6502 Microprocessor

 Most of the following information has been taking out of the "Commodore 64
Programmers Reference Manual" simply because it was available in electronic
form and there appears to be no difference between this documentation and
the 6502 documentation, they are both from the 6500 family after all. I've
made changes and additions where appropriate.

 In theory you should be able to use any code you can find for emulating
the 6510 (the C64 processor).



  THE REGISTERS INSIDE THE 6502 MICROPROCESSOR

    Almost all calculations are done in the microprocessor. Registers are
  special pieces of memory in the processor which are used to carry out, and
  store information about calculations. The 6502 has the following registers:


  THE ACCUMULATOR

    This is THE most important register in the microprocessor. Various ma-
  chine language instructions allow you to copy the contents of a memory
  location into the accumulator, copy the contents of the accumulator into
  a memory location, modify the contents of the accumulator or some other
  register directly, without affecting any memory. And the accumulator is
  the only register that has instructions for performing math.


  THE X INDEX REGISTER

    This is a very important register. There are instructions for nearly
  all of the transformations you can make to the accumulator. But there are
  other instructions for things that only the X register can do. Various
  machine language instructions allow you to copy the contents of a memory
  location into the X register, copy the contents of the X register into a
  memory location, and modify the contents of the X, or some other register
  directly.


  THE Y INDEX REGISTER

    This is a very important register. There are instructions for nearly
  all of the transformations you can make to the accumulator, and the X
  register. But there are other instructions for things that only the Y
  register can do. Various machine language instructions allow you to copy
  the contents of a memory location into the Y register, copy the contents
  of the Y register into a memory location, and modify the contents of the
  Y, or some other register directly.


  THE STATUS REGISTER

    This register consists of eight "flags" (a flag = something that indi-
  cates whether something has, or has not occurred). Bits of this register
  are altered depending on the result of arithmetic and logical operations.
  These bits are described below:

     Bit No.       7   6   5   4   3   2   1   0
                   S   V       B   D   I   Z   C

   Bit0 - C - Carry flag: this holds the carry out of the most significant
   bit in any arithmetic operation. In subtraction operations however, this
   flag is cleared - set to 0 - if a borrow is required, set to 1 - if no
   borrow is required. The carry flag is also used in shift and rotate
   logical operations.

   Bit1 - Z - Zero flag: this is set to 1 when any arithmetic or logical
   operation produces a zero result, and is set to 0 if the result is
   non-zero.

   Bit 2 - I: this is an interrupt enable/disable flag. If it is set,
   interrupts are disabled. If it is cleared, interrupts are enabled.

   Bit 3 - D: this is the decimal mode status flag. When set, and an Add with
   Carry or Subtract with Carry instruction is executed, the source values are
   treated as valid BCD (Binary Coded Decimal, eg. 0x00-0x99 = 0-99) numbers.
   The result generated is also a BCD number.

   Bit 4 - B: this is set when a software interrupt (BRK instruction) is
   executed.

   Bit 5: not used. Supposed to be logical 1 at all times.

   Bit 6 - V - Overflow flag: when an arithmetic operation produces a result
   too large to be represented in a byte, V is set.

   Bit 7 - S - Sign flag: this is set if the result of an operation is
   negative, cleared if positive.

   The most commonly used flags are C, Z, V, S.

  

  THE PROGRAM COUNTER

    This contains the address of the current machine language instruction
  being executed. Since the operating system is always "RUN"ning in the
  Commodore VIC-20 (or, for that matter, any computer), the program counter
  is always changing. It could only be stopped by halting the microprocessor
  in some way.


  THE STACK POINTER

    This register contains the location of the first empty place on the
  stack. The stack is used for temporary storage by machine language pro-
  grams, and by the computer.




  ADDRESSING MODES

   Instructions need operands to work on. There are various ways of
  indicating where the processor is to get these operands. The different
  methods used to do this are called addressing modes. The 6502 offers 11
  modes, as described below.

