you probably want to write your own library for Clipper,
or perhaps individual functions to cater for circumstances which Clipper
cannot handle, such as high resolution graphics.

Throughout this section, Clipper refers to the Summer `87 Clipper
compiler, although initial tests show that the functions described here
work perfectly well with the new Clipper 5 compiler also, we are not in a
position to guarrantee success!


COMPILING AND LINKING
The  Clipper extend functions allow user defined functions to be written
in C, linked with and used by the Clipper application. The  first
problem a programmer must address when writing functions in C  to link
with  a  Clipper  application is that of the  C  compiler's  run  time
libraries.

If one is writing functions with Microsoft C,  then most of the required
run time  library  functions  will be found in the  Clipper.lib  and
Extend.lib libraries which are part of Clipper.

If,  however, one is using a different C compiler, such as Borland's
Turbo C then the run time library routines must be supplied on the link
line.

All C functions must be compiled using the large memory model the
following line is used with Microsoft C


     cl /c /AL /Zl /Oalt /FPa /Gs <program.c>

and this compile line may be used with Turbo C

     tcc -c -ml <program>

simply substitute <program> for the program name to be compiled.

Having compiled a C function it must be linked in with the application.
If the C function was compiled with Microsoft C then the link line will
look a little like this;


     LINK /SE:500 /NOE program.obj cfunc.obj,,,Clipper Extend

If  the C function was linked with another C compiler you will also need
to link in the C run time libraries,  for example to link in the Turbo C
large memory mode library use the following link line;


     LINK /SE:500 /NOE program.obj cfunc.obj,,,Clipper Extend cl

If one is using a number of separately compiled C functions it is a good
idea to  collect  them in a library.  If you are using Microsoft C then
you  can simply  create  the  library by using Microsoft Lib.exe with
the  following command line;


      LIB mylib +prog1 +prog2, NUL, NUL

This tells the librarian to add prog1.obj and prog2.obj to a library
called mylib.lib,   creating  it  if it does not exist.  The NUL
parameter  is  for supressing the listing file.


If you have been using another C compiler you should copy the C large
memory model run time library before adding your functions to it for
example;


      COPY C:\TURBOC\LIB\cl.lib mylib.lib
      LIB mylib +prog1 +prog2, NUL, NUL

Then when you link your Clipper application you will use a link line
similar to;

     LINK /SE:500 /NOE myprog,,,Clipper Extend Mylib

Often  when  linking C functions with  Clipper applications link errors
will occur such as those shown below;

     
     Microsoft (R) Overlay Linker  Version 3.65
     Copyright (C) Microsoft Corp 1983-1988.  All rights reserved.
     
     
     LINK : error L2029: Unresolved externals:
     
     
     FIWRQQ in file(s):
      M:SLIB.LIB(TEST)
     FIDRQQ in file(s):
      M:SLIB.LIB(TEST)
     
     There were 2 errors detected
     

Example Link Errors

The  errors  shown  here  are  `Unresolved externals',   that  is  they
are references to functions which are not found in any of the object
modules  or libraries  specified on the link line.  These occur because
the C  compilers often  scatter  functions and variables through a number
of  libraries.   In tracking  these  functions down  use may be made of
the Microsoft  librarian list file option.  If you run Lib.Exe on the
Turbo C `emu.lib'  library file and specify a listing file as follows;


     LIB emu,emu.lst

The  librarian  will create an ascii file which contains the names  of
each object module contained in the specified library file, and the names
of each function and public variable declared in each object module, as
shown in this listing of Borland's EMU.LIB library.


     e086_Entry........EMU086      e086_Shortcut.....EMU086
     e087_Entry........EMU087      e087_Shortcut.....EMU087
     FIARQQ............EMUINIT          FICRQQ............EMUINIT
     FIDRQQ............EMUINIT          FIERQQ............EMUINIT
     FISRQQ............EMUINIT          FIWRQQ............EMUINIT
     FJARQQ............EMUINIT          FJCRQQ............EMUINIT
     FJSRQQ............EMUINIT          __EMURESET........EMUINIT
     
     
     EMUINIT        Offset: 00000010H  Code and data size: 1a2H
     FIARQQ         FICRQQ              FIDRQQ              FIERQQ
     FISRQQ         FIWRQQ              FJARQQ              FJCRQQ
     FJSRQQ         __EMURESET
     
     EMU086         Offset: 00000470H  Code and data size: 2630H
       e086_Entry        e086_Shortcut
     
     EMU087         Offset: 00003200H  Code and data size: 417H
       e087_Entry        e087_Shortcut
     


Receiving Parameters

Clipper provides six different functions for receiving parameters in a C
function. These functions are;


  Receive a string      char * _parc(int,[int])
  Receive a Date string char * _pards(int,[int])
  Receive a logical     int _parl(int,[int])
  Receive an integer         int _parni(int,[int])
  Receive a long        long _parnl(int,[int])
  Receive a double      double _parnd(int,[int])



To  illustrate  simple  parameter  receiving in a C  function  I  offer
the following  simple C function which receives two numeric parameters
from  the calling  Clipper program,  and uses these two numeric
parameters to set  the size of the cursor.


     #include <nandef.h>            /* Clipper header files */
     #include <extend.h>
     #include <dos.h>                   /* Header file to define REGS */
     
     CLIPPER s_curset()
     {
          /* Demonstration function to set cursor shape */
     
          union REGS inreg,outreg;
     
          inreg.h.ah = 0x01;
          inreg.h.ch = _parni(1);   /* Get integer parameter 1 */
          inreg.h.cl = _parni(2);   /* Get integer parameter 2 */
          int86(0x10,&inreg,&outreg);
          _ret();                        /* Return to Clipper */
     }

Clipper  provides four more functions for dealing with received
parameters;

_parclen(int,[int])  which returns the length of a string including
imbedded `\0's,   _parcsiz(int[int])  which returns the length of a
character  string passed by reference from Clipper, _parinfa(int,[int])
which returns the type of  a  specified  array  element  or the length of
an  array,   and  finally _parinfo(int) whic returns the type of a
parameter.

The following example function uses _parinfa()  to determine both the
length of an array passed from a Clipper program,  and the type of each
element  in the array.  The function then returns to Clipper an integer
representing the number of defined elements in the array.



     #include <nandef.h>
     #include <extend.h>
     
     CLIPPER s_alen()
     {
          int total;
          int n;
          int defined;
          int type;
     
          /* Return the number of defined elements in an array */
          /* From Clipper use defined = s_alen(arr) */
     
          total = _parinfa(1,0); /* Get declared number of elements in
     array */
     
          defined = 0;
     
          for (n = 1; n <= total; n++){
               type = _parinfa(1,n);   /* Get array parameter type */
               if (type)
                    defined++;
          }
          _retni(defined);              /* Return an integer to Clipper
     */
     }
     

This  function  goes  one step further to return the  mean  average  of
all numeric  values  in an array.  Notice the use of _parnd()  to
retrieve  the numeric  values as doubles.  You may find that because of
the floating point arithmetic  in  this  function  that it will  only
work  if  compiled  with Microsoft C.


     #include <nandef.h>
     #include <extend.h>
     
     CLIPPER s_aave()
     {
          int total;
          int defined;
          int n;
          int type;
          double sum;
     
          /* Return the mean average value of numbers in array */
          /* From Clipper use mean = s_aave(arr)
     
     
          total = _parinfa(1,0);               /* Get declared number of
     elements */
     
          defined = 0;
     
          for (n = 1; n <= total; n++){    /* Determine number of defined
     */
               type = _parinfa(1,n);            /* elements */
               if (type == 2)
                    defined++;
          }
     
          sum = 0;
     
          for (n = 1; n <= total; n++){
               type = _parinfa(1,n);
               if (type == 2)                  /* Only sum numeric values
     */
                    sum += _parnd(1,n);
          }
          _retnd(sum / defined);               /* Return a double to
     Clipper */
     }
     
     

Returning Values

The  Clipper  manual  lists seven functions for returning  from  a
function written in another language. These return functions for C are as
follows;

  character        _retc(char *)
  date        _retds(char *)
  logical     _retl(int)
  numeric (int)    _retni(int)
  numeric (long)   _retnl(long)
  numeric  (double)     _retnd(double)
  nothing     _ret(void)

Omitted  from  the  Clipper manual is the information that  you  may
return different types of value back from a function! For example,  you
may wish to return a character string under normal circumstances,  fine
use _retc().  On error occurences however you can return a logical using
_retl(). The Clipper program  will  assign the received value to the
receiving  variable  in  the correct manner.

The following simple C function returns a random number.  Notice the use
of integers  which  limits  the range of the function to +-32767.   For
larger values you should use longs instead of integers.


     #include <nandef.h>
     #include <extend.h>
     #include <dos.h>
     
     CLIPPER s_random()
     {
          /* Returns a random number between 0 and param1 - 1 */
          /* From Clipper use x = s_random(param1) */
     
          int param1;
          int x;
     
          param1 = _parni(1);
     
          x = rand() % param1;
          _retni(x);
     }
     

This function receives a string from Clipper,  and passes back an upper
case copy of the string,  leaving the original unchanged.  The maximum
length  of the  string  which can be processed is determined by the size
of  target[], here set to 5000 characters.


     #include <nandef.h>
     #include <extend.h>
     
     CLIPPER s_upper()
     {
          /* Returns an upper case copy of string */
          /* From Clipper use ? s_upper("this is a string")
     
          char *p;
          char *q;
          char *string;
          char target[5000];
          int n;
     
          string = _parc(1);
     
          p = string;
          q = target;
     
          while(*string){
               *q++ = toupper(*string);
               string++;
          }
          *q = '\0';
          string = p;
          _retc(target);
     }
     
     
This  function  may be used to change the current DOS directory.  If  it
is successful  it  returns .T.  to the calling Clipper program,
otherwise  it returns .F.

     #include <nandef.h>
     #include <extend.h>
     #include <dos.h>
     
     CLIPPER s_chdir()
     {
          /* Attempts to change the current DOS directory */
          /* From Clipper use result = s_chdir(path) */
     
          union REGS inreg,outreg;
          struct SREGS segreg;
     
          char *path;
          int x;
     
          path = _parc(1);              /* Retrieve string from Clipper
     */
     
          inreg.h.ah = 0x3b;
          segreg.ds = FP_SEG(path);
          inreg.x.dx = FP_OFF(path);
          intdosx(&inreg,&outreg,&segreg);
     
          x = outreg.x.ax;
     
          if (x == 3)
               _retl(0);    /* Return logical .F. back to Clipper */
          else
               _retl(1);    /* Return logical .T. back to Clipper */
     }
     


Avoiding Unresolved Externals

As we have already seen, a common problem plaguing the programmer
interfacing C functions with Clipper programs is Unresolved Externals.

The  following  example  C function called s_print()   will  not  link
with Clipper.

     #include <nandef.h>
     #include <extend.h>
     #include <stdio.h>
     
     CLIPPER s_print()
     {
          char *x;
     
          x = _parc(1);
     
          printf("\nI received %s from Clipper.\n",x);
     
          _ret();
     }
     

The linker gives you the following reply;

     Microsoft (R) Overlay Linker  Version 3.65
     Copyright (C) Microsoft Corp 1983-1988.  All rights reserved.
     
     M:SLIB.LIB(IOERROR) : error L2025: __doserrno : symbol defined more
     than once
      pos: 16C6F Record type: 53C6
     
     LINK : error L2029: Unresolved externals:
     
     

     __RealCvtVector in file(s):
      M:SLIB.LIB(REALCVT)
     _abort in file(s):
      M:SLIB.LIB(CVTFAK)
     
     There were 3 errors detected
     

The error L2025 `symbol defined more than once' can in this case be
ignored.  However,   the unresolved externals `RealCvtVector'  and
`abort'  cannot  be ignored.  These two functions are referenced by the
function printf()  which has  been  included in the C function.  The
answer is to use as few  of  the compiler's  run time library functions
as possible,  use ROM  calls  instead with INT86() and INTDOSX() etc.