  1) Immediate
  In this mode the operand's value is given in the instruction itself. In
  assembly language this is indicated by "#" before the operand.
  eg.  LDA #$0A - means "load the accumulator with the hex value 0A"
  In machine code different modes are indicated by different codes. So LDA
  would be translated into different codes depending on the addressing mode.
  In this mode, it is: $A9 $0A

  2 & 3) Absolute and Zero-page Absolute
  In these modes the operands address is given.
  eg.  LDA $31F6 - (assembler)
       $AD $31F6 - (machine code)
  If the address is on zero page - i.e. any address where the high byte is
  00 - only 1 byte is needed for the address. The processor automatically
  fills the 00 high byte.
  eg.  LDA $F4
       $A5 $F4
  Note the different instruction codes for the different modes.
  Note also that for 2 byte addresses, the low byte is store first, eg.
  LDA $31F6 is stored as three bytes in memory, $AD $F6 $31.
  Zero-page absolute is usually just called zero-page.

  4) Implied
  No operand addresses are required for this mode. They are implied by the
  instruction.
  eg.  TAX - (transfer accumulator contents to X-register)
       $AA

  5) Accumulator
  In this mode the instruction operates on data in the accumulator, so no
  operands are needed.
  eg.  LSR - logical bit shift right
       $4A

  6 & 7) Indexed and Zero-page Indexed
  In these modes the address given is added to the value in either the X or
  Y index register to give the actual address of the operand.
  eg.  LDA $31F6, Y
       $D9 $31F6
       LDA $31F6, X
       $DD $31F6
  Note that the different operation codes determine the index register used.
  In the zero-page version, you should note that the X and Y registers are
  not interchangeable. Most instructions which can be used with zero-page
  indexing do so with X only.
  eg.  LDA $20, X
       $B5 $20

  8) Indirect
  This mode applies only to the JMP instruction - JuMP to new location. It is
  indicated by parenthesis around the operand. The operand is the address of
  the bytes whose value is the new location.
  eg.  JMP ($215F)
  Assume the following -        byte      value
                                $215F     $76
                                $2160     $30
  This instruction takes the value of bytes $215F, $2160 and uses that as the
  address to jump to - i.e. $3076 (remember that addresses are stored with
  low byte first).

  9) Pre-indexed indirect
  In this mode a zer0-page address is added to the contents of the X-register
  to give the address of the bytes holding the address of the operand. The
  indirection is indicated by parenthesis in assembly language.
  eg.  LDA ($3E, X)
       $A1 $3E
  Assume the following -        byte      value
                                X-reg.    $05
                                $0043     $15
                                $0044     $24
                                $2415     $6E

  Then the instruction is executed by:
  (i)   adding $3E and $05 = $0043
  (ii)  getting address contained in bytes $0043, $0044 = $2415
  (iii) loading contents of $2415 - i.e. $6E - into accumulator

  Note a) When adding the 1-byte address and the X-register, wrap around
          addition is used - i.e. the sum is always a zero-page address.
          eg. FF + 2 = 0001 not 0101 as you might expect.
          DON'T FORGET THIS WHEN EMULATING THIS MODE.
       b) Only the X register is used in this mode.

  10) Post-indexed indirect
  In this mode the contents of a zero-page address (and the following byte)
  give the indirect addressm which is added to the contents of the Y-register
  to yield the actual address of the operand. Again, inassembly language,
  the instruction is indicated by parenthesis.
  eg.  LDA ($4C), Y
  Note that the parenthesis are only around the 2nd byte of the instruction
  since it is the part that does the indirection.
  Assume the following -        byte       value
                                $004C      $00
                                $004D      $21
                                Y-reg.     $05
                                $2105      $6D
  Then the instruction above executes by:
  (i)   getting the address in bytes $4C, $4D = $2100
  (ii)  adding the contents of the Y-register = $2105
  (111) loading the contents of the byte $2105 - i.e. $6D into the
        accumulator.
  Note: only the Y-register is used in this mode.

  11) Relative
  This mode is used with Branch-on-Condition instructions. It is probably
  the mode you will use most often. A 1 byte value is added to the program
  counter, and the program continues execution from that address. The 1
  byte number is treated as a signed number - i.e. if bit 7 is 1, the number
  given byt bits 0-6 is negative; if bit 7 is 0, the number is positive. This
  enables a branch displacement of up to 127 bytes in either direction.
  eg  bit no.  7 6 5 4 3 2 1 0    signed value          unsigned value
      value    1 0 1 0 0 1 1 1    -39                   $A7
      value    0 0 1 0 0 1 1 1    +39                   $27
  Instruction example:
    BEQ $A7
    $F0 $A7
  This instruction will check the zero status bit. If it is set, 39 decimal
  will be subtracted from the program counter and execution continues from
  that address. If the zero status bit is not set, execution continues from
  the following instruction.
  Notes:  a) The program counter points to the start of the instruction
  after the branch instruction before the branch displacement is added.
  Remember to take this into account when calculating displacements.
          b) Branch-on-condition instructions work by checking the relevant
  status bits in the status register. Make sure that they have been set or
  unset as you want them. This is often done using a CMP instruction.
          c) If you find you need to branch further than 127 bytes, use the
  opposite branch-on-condition and a JMP.