Adding High Resolution Graphics To Clipper With C

The most annoying omission from Clipper, in my opinion, is the lack of
high resolution graphics facilities. The following functions, written in
Turbo C, provide high resolution graphics to Clipper.

First we require a means to change the video display mode to a high
resolution graphics mode, and back to text mode. The IBM PC BIOS provides
the means for this and can be called from C as follows;




     /*         Servile Software Library For Clipper              */
     
     #include <nandef.h>
     #include <extend.h>
     #include <dos.h>
     
     CLIPPER s_smode()
     {
          /* Set Video Mode */
          /* From Clipper use s_smode(mode) */
     
          union REGS inreg,outreg;
     
          inreg.h.al = _parni(1);
          inreg.h.ah = 0x00;
          int86 (0x10, &inreg, &outreg);
     
     
     /*  1 40x25 colour text
          2 40x25 bw text
          3 80x25 colour text
          4 320x200 4 colour graphics
          5 320x200 4 colour graphics colour burst off
          6 640x200 2 colour graphics
          etc
     */
          _ret();
     }

Having set the computer into graphics mode, how about setting pixels to a
specified colour?

     
     /*         Servile Software Library For Clipper              */
     
     #include <nandef.h>
     #include <extend.h>
     #include <dos.h>
     
     CLIPPER s_plot()
     {
     
          union REGS inreg,outreg;
     
          /* Sets a pixel at the specified coordinates to the specified
     colour. */
     
          inreg.h.bh = 0x00;
          inreg.x.cx = _parni(1);
          inreg.x.dx = _parni(2);
          inreg.h.al = _parni(3);
          inreg.h.ah = 0x0C;
          int86(0x10, &inreg, &outreg);
     }
     
Line drawing and circles are handled by these two functions;

     
     /*         Servile Software Library For Clipper              */
     
     #include <nandef.h>
     #include <extend.h>
     #include <dos.h>
     
     CLIPPER s_line()
     {
     
          union REGS inreg,outreg;
     
          /* Draws a straight line from (a,b) to (c,d) in colour col */
     
          int a;
          int b;
          int c;
          int d;
          int col;
          int u;
          int v;
          int d1x;
          int d1y;
          int d2x;
          int d2y;
          int m;
          int n;
          int s;
          int i;
     
          a = _parni(1);
          b = _parni(2);
          c = _parni(3);
          d = _parni(4);
          col = _parni(5);
     
          u = c - a;
          v = d - b;
          if (u == 0)
          {
               d1x = 0;
               m = 0;
          }
          else
          {
               m = abs(u);
               if (u < 0)
                    d1x = -1;
               else
                    if (u > 0)
                         d1x = 1;
          }
          if ( v == 0)
          {
               d1y = 0;
               n = 0;
          }
          else
          {
               n = abs(v);
               if (v < 0)
                    d1y = -1;
               else
                    if (v > 0)
                         d1y = 1;
          }
          if (m > n)
          {
               d2x = d1x;
               d2y = 0;
          }
          else
          {
               d2x = 0;
               d2y = d1y;
               m = n;
               n = abs(u);
          }
          s = (m / 2);
     
          inreg.h.al = (unsigned char)col;
          inreg.h.bh = 0x00;
          inreg.h.ah = 0x0C;
          for (i = 0; i <= m; i++)
          {
               inreg.x.cx = (unsigned int)(a);
               inreg.x.dx = (unsigned int)(b);
               int86(0x10, &inreg, &outreg);
               s += n;
               if (s >= m)
               {
                    s -= m;
                    a += d1x;
                    b += d1y;
               }
               else
               {
                    a += d2x;
                    b += d2y;
               }
          }
     }
     
     

This circle drawing function uses in-line assembler to speed up the
drawing process. It can easily be replaced with inreg and outreg
parameters as in the other functions, or the other functions can be
changed to in-line assembler.  Both methods are shown to illustrate
different ways of achieving the same result.


     /*         Servile Software Library For Clipper              */
     
     #include <nandef.h>
     #include <extend.h>
     #include <dos.h>
     
     
     void plot(int x, int y, unsigned char colour)
     {
          asm mov al , colour;
          asm mov bh , 00;
          asm mov cx , x;
          asm mov dx , y;
          asm mov ah , 0Ch;
          asm int 10h;
     }
     
     int getmode()
     {
          /* Returns current video mode  and number of columns in ncols
     */
     
          asm mov ah , 0Fh;
          asm int 10h;
          return(_AL);
     }
     
     
     CLIPPER s_circle()
     {
          int x_centre;
          int y_centre;
          int radius;
          int colour;
          int x,y,delta;
          int startx,endx,x1,starty,endy,y1;
          int asp_ratio;
     
          x_centre = _parni(1);
          y_centre = _parni(2);
          radius = _parni(3);
          colour = _parni(4);
     
     
     
     
          if (getmode() == 6)
               asp_ratio = 22;
          else
               asp_ratio = 13;
     
          y = radius;
          delta = 3 - 2 * radius;
     
          for(x = 0; x < y; )
          {
               starty = y * asp_ratio / 10;
               endy = (y + 1) * asp_ratio / 10;
               startx = x * asp_ratio / 10;
               endx = (x + 1) * asp_ratio / 10;
     
               for(x1 = startx; x1 < endx; ++x1)
               {
                    plot(x1+x_centre,y+y_centre,colour);
                    plot(x1+x_centre,y_centre - y,colour);
                    plot(x_centre - x1,y_centre - y,colour);
                    plot(x_centre - x1,y + y_centre,colour);
               }
     
               for(y1 = starty; y1 < endy; ++y1)
               {
                    plot(y1+x_centre,x+y_centre,colour);
                    plot(y1+x_centre,y_centre - x,colour);
                    plot(x_centre - y1,y_centre - x,colour);
                    plot(x_centre - y1,x + y_centre,colour);
               }
     
               if (delta < 0)
                    delta += 4 * x + 6;
               else
               {
                    delta += 4*(x-y)+10;
                    y--;
               }
               x++;
          }
     
     
     
     
          if(y)
          {
               starty = y * asp_ratio / 10;
               endy = (y + 1) * asp_ratio / 10;
               startx = x * asp_ratio / 10;
               endx = (x + 1) * asp_ratio / 10;
               for(x1 = startx; x1 < endx; ++x1)
               {
                    plot(x1+x_centre,y+y_centre,colour);
                    plot(x1+x_centre,y_centre - y,colour);
                    plot(x_centre - x1,y_centre - y,colour);
                    plot(x_centre - x1,y + y_centre,colour);
               }
     
               for(y1 = starty; y1 < endy; ++y1)
               {
                    plot(y1+x_centre,x+y_centre,colour);
                    plot(y1+x_centre,y_centre - x,colour);
                    plot(x_centre - y1,y_centre - x,colour);
                    plot(x_centre - y1,x + y_centre,colour);
               }
          }
     }
     

The Clipper facilities for displaying text on the screen, @....SAY and ?
do not work when the monitor is in graphics mode. You then need the
following function to allow text to be displayed in a graphics mode;

     
     /*         Servile Software Library For Clipper              */
     
     #include <nandef.h>
     #include <extend.h>
     #include <dos.h>
     
     int sgetmode(int *ncols)
     {
          /* Returns current video mode  and number of columns in ncols
     */
     
          union REGS inreg,outreg;
     
          inreg.h.ah = 0x0F;
          int86(0x10, &inreg, &outreg);
          *ncols = outreg.h.ah;
          return(outreg.h.al);
     }
     
     void at(int row, int col)
     {
          asm mov bh , 0;
          asm mov dh , row;
          asm mov dl , col;
          asm mov ah , 02h;
          asm int 10h;
     }
     
     
     
     
     CLIPPER s_say()
     {
          char *output;
          int p = 0;
          unsigned char page;
          unsigned char text;
          int n;
          int r;
          int c;
          int attribute;
     
          output = _parc(1);
          r = _parni(2);
          c = _parni(3);
          attribute = _parni(4);
     
          asm mov ah , 0Fh;
          asm int 10h;
          asm mov page, bh;
     
          sgetmode(&n);
     
          at(r,c);
     
          while (output[p])
          {
               text = output[p++];
               asm mov bh , page;
               asm mov bl , attribute;
               asm mov cx , 01h;
               asm mov ah , 09h;
               asm mov al , text;
               asm int 10h;
               c++;
               if (c < (n-1))
                    at( r, c);
               else
               {
                    c = 0;
                    at(++r,0);
               }
          }
     }
     



When drawing graphs, it is often required to fill in areas of the graph
in different patterns. This is a graphics function to fill boundered
shapes with a specified hatching pattern providing a means to achieve
more usable graphs;


     
     /*         Servile Software Library For Clipper              */
     
     #include <nandef.h>
     #include <extend.h>
     #include <dos.h>
     
     int pixset(int x, int y)
     {
          /* Returns the colour of the specified pixel */
     
          asm mov cx ,x;
          asm mov dx ,y;
          asm mov ah ,0Dh;
          asm int 10h;
          return(_AL);
     }
     
     CLIPPER s_fill()
     {
          /* Fill a boundered shape using a hatch pattern */
     
          int mode;
          int xa;
          int ya;
          int bn;
          int byn;
          int x;
          int y;
          int col;
          int pattern;
          int maxx;
          int maxy;
          int hatch[10][8] = { 255,255,255,255,255,255,255,255,
                                   128,64,32,16,8,4,2,1,
                                   1,2,4,8,16,32,64,128,
                                   1,2,4,8,8,4,2,1,
                                   238,238,238,238,238,238,238,238,
                                   170,85,170,85,170,85,170,85,
                                   192,96,48,24,12,6,3,1,
                                   62,62,62,0,227,227,227,0,
                                   129,66,36,24,24,36,66,129,
                                   146,36,146,36,146,36,146,36};
     
          /* Patterns for fill, each integer describes a row of dots */
     
     
     
     
          x = _parni(1);
          y = _parni(2);
          col = _parni(3);
          pattern = _parni(4);
     
          mode = getmode();
     
          switch(mode)
          {
               case 0:
               case 1:
               case 2:
               case 3: break;
               case 4:
               case 9:
               case 13:
               case 19:
               case 5: maxx = 320;
                         maxy = 200;
                         break;
               case 14:
               case 10:
               case 6: maxx = 640;
                         maxy = 200;
                         break;
               case 7: maxx = 720;
                         maxy = 400;
                         break;
               case 8: maxx = 160;
                         maxy = 200;
                         break;
               case 15:
               case 16: maxx = 640;
                          maxy = 350;
                          break;
               case 17:
               case 18: maxx = 640;
                          maxy = 480;
                          break;
     
          }
     
          xa = x;
          ya = y;  /* Save Origin */
     
          if(pixset(x,y))
               return;
     
          bn = 1;
          byn = 0;
     
     
     
     
          do
          {
               if (hatch[pattern][byn] != 0)
               {  /* If blank ignore */
                    do
                    {
                         if ((bn & hatch[pattern][byn]) == bn)
                         {
                              asm mov al , col;
                              asm mov bh , 00;
                              asm mov cx , x;
                              asm mov dx , y;
                              asm mov ah , 0Ch;
                              asm int 10h;
                         }
                         x--;
                         bn <<= 1;
                         if (bn > 128)
                              bn = 1;
                    }
                    while(!pixset(x,y) && (x > -1));
     
                    x = xa + 1;
                    bn = 128;
     
                    do
                    {
                         if ((bn & hatch[pattern][byn]) == bn)
                         {
                              asm mov al , col;
                              asm mov bh , 00;
                              asm mov cx , x;
                              asm mov dx , y;
                              asm mov ah , 0Ch;
                              asm int 10h;
                         }
                         x++;
                         bn >>=1;
                         if (bn <1)
                              bn = 128;
                    }
                    while((!pixset(x,y)) && (x <= maxx));
               }
               x = xa;
               y--;
               bn = 1;
               byn++;
               if (byn > 7)
                    byn = 0;
     