  +------------------------------------------------------------------------
  |
  |      MCS6502 MICROPROCESSOR INSTRUCTION SET - ALPHABETIC SEQUENCE
  |
  +------------------------------------------------------------------------
  |
  |     ADC   Add Memory to Accumulator with Carry
  |     AND   "AND" Memory with Accumulator
  |     ASL   Shift Left One Bit (Memory or Accumulator)
  |
  |     BCC   Branch on Carry Clear
  |     BCS   Branch on Carry Set
  |     BEQ   Branch on Result Zero
  |     BIT   Test Bits in Memory with Accumulator
  |     BMI   Branch on Result Minus
  |     BNE   Branch on Result not Zero
  |     BPL   Branch on Result Plus
  |     BRK   Force Break
  |     BVC   Branch on Overflow Clear
  |     BVS   Branch on Overflow Set
  |
  |     CLC   Clear Carry Flag
  |     CLD   Clear Decimal Mode
  |     CLI   Clear interrupt Disable Bit
  |     CLV   Clear Overflow Flag
  |     CMP   Compare Memory and Accumulator
  |     CPX   Compare Memory and Index X
  |     CPY   Compare Memory and Index Y
  |
  |     DEC   Decrement Memory by One
  |     DEX   Decrement Index X by One
  |     DEY   Decrement Index Y by One
  |
  |     EOR   "Exclusive-Or" Memory with Accumulator
  |
  |     INC   Increment Memory by One
  |     INX   Increment Index X by One
  |     INY   Increment Index Y by One
  |
  |     JMP   Jump to New Location
  |
  +------------------------------------------------------------------------


  ------------------------------------------------------------------------+
                                                                          |
         MCS6502 MICROPROCESSOR INSTRUCTION SET - ALPHABETIC SEQUENCE     |
                                                                          |
  ------------------------------------------------------------------------+
                                                                          |
        JSR   Jump to New Location Saving Return Address                  |
                                                                          |
        LDA   Load Accumulator with Memory                                |
        LDX   Load Index X with Memory                                    |
        LDY   Load Index Y with Memory                                    |
        LSR   Shift Right One Bit (Memory or Accumulator)                 |
                                                                          |
        NOP   No Operation                                                |
                                                                          |
        ORA   "OR" Memory with Accumulator                                |
                                                                          |
        PHA   Push Accumulator on Stack                                   |
        PHP   Push Processor Status on Stack                              |
        PLA   Pull Accumulator from Stack                                 |
        PLP   Pull Processor Status from Stack                            |
                                                                          |
        ROL   Rotate One Bit Left (Memory or Accumulator)                 |
        ROR   Rotate One Bit Right (Memory or Accumulator)                |
        RTI   Return from Interrupt                                       |
        RTS   Return from Subroutine                                      |
                                                                          |
        SBC   Subtract Memory from Accumulator with Borrow                |
        SEC   Set Carry Flag                                              |
        SED   Set Decimal Mode                                            |
        SEI   Set Interrupt Disable Status                                |
        STA   Store Accumulator in Memory                                 |
        STX   Store Index X in Memory                                     |
        STY   Store Index Y in Memory                                     |
                                                                          |
        TAX   Transfer Accumulator to Index X                             |
        TAY   Transfer Accumulator to Index Y                             |
        TSX   Transfer Stack Pointer to Index X                           |
        TXA   Transfer Index X to Accumulator                             |
        TXS   Transfer Index X to Stack Pointer                           |
        TYA   Transfer Index Y to Accumulator                             |
  ------------------------------------------------------------------------+


                The following notation applies to this summary:


     A       Accumulator                  EOR     Logical Exclusive Or

     X, Y    Index Registers              fromS   Transfer from Stack

     M       Memory                       toS     Transfer to Stack

     P       Processor Status Register    ->      Transfer to

     S       Stack Pointer                <-      Transfer from

     /       Change                       V       Logical OR

     _       No Change                    PC      Program Counter

     +       Add                          PCH     Program Counter High

     /\      Logical AND                  PCL     Program Counter Low

     -       Subtract                     OPER    OPERAND

                                          #       IMMEDIATE ADDRESSING MODE



  Note: At the top of each table is located in parentheses a reference
        number (Ref: XX) which directs the user to that Section in the
        MCS6500 Microcomputer Family Programming Manual in which the
        instruction is defined and discussed.




  ADC               Add memory to accumulator with carry                ADC

  Operation:  A + M + C -> A, C                         N Z C I D V
                                                        / / / _ _ /
                                (Ref: 2.2.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   ADC #Oper           |    69   |    2    |    2     |
  |  Zero Page     |   ADC Oper            |    65   |    2    |    3     |
  |  Zero Page,X   |   ADC Oper,X          |    75   |    2    |    4     |
  |  Absolute      |   ADC Oper            |    60   |    3    |    4     |
  |  Absolute,X    |   ADC Oper,X          |    70   |    3    |    4*    |
  |  Absolute,Y    |   ADC Oper,Y          |    79   |    3    |    4*    |
  |  (Indirect,X)  |   ADC (Oper,X)        |    61   |    2    |    6     |
  |  (Indirect),Y  |   ADC (Oper),Y        |    71   |    2    |    5*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if page boundary is crossed.


  AND                  "AND" memory with accumulator                    AND

  Operation:  A /\ M -> A                               N Z C I D V
                                                        / / _ _ _ _
                               (Ref: 2.2.3.0)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   AND #Oper           |    29   |    2    |    2     |
  |  Zero Page     |   AND Oper            |    25   |    2    |    3     |
  |  Zero Page,X   |   AND Oper,X          |    35   |    2    |    4     |
  |  Absolute      |   AND Oper            |    2D   |    3    |    4     |
  |  Absolute,X    |   AND Oper,X          |    3D   |    3    |    4*    |
  |  Absolute,Y    |   AND Oper,Y          |    39   |    3    |    4*    |
  |  (Indirect,X)  |   AND (Oper,X)        |    21   |    2    |    6     |
  |  (Indirect,Y)  |   AND (Oper),Y        |    31   |    2    |    5     |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if page boundary is crossed.


  ASL          ASL Shift Left One Bit (Memory or Accumulator)           ASL
                   +-+-+-+-+-+-+-+-+
  Operation:  C <- |7|6|5|4|3|2|1|0| <- 0
                   +-+-+-+-+-+-+-+-+                    N Z C I D V
                                                        / / / _ _ _
                                 (Ref: 10.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Accumulator   |   ASL A               |    0A   |    1    |    2     |
  |  Zero Page     |   ASL Oper            |    06   |    2    |    5     |
  |  Zero Page,X   |   ASL Oper,X          |    16   |    2    |    6     |
  |  Absolute      |   ASL Oper            |    0E   |    3    |    6     |
  |  Absolute, X   |   ASL Oper,X          |    1E   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+


  BCC                     BCC Branch on Carry Clear                     BCC
                                                        N Z C I D V
  Operation:  Branch on C = 0                           _ _ _ _ _ _
                               (Ref: 4.1.1.3)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Relative      |   BCC Oper            |    90   |    2    |    2*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if branch occurs to same page.
  * Add 2 if branch occurs to different page.


  BCS                      BCS Branch on carry set                      BCS

  Operation:  Branch on C = 1                           N Z C I D V
                                                        _ _ _ _ _ _
                               (Ref: 4.1.1.4)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Relative      |   BCS Oper            |    B0   |    2    |    2*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if branch occurs to same  page.
  * Add 2 if branch occurs to next  page.


  BEQ                    BEQ Branch on result zero                      BEQ
                                                        N Z C I D V
  Operation:  Branch on Z = 1                           _ _ _ _ _ _
                               (Ref: 4.1.1.5)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Relative      |   BEQ Oper            |    F0   |    2    |    2*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if branch occurs to same  page.
  * Add 2 if branch occurs to next  page.


  BIT             BIT Test bits in memory with accumulator              BIT

  Operation:  A /\ M, M7 -> N, M6 -> V

  Bit 6 and 7 are transferred to the status register.   N Z C I D V
  If the result of A /\ M is zero then Z = 1, otherwise M7/ _ _ _ M6
  Z = 0
                               (Ref: 4.2.1.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Zero Page     |   BIT Oper            |    24   |    2    |    3     |
  |  Absolute      |   BIT Oper            |    2C   |    3    |    4     |
  +----------------+-----------------------+---------+---------+----------+


  BMI                    BMI Branch on result minus                     BMI

  Operation:  Branch on N = 1                           N Z C I D V
                                                        _ _ _ _ _ _
                               (Ref: 4.1.1.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Relative      |   BMI Oper            |    30   |    2    |    2*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if branch occurs to same page.
  * Add 1 if branch occurs to different page.


  BNE                   BNE Branch on result not zero                   BNE

  Operation:  Branch on Z = 0                           N Z C I D V
                                                        _ _ _ _ _ _
                               (Ref: 4.1.1.6)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Relative      |   BMI Oper            |    D0   |    2    |    2*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if branch occurs to same page.
  * Add 2 if branch occurs to different page.


  BPL                     BPL Branch on result plus                     BPL

  Operation:  Branch on N = 0                           N Z C I D V
                                                        _ _ _ _ _ _
                               (Ref: 4.1.1.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Relative      |   BPL Oper            |    10   |    2    |    2*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if branch occurs to same page.
  * Add 2 if branch occurs to different page.


  BRK                          BRK Force Break                          BRK

  Operation:  Forced Interrupt PC + 2 toS P toS         N Z C I D V
                                                        _ _ _ 1 _ _
                                 (Ref: 9.11)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   BRK                 |    00   |    1    |    7     |
  +----------------+-----------------------+---------+---------+----------+
  1. A BRK command cannot be masked by setting I.


  BVC                   BVC Branch on overflow clear                    BVC

  Operation:  Branch on V = 0                           N Z C I D V
                                                        _ _ _ _ _ _
                               (Ref: 4.1.1.8)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Relative      |   BVC Oper            |    50   |    2    |    2*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if branch occurs to same page.
  * Add 2 if branch occurs to different page.


  BVS                    BVS Branch on overflow set                     BVS

  Operation:  Branch on V = 1                           N Z C I D V
                                                        _ _ _ _ _ _
                               (Ref: 4.1.1.7)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Relative      |   BVS Oper            |    70   |    2    |    2*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if branch occurs to same page.
  * Add 2 if branch occurs to different page.


  CLC                       CLC Clear carry flag                        CLC

  Operation:  0 -> C                                    N Z C I D V
                                                        _ _ 0 _ _ _
                                (Ref: 3.0.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   CLC                 |    18   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  CLD                      CLD Clear decimal mode                       CLD

  Operation:  0 -> D                                    N A C I D V
                                                        _ _ _ _ 0 _
                                (Ref: 3.3.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   CLD                 |    D8   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  CLI                  CLI Clear interrupt disable bit                  CLI

  Operation: 0 -> I                                     N Z C I D V
                                                        _ _ _ 0 _ _
                                (Ref: 3.2.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   CLI                 |    58   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  CLV                      CLV Clear overflow flag                      CLV

  Operation: 0 -> V                                     N Z C I D V
                                                        _ _ _ _ _ 0
                                (Ref: 3.6.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   CLV                 |    B8   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  CMP                CMP Compare memory and accumulator                 CMP

  Operation:  A - M                                     N Z C I D V
                                                        / / / _ _ _
                                (Ref: 4.2.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   CMP #Oper           |    C9   |    2    |    2     |
  |  Zero Page     |   CMP Oper            |    C5   |    2    |    3     |
  |  Zero Page,X   |   CMP Oper,X          |    D5   |    2    |    4     |
  |  Absolute      |   CMP Oper            |    CD   |    3    |    4     |
  |  Absolute,X    |   CMP Oper,X          |    DD   |    3    |    4*    |
  |  Absolute,Y    |   CMP Oper,Y          |    D9   |    3    |    4*    |
  |  (Indirect,X)  |   CMP (Oper,X)        |    C1   |    2    |    6     |
  |  (Indirect),Y  |   CMP (Oper),Y        |    D1   |    2    |    5*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if page boundary is crossed.