     
     
     
          }
          while(!pixset(x,y) && ( y > -1));
     
          /* Now travel downwards */
     
          y = ya + 1;
     
          byn = 7;
          bn = 1;
          do
          {
               /* Travel left */
               if (hatch[pattern][byn] !=0)
               {
                    do
                    {
                         if ((bn & hatch[pattern][byn]) == bn)
                         {
                              asm mov al , col;
                              asm mov bh , 00;
                              asm mov cx , x;
                              asm mov dx , y;
                              asm mov ah , 0Ch;
                              asm int 10h;
                         }
                         x--;
                         bn <<= 1;
                         if (bn > 128)
                              bn = 1;
                    }
                    while(!pixset(x,y) && (x > -1));
     
                    /* Back to x origin */
                    x = xa + 1 ;
                    bn = 128;
     
                    /* Travel right */
                    do
                    {
                         if ((bn & hatch[pattern][byn]) == bn)
                         {
                              asm mov al , col;
                              asm mov bh , 00;
                              asm mov cx , x;
                              asm mov dx , y;
                              asm mov ah , 0Ch;
                              asm int 10h;
                         }
                         x++;
                         bn >>=1;
     
     
     
     
                         if (bn <1)
                              bn = 128;
                    }
                    while((!pixset(x,y)) && (x <= maxx));
               }
               x = xa;
               bn = 1;
               y++;
               byn--;
               if (byn < 0)
                    byn = 7;
          }
          while((!pixset(x,y)) && (y <= maxy));
     }
     
     
     
                                    
                       SPELL - AN EXAMPLE PROGRAM

It has been said that example programs provide a good way of learning a
new computer language. On that basis the following simple program is
offered as an example of making use of the dynamic memory allocation
provided by DOS.

This is a spell checker for ASCII (text) documents, written in, and
making use of Borland's Turbo C text graphics facilities for displaying
windows of text.


     /* Spell checker for ascii documents */
     /* Compile with -mc (compact memory model) and unsigned characters
     */
     
     
     #include <stdio.h>
     #include <conio.h>
     #include <fcntl.h>
     #include <io.h>
     #include <dos.h>
     #include <string.h>
     #include <alloc.h>
     #include <ctype.h>
     #include <stdlib.h>
     #include <bios.h>
     #include <dir.h>
     #include <stat.h>
     
     #define  ON           0x06
     #define  OFF         0x20
     #define  MAXIMUM     15000
     #define  WORDLEN     20
     #define  LEFTMARGIN  1
     
     union REGS inreg,outreg;
     
     char *dicname;
     char *dic[MAXIMUM];      /* Array of text lines */
     char word[31];
     char comp[31];
     char fname[160];
     int lastelem;
     char changed;
     char *ignore[100];
     int lastign;
     int insert;
     int n;
     int bp;
     int mp;
     int tp;
     int result;
     
     
     
     
     char *text;
     char *textsav;
     
     void AT(int, int);
     void BANNER(void);
     int COMPARE(void);
     void CORRECT(void);
     void FATAL(char *);
     void FILERR(char *);
     void GETDIC(void);
     void IGNORE(void);
     void INSERT(void);
     int MATCHSTR(char *, char *);
     void SPELL(void);
     void UPDATE(void);
     
     void CURSOR(char status)
     {
          /* Toggle cursor display on and off */
     
          union REGS inreg,outreg;
     
          inreg.h.ah = 1;
          inreg.h.ch = (unsigned char)status;
          inreg.h.cl = 7;
          int86(0x10,&inreg,&outreg);
     }
     
     void DISPLAY(char *text)
     {
          /* Display 'text' expanding tabs and newline characters */
     
          while(*text)
          {
               switch(*text)
               {
                    case '\n':  cputs("\r\n");
                                   break;
                    case '\t':  cputs("      ");
                                   break;
                    default:  putch(*text);
               }
               text++;
          }
     }
     
     
     
     
     void GETDIC()
     {
          /* Read dictionary into memory */
     
          FILE *fp;
          char *p;
          int poscr;
          int handle;
     
          window(1,22,80,24);
          clrscr();
          gotoxy(28,2);
          cprintf("Reading Dictionary....");
     
          changed = 0;
          lastelem = 0;
     
          dicname = searchpath("spell.dic");
          handle = open(dicname,O_RDWR);
          if (handle < 0)
               FILERR("spell.dic");
     
          fp = fdopen(handle,"r");
          if (fp == NULL)
               FILERR("spell.dic");
     
          do
          {
               dic[lastelem] = calloc(WORDLEN,1);
               if (dic[lastelem])
               {
                    p = fgets(dic[lastelem],79,fp);
                    /* Remove carriage return from end of text line */
                    poscr = (int)strlen(dic[lastelem]) - 1;
                    if (dic[lastelem][poscr] == '\n')
                         dic[lastelem][poscr] = 0;
               }
               else
                    FATAL("Unable To Allocate Memory");
          }
          while((p != NULL) && (lastelem++ < MAXIMUM));
     
          lastelem--;
     
          fclose(fp);
     }
     
     
     
     
     void UPDATE()
     {
          FILE *fp;
          int n;
     
          if (changed)
          {
               window(1,22,80,24);
               clrscr();
               gotoxy(27,2);
               cprintf("Updating Dictionary....");
     
               fp = fopen(dicname,"w+");
               if (fp == NULL)
                    FILERR("spell.dic");
     
               for(n = 0; n <= lastelem; n++)
                    fprintf(fp,"%s\n",dic[n]);
     
               fclose(fp);
          }
     }
     
     void IGNORE()
     {
          /* Add a word to the ignore table */
     
          if (lastign < 100)
          {
               ignore[lastign] = calloc(strlen(word) + 1,1);
               if (ignore[lastign])
                    strcpy(ignore[lastign++],comp);
               else
               {
                    clrscr();
                    cprintf("No available memory for new words!\r\nPress
     A key....");
                    bioskey(0);
               }
           }
           else
           {
               clrscr();
               cprintf("No available memory for new words!\r\nPress A
     key....");
               bioskey(0);
           }
     }
     
     
     
     
     void FATAL(char *text)
     {
          /* Fatal error drop out */
     
          textcolor(LIGHTGRAY);
          textbackground(BLACK);
          window(1,1,80,25);
          clrscr();
          printf("SERVILE SOFTWARE\n\nSPELL V1.7\nFATAL ERROR:
     %s\n\n",text);
          CURSOR(ON);
          exit(0);
     }
     
     void FILERR(char *fname)
     {
          char text[60];
     
          strcpy(text,"Unable To Access: ");
          strcat(text,fname);
          FATAL(text);
     }
     
     int COMPARE()
     {
          char **p;
     
          /* Check Ignore table */
          for(p = ignore; p <= &ignore[lastign]; p++)
               if (strcmp(comp,*p) == 0)
                    return(1);
     
          /* Binary search of dictionary file */
          bp = 0;
          tp = lastelem;
          mp = (tp + bp) / 2;
     
     
     
     
          while((result = strcmp(dic[mp],comp)) != 0)
          {
               if (mp >= tp)
               {
                    /* Not found! */
                    insert = mp;
                    if (result > 0)
                         insert--;
                    return(0);
               }
               if (result < 0)
                    bp = mp + 1;
               else
                    tp = mp - 1;
     
               mp = (bp + tp) / 2;
          }
          return(1);
     }
     
     void INSERT()
     {
          int n;
     
          changed = 1;
          lastelem++;
          n = lastelem;
     
          dic[n] = calloc(WORDLEN,1);
     
          if (dic[n] == NULL)
          {
               clrscr();
               cprintf("No available memory for new words!\r\nPress A
     key....");
               bioskey(0);
               free(dic[n]);
               lastelem--;
               return;
          }
     
          while(n > (insert + 1))
          {
               strcpy(dic[n],dic[n-1]);
               n--;
          };
     
          strcpy(dic[insert + 1],comp);
     }
     
     
     
     
     void SPELL()
     {
          FILE *target;
          FILE *source;
          char *p;
          char *x;
          char temp[256];
          char dat1[1250];
          char dat2[1250];
          int c;
          int m;
          int found;
          int curpos;
          int key;
          int row;
          int col;
          int srow;
          int scol;
     
          window(1,1,80,20);
          textcolor(BLACK);
          textbackground(WHITE);
     
          /* Open temporary file to take spell checked copy */
          target = fopen("spell.$$$","w+");
     
          source = fopen(fname,"r");
     
          if (source == NULL)
               FILERR(fname);
     
          lastign = 0;
     
          do
          {
               clrscr();
     
               text = dat1;
     
               p = text;
     
               textsav = dat2;
     
               strcpy(text,"");
     
               /* Display read text */
               row = wherey();
               col = wherex();
     
     
     
     
               for(m = 0; m < 15; m++)
               {
                    x = fgets(temp,200,source);
                    if (x)
                    {
                         strcat(text,temp);
                         DISPLAY(temp);
                    }
                    if (wherey() > 18)
                         break;
               }
     
               /* return cursor to start position */
               gotoxy(col,row);
     
               do
               {
                    memset(word,32,30);
                    curpos = 0;
                    do
                    {
                         c = *text++;
                         if ((isalpha(c)) || (c == '-') && (curpos != 0))
                              word[curpos++] = c;
                    }
                    while(((isalpha(c)) || (c == '-') && (curpos != 0))
                            && (curpos < 30));
                    word[curpos] = 0;
                    strcpy(comp,word);
                    strupr(comp);
     
                    if (*comp != 0)
                    {
                         found = COMPARE();
                         if (!found){
                              textbackground(RED);
                              textcolor(WHITE);
                         }
                    }
                    else
                         found = 1;
     
                    srow = wherey();
                    scol = wherex();
     
                    cputs(word);
                    textbackground(WHITE);
                    textcolor(BLACK);
     
     
     
     
     
                    switch(c)
                    {
                         case '\n': cputs("\r\n");
                                      break;
                         case '\t': cputs("       ");
                                      break;
                         default: putch(c);
                    }
     
                    row = wherey();
                    col = wherex();
     
                    if (!found)
                    {
                         window(1,22,80,24);
                         clrscr();
                         cputs("Unknown word ");
                         textcolor(BLUE);
                         cprintf("%s ",word);
                         textcolor(BLACK);
                         cputs("[A]dd  [I]gnore  [C]orrect  [S]kip");
                         do
                         {
                              key = toupper(getch());
                              if (key == 27)
                                   key = 'Q';
                         }
                         while(strchr("AICSQ",key) == NULL);
     
                         switch(key)
                         {
                              case 'A':INSERT();
                                         break;
     
                              case 'C':CORRECT();
                                         break;
     
                              case 'I':IGNORE();
                                         break;
                         }
     
     
     
     
                         if (key == 'C')
                         {
                              clrscr();
                              gotoxy(1,1);
                              strcpy(textsav,--text);
                              /* Delete old word */
                              text -= strlen(comp);
                              *text = 0;
                              /* Insert new word */
                              strcat(text,word);
                              /* Append remainder of text */
                              strcat(text,textsav);
                              text += strlen(word);
                              text++;
                              /* Length of text may have changed ! */
                              if (strlen(word) < strlen(comp))
                                   col -= (strlen(comp) - strlen(word));
                              window(1,1,80,20);
                              clrscr();
                              DISPLAY(p);
                         }
                         else
                         {
                              clrscr();
                              gotoxy(29,2);
                              cputs("Checking Spelling....");
                              window(1,1,80,20);
                              gotoxy(scol,srow);
                              cputs(word);
                         }
                         window(1,1,80,20);
                         gotoxy(col,row);
                    }
               }
               while((*text) && (key != 'Q'));
               fprintf(target,"%s",p);
          }
          while((x != NULL) && (key != 'Q'));
     
          window(1,22,80,24);
          clrscr();
          gotoxy(27,2);
          cprintf("Writing Updated File....");
     