  CPX                  CPX Compare Memory and Index X                   CPX
                                                        N Z C I D V
  Operation:  X - M                                     / / / _ _ _
                                 (Ref: 7.8)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   CPX *Oper           |    E0   |    2    |    2     |
  |  Zero Page     |   CPX Oper            |    E4   |    2    |    3     |
  |  Absolute      |   CPX Oper            |    EC   |    3    |    4     |
  +----------------+-----------------------+---------+---------+----------+

  CPY                  CPY Compare memory and index Y                   CPY
                                                        N Z C I D V
  Operation:  Y - M                                     / / / _ _ _
                                 (Ref: 7.9)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   CPY *Oper           |    C0   |    2    |    2     |
  |  Zero Page     |   CPY Oper            |    C4   |    2    |    3     |
  |  Absolute      |   CPY Oper            |    CC   |    3    |    4     |
  +----------------+-----------------------+---------+---------+----------+


  DEC                   DEC Decrement memory by one                     DEC

  Operation:  M - 1 -> M                                N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 10.7)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Zero Page     |   DEC Oper            |    C6   |    2    |    5     |
  |  Zero Page,X   |   DEC Oper,X          |    D6   |    2    |    6     |
  |  Absolute      |   DEC Oper            |    CE   |    3    |    6     |
  |  Absolute,X    |   DEC Oper,X          |    DE   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+


  DEX                   DEX Decrement index X by one                    DEX

  Operation:  X - 1 -> X                                N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 7.6)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   DEX                 |    CA   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  DEY                   DEY Decrement index Y by one                    DEY

  Operation:  X - 1 -> Y                                N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 7.7)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   DEY                 |    88   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  EOR            EOR "Exclusive-Or" memory with accumulator             EOR

  Operation:  A EOR M -> A                              N Z C I D V
                                                        / / _ _ _ _
                               (Ref: 2.2.3.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   EOR #Oper           |    49   |    2    |    2     |
  |  Zero Page     |   EOR Oper            |    45   |    2    |    3     |
  |  Zero Page,X   |   EOR Oper,X          |    55   |    2    |    4     |
  |  Absolute      |   EOR Oper            |    40   |    3    |    4     |
  |  Absolute,X    |   EOR Oper,X          |    50   |    3    |    4*    |
  |  Absolute,Y    |   EOR Oper,Y          |    59   |    3    |    4*    |
  |  (Indirect,X)  |   EOR (Oper,X)        |    41   |    2    |    6     |
  |  (Indirect),Y  |   EOR (Oper),Y        |    51   |    2    |    5*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if page boundary is crossed.

  INC                    INC Increment memory by one                    INC
                                                        N Z C I D V
  Operation:  M + 1 -> M                                / / _ _ _ _
                                 (Ref: 10.6)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Zero Page     |   INC Oper            |    E6   |    2    |    5     |
  |  Zero Page,X   |   INC Oper,X          |    F6   |    2    |    6     |
  |  Absolute      |   INC Oper            |    EE   |    3    |    6     |
  |  Absolute,X    |   INC Oper,X          |    FE   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+

  INX                    INX Increment Index X by one                   INX
                                                        N Z C I D V
  Operation:  X + 1 -> X                                / / _ _ _ _
                                 (Ref: 7.4)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   INX                 |    E8   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  INY                    INY Increment Index Y by one                   INY

  Operation:  X + 1 -> X                                N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 7.5)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   INY                 |    C8   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  JMP                     JMP Jump to new location                      JMP

  Operation:  (PC + 1) -> PCL                           N Z C I D V
              (PC + 2) -> PCH   (Ref: 4.0.2)            _ _ _ _ _ _
                                (Ref: 9.8.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Absolute      |   JMP Oper            |    4C   |    3    |    3     |
  |  Indirect      |   JMP (Oper)          |    6C   |    3    |    5     |
  +----------------+-----------------------+---------+---------+----------+


  JSR          JSR Jump to new location saving return address           JSR

  Operation:  PC + 2 toS, (PC + 1) -> PCL               N Z C I D V
                          (PC + 2) -> PCH               _ _ _ _ _ _
                                 (Ref: 8.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Absolute      |   JSR Oper            |    20   |    3    |    6     |
  +----------------+-----------------------+---------+---------+----------+