     
     
     
          do
          {
               p = fgets(temp,200,source);
               if (p)
                    fprintf(target,"%s",temp);
          }
          while(p);
     
          fclose(target);
          fclose(source);
     
          /* Now transfer spell.$$$ to fname */
          unlink(fname);
          rename("SPELL.$$$",fname);
     }
     
     void CORRECT()
     {
          /* Locate a good match and return word */
     
          char text[51];
          int m;
          int n;
          int key;
     
          window(1,22,80,24);
          clrscr();
          gotoxy(25,2);
          cprintf("Searching For Alternatives....");
     
          /* Remove any pending key strokes from keyboard buffer */
          while(kbhit())
               getch();
     
          for(n = 0; n <= lastelem; n++)
          {
               if (MATCHSTR(dic[n],comp))
               {
                    strcpy(text,dic[n]);
                    if (strlen(word) <= strlen(text))
                    {
                         for (m = 0; m < strlen(word); m++)
                         {
                              if (isupper(word[m]))
                                   text[m] = toupper(text[m]);
                              else
                                   text[m] = tolower(text[m]);
                         }
     
     
     
     
                         for(m = strlen(word); m < strlen(text); m++)
                              if (isupper(word[strlen(word)]))
                                   text[m] = toupper(text[m]);
                              else
                                   text[m] = tolower(text[m]);
                    }
                    else
                    {
                         for (m = 0; m < strlen(text); m++)
                         {
                              if (isupper(word[m]))
                                   text[m] = toupper(text[m]);
                              else
                                   text[m] = tolower(text[m]);
                         }
                    }
                    clrscr();
                    cprintf("Replace ");
                    textcolor(BLUE);
                    cprintf("%s ",word);
                    textcolor(BLACK);
                    cprintf("With ");
                    textcolor(BLUE);
                    cprintf("%s",text);
                    textcolor(BLACK);
                    cprintf(" Yes No Continue");
                    do
                    {
                         key = toupper(getch());
                    }
                    while(strchr("YNC",key) == NULL);
                    if (key == 'Y')
                    {
                         strcpy(word,text);
                         return;
                    }
                    clrscr();
                    gotoxy(25,2);
                    cprintf("Searching For Alternatives....");
     
                    /* Remove any pending key strokes from keyboard
     buffer */
                    while(kbhit())
                         getch();
     
                    if (key == 'C')
                         return;
               }
          }
          clrscr();
          gotoxy(23,2);
     
     
     
     
          cprintf("NO ALTERNATIVES FOUND! (Press a key)");
          bioskey(0);
          return;
     }
     
     
     int MATCHSTR(char *src, char *tgt)
     {
          /* Compare two words and return non zero if they are similar */
     
          int match;
          int result;
          int strsrc;
          int strtgt;
          int longest;
     
          strtgt = strlen(strupr(tgt));
          strsrc = strlen(strupr(src));
     
          longest = max(strtgt,strsrc);
     
          match = 0;
     
          if(strtgt > strsrc)
          {
               for(; *src ; match += (*src++ == *tgt++))
                    ;
          }
          else
          {
               for(; *tgt ; match += (*src++ == *tgt++))
                    ;
          }
     
          result = (match * 100 / longest);
     
          /* result holds percentage similarity */
     
          if (result > 50)
               return(1);
          return(0);
     }
     
     void AT(int row, int col)
     {
          /* Position the text cursor */
          inreg.h.bh = 0;
          inreg.h.dh = row;
          inreg.h.dl = col;
          inreg.h.ah = 0x02;
          int86 (0x10, &inreg, &outreg);
     }
     
     void WRTCHA (unsigned char ch, unsigned char attrib,  int num)
     {
          /* Display a character num times in colour attrib */
          /* via the BIOS */
     
          inreg.h.al = ch;
          inreg.h.bh = 0;
          inreg.h.bl = attrib;
          inreg.x.cx = num;
          inreg.h.ah = 0x09;
          int86 (0x10, &inreg, &outreg);
     }
     
     void SHADE_BLOCK(int left,int top,int right,int bottom)
     {
          int c;
     
          AT(bottom,right);
          WRTCHA(223,56,1);
          AT(top,right);
          WRTCHA('á',7,1);
          for (c = top+1; c < bottom; c++)
          {
               AT(c,right);
               WRTCHA(' ',7,1);
          }
          AT(bottom,left+1);
          WRTCHA('š',7,right-left);
     }
     
     
     
     
     void BOX(int l, int t, int r, int b)
     {
          /* Draws a single line box around a described area */
     
          int n;
          char top[81];
          char bottom[81];
          char tolc[5];
          char torc[5];
          char bolc[5];
          char borc[5];
          char hoor[5];
     
          sprintf(tolc,"%c",218);
          sprintf(bolc,"%c",192);
          sprintf(hoor,"%c",196);
          sprintf(torc,"%c",191);
          sprintf(borc,"%c",217);
     
               strcpy(top,tolc);
          strcpy(bottom,bolc);
          for(n = l + 1; n < r; n++)
          {
               strcat(top,hoor);
               strcat(bottom,hoor);
          }
          strcat(top,torc);
          strcat(bottom,borc);
     
          window(1,1,80,25);
               gotoxy(l,t);
          cputs(top);
          for (n = t + 1; n < b; n++)
          {
               gotoxy(l,n);
               putch(179);
               gotoxy(r,n);
               putch(179);
          }
          gotoxy(l,b);
          cputs(bottom);
     }
     