  LDA                  LDA Load accumulator with memory                 LDA

  Operation:  M -> A                                    N Z C I D V
                                                        / / _ _ _ _
                                (Ref: 2.1.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   LDA #Oper           |    A9   |    2    |    2     |
  |  Zero Page     |   LDA Oper            |    A5   |    2    |    3     |
  |  Zero Page,X   |   LDA Oper,X          |    B5   |    2    |    4     |
  |  Absolute      |   LDA Oper            |    AD   |    3    |    4     |
  |  Absolute,X    |   LDA Oper,X          |    BD   |    3    |    4*    |
  |  Absolute,Y    |   LDA Oper,Y          |    B9   |    3    |    4*    |
  |  (Indirect,X)  |   LDA (Oper,X)        |    A1   |    2    |    6     |
  |  (Indirect),Y  |   LDA (Oper),Y        |    B1   |    2    |    5*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 if page boundary is crossed.


  LDX                   LDX Load index X with memory                    LDX

  Operation:  M -> X                                    N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 7.0)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   LDX #Oper           |    A2   |    2    |    2     |
  |  Zero Page     |   LDX Oper            |    A6   |    2    |    3     |
  |  Zero Page,Y   |   LDX Oper,Y          |    B6   |    2    |    4     |
  |  Absolute      |   LDX Oper            |    AE   |    3    |    4     |
  |  Absolute,Y    |   LDX Oper,Y          |    BE   |    3    |    4*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 when page boundary is crossed.


  LDY                   LDY Load index Y with memory                    LDY
                                                        N Z C I D V
  Operation:  M -> Y                                    / / _ _ _ _
                                 (Ref: 7.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   LDY #Oper           |    A0   |    2    |    2     |
  |  Zero Page     |   LDY Oper            |    A4   |    2    |    3     |
  |  Zero Page,X   |   LDY Oper,X          |    B4   |    2    |    4     |
  |  Absolute      |   LDY Oper            |    AC   |    3    |    4     |
  |  Absolute,X    |   LDY Oper,X          |    BC   |    3    |    4*    |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 when page boundary is crossed.


  LSR          LSR Shift right one bit (memory or accumulator)          LSR

                   +-+-+-+-+-+-+-+-+
  Operation:  0 -> |7|6|5|4|3|2|1|0| -> C               N Z C I D V
                   +-+-+-+-+-+-+-+-+                    0 / / _ _ _
                                 (Ref: 10.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Accumulator   |   LSR A               |    4A   |    1    |    2     |
  |  Zero Page     |   LSR Oper            |    46   |    2    |    5     |
  |  Zero Page,X   |   LSR Oper,X          |    56   |    2    |    6     |
  |  Absolute      |   LSR Oper            |    4E   |    3    |    6     |
  |  Absolute,X    |   LSR Oper,X          |    5E   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+


  NOP                         NOP No operation                          NOP
                                                        N Z C I D V
  Operation:  No Operation (2 cycles)                   _ _ _ _ _ _

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   NOP                 |    EA   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  ORA                 ORA "OR" memory with accumulator                  ORA

  Operation: A V M -> A                                 N Z C I D V
                                                        / / _ _ _ _
                               (Ref: 2.2.3.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   ORA #Oper           |    09   |    2    |    2     |
  |  Zero Page     |   ORA Oper            |    05   |    2    |    3     |
  |  Zero Page,X   |   ORA Oper,X          |    15   |    2    |    4     |
  |  Absolute      |   ORA Oper            |    0D   |    3    |    4     |
  |  Absolute,X    |   ORA Oper,X          |    10   |    3    |    4*    |
  |  Absolute,Y    |   ORA Oper,Y          |    19   |    3    |    4*    |
  |  (Indirect,X)  |   ORA (Oper,X)        |    01   |    2    |    6     |
  |  (Indirect),Y  |   ORA (Oper),Y        |    11   |    2    |    5     |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 on page crossing


  PHA                   PHA Push accumulator on stack                   PHA

  Operation:  A toS                                     N Z C I D V
                                                        _ _ _ _ _ _
                                 (Ref: 8.5)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   PHA                 |    48   |    1    |    3     |
  +----------------+-----------------------+---------+---------+----------+