     
     
     
     void BANNER()
     {
          window (2,2,78,4);
          textcolor(BLACK);
          textbackground(GREEN);
          clrscr();
          SHADE_BLOCK(1,1,78,4);
          BOX(2,2,78,4);
          gotoxy(4,3);
          cprintf("Servile Software                SPELL CHECKER V1.7
                     (c)1992");
     }
     
     
     void main(int argc, char *argv[])
     {
          char *p;
          char tmp_name[160];
          char tmp_fname[160];
     
          if (argc != 2)
          {
               puts("\nERROR: Usage is SPELL document");
               exit(1);
          }
          else
               strcpy(fname,argv[1]);
     
          CURSOR(OFF);
     
          GETDIC();
     
          window(1,22,80,24);
          clrscr();
          gotoxy(28,2);
          cprintf("Making Backup File....");
     
          strcpy(tmp_fname,argv[1]);
     
          /* Remove extension from tmp_fname */
          p = strchr(tmp_fname,'.');
          if(p)
               *p = 0;
     
          /* Create backup file name using DOS */
          sprintf(tmp_name,"copy %s %s.!s! > NUL",argv[1],tmp_fname);
     
          system(tmp_name);
     
          window(1,1,80,25);
     
     
     
     
          textcolor(WHITE);
          textbackground(BLACK);
          clrscr();
          gotoxy(29,2);
          cprintf("Checking Spelling....");
     
          SPELL();
     
          UPDATE();
          window(1,1,80,25);
          textcolor(LIGHTGRAY);
          textbackground(BLACK);
          clrscr();
          CURSOR(ON);
     }
     
                                    
                         APPENDIX A - USING LINK



General Syntax:

     LINK [options] obj[,[exe][,[map][,[lib]]]][;]

`obj' is a list of object files to be linked. Each obj file name must be
separated by a + or a space. If you do not specify an extension, LINK
will assume .OBJ. `exe' allows you to specify a name for the executable
file. If this file name is ommited, LINK will use the first obj file name
and suffix it with .EXE. `map' is an optional map file name. If you
specify the name `NUL', no map file is produced. `lib' is a list of
library files to link. LINK searches each library file and only links in
modules which are referenced.


eg:

     LINK filea+fileb,myfile,NUL;

Links .obj files `filea.obj' and `fileb.obj' into .exe file `myfile.exe'
with no map file produced. The ; at the end of the line tells LINK that
there are no more parameters.



Using Overlays

Overlay .obj modules are specified by encasing the .obj name in
parenthesis in the link line.

eg:

     LINK filea + (fileb) + (filec),myfile,NUL;

Will link filea.obj fileb.obj and filec.obj with modules fileb.obj and
filec.obj as overlay code.


Overlay modules must use FAR call/return instructions.


Linker Options

All LINK options commence with a forward slash `/'. Options which accept
a number can accept a decimal number or a hex number prefixed 0X. eg:
0X10 is interpreted as 10h, decimal 16.



Pause during Linking (/PAU)

     Tells LINK to wait before writing the .exe file to disk. LINK
displays a
     message and waits for you to press enter.

Display Linker Process Information (/I)

     Tells LINK to display information about the link process.

Pack Executable File (/E)

     Tells LINK to remove sequences of repeated bytes and to optimise the
load-time
     relocation table before creating the executable file. Symbolic debug
     information is stripped out of the file.

List Public Symbols (/M)

     Tells LINK to create a list of all public symbols defined in the
     object files
     in the MAP file.

Include Line Numbers In Map File (/LI)

     Tells LINK to include line numbers and associated addresses of the
source
     program in the MAP file.

Preserve Case Sensitivity (/NOI)

     By default LINK treats uppercase and lowercase letters as the same.
This
     option tells LINK that they are different.

Ignore Default Libraries (/NOD)

     Tells LINK not to search any library specified in the object files
to resolve
     external references.

Controlling Stack Size (/ST:n)

     Specifies the size of the stack segment where 'n' is the number of
bytes.

Setting Maximum Allocation Space (/CP:n)

     Tells LINK to write the parameter 'n' into the exe file header. When
the exe
     file is executed by DOS, 'n' 16 byte paragraphs of memory are
reserved. If 'n'
     is less than the minimum required, it will be set to the minimum.
This option
     is ESSENTIAL to free memory from the program. C programs free memory
     automatically on start-up, assembly language programs which want to
use
     dynamic memory allocation must be linked with this option set to a
minimum.

Setting Maximum Number Of Segments (/SE:n)

     Tells LINK how many segments a program is allowed to have. The
default is 128
     but 'n' can be any number between 1 and 3072.


Setting Overlay Interrupt (/O:n)

     Tells LINK which interrupt number will be used for passing control
to
     overlays. The default is 63. Valid values for 'n' are 0 through 255.

Ordering Segments (/DO)

     Tells LINK to use DOS segment ordering. This option is also enabled
by the
     MASM directive .DOSSEG.

Controlling Data Loading (/DS)

     By default LINK loads all data starting at the low end of the data
segment. At
     run time the DS register is set to the lowest possible address to
allow the
     entire data segment to be used. This option tells LINK to load all
data
     starting at the high end of the data segment.

Control Exe File Loading (/HI)

     Tells LINK to place the exe file as high as possible in memory.

Prepare for Debugging (/CO)

     Tells LINK to include symbolic debug information for use by
codeview.

Optimising Far Calls (/F)

     Tells LINK to translate FAR calls to NEAR calls where possible. This
results
     in faster code.

Disabling Far Call Optimisation (/NOF)

     Tells LINK not to translate FAR calls. This option is specified by
default.

Packing Contiguous Segments (/PAC:n)

     Tells LINK to group together neighbouring code segments, providing
more
     oportunities for FAR call translation. 'n' specifies the maximum
size of a
     segment. By default 'n' is 65530. This option is only relevant to
obj files
     created using FAR calls.

Disabling Segment Packing (/NOP)

     Disables segment packing. This option is specified by default.



Using Response Files

Linker options and file names may be specified in a response file. Each
file list starting on a new line instead of being separated by a comma.

eg:

     filea.obj fileb.obj
     myfile.exe
     NUL
     liba.lib libb.lib
     
A response file is specified to LINK by prefixing the response file name
with '@'.
eg:

     LINK @response