  PHP                 PHP Push processor status on stack                PHP

  Operation:  P toS                                     N Z C I D V
                                                        _ _ _ _ _ _
                                 (Ref: 8.11)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   PHP                 |    08   |    1    |    3     |
  +----------------+-----------------------+---------+---------+----------+


  PLA                 PLA Pull accumulator from stack                   PLA

  Operation:  A fromS                                   N Z C I D V
                                                        _ _ _ _ _ _
                                 (Ref: 8.6)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   PLA                 |    68   |    1    |    4     |
  +----------------+-----------------------+---------+---------+----------+


  PLP               PLP Pull processor status from stack                PLA

  Operation:  P fromS                                   N Z C I D V
                                                         From Stack
                                 (Ref: 8.12)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   PLP                 |    28   |    1    |    4     |
  +----------------+-----------------------+---------+---------+----------+


  ROL          ROL Rotate one bit left (memory or accumulator)          ROL

               +------------------------------+
               |         M or A               |
               |   +-+-+-+-+-+-+-+-+    +-+   |
  Operation:   +-< |7|6|5|4|3|2|1|0| <- |C| <-+         N Z C I D V
                   +-+-+-+-+-+-+-+-+    +-+             / / / _ _ _
                                 (Ref: 10.3)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Accumulator   |   ROL A               |    2A   |    1    |    2     |
  |  Zero Page     |   ROL Oper            |    26   |    2    |    5     |
  |  Zero Page,X   |   ROL Oper,X          |    36   |    2    |    6     |
  |  Absolute      |   ROL Oper            |    2E   |    3    |    6     |
  |  Absolute,X    |   ROL Oper,X          |    3E   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+


  ROR          ROR Rotate one bit right (memory or accumulator)         ROR

               +------------------------------+
               |                              |
               |   +-+    +-+-+-+-+-+-+-+-+   |
  Operation:   +-> |C| -> |7|6|5|4|3|2|1|0| >-+         N Z C I D V
                   +-+    +-+-+-+-+-+-+-+-+             / / / _ _ _
                                 (Ref: 10.4)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Accumulator   |   ROR A               |    6A   |    1    |    2     |
  |  Zero Page     |   ROR Oper            |    66   |    2    |    5     |
  |  Zero Page,X   |   ROR Oper,X          |    76   |    2    |    6     |
  |  Absolute      |   ROR Oper            |    6E   |    3    |    6     |
  |  Absolute,X    |   ROR Oper,X          |    7E   |    3    |    7     |
  +----------------+-----------------------+---------+---------+----------+

    Note: ROR instruction is available on MCS650X microprocessors after
          June, 1976.


  RTI                    RTI Return from interrupt                      RTI
                                                        N Z C I D V
  Operation:  P fromS PC fromS                           From Stack
                                 (Ref: 9.6)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   RTI                 |    4D   |    1    |    6     |
  +----------------+-----------------------+---------+---------+----------+


  RTS                    RTS Return from subroutine                     RTS
                                                        N Z C I D V
  Operation:  PC fromS, PC + 1 -> PC                    _ _ _ _ _ _
                                 (Ref: 8.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   RTS                 |    60   |    1    |    6     |
  +----------------+-----------------------+---------+---------+----------+


  SBC          SBC Subtract memory from accumulator with borrow         SBC
                      -
  Operation:  A - M - C -> A                            N Z C I D V
         -                                              / / / _ _ /
    Note:C = Borrow             (Ref: 2.2.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   SBC #Oper           |    E9   |    2    |    2     |
  |  Zero Page     |   SBC Oper            |    E5   |    2    |    3     |
  |  Zero Page,X   |   SBC Oper,X          |    F5   |    2    |    4     |
  |  Absolute      |   SBC Oper            |    ED   |    3    |    4     |
  |  Absolute,X    |   SBC Oper,X          |    FD   |    3    |    4*    |
  |  Absolute,Y    |   SBC Oper,Y          |    F9   |    3    |    4*    |
  |  (Indirect,X)  |   SBC (Oper,X)        |    E1   |    2    |    6     |
  |  (Indirect),Y  |   SBC (Oper),Y        |    F1   |    2    |    5     |
  +----------------+-----------------------+---------+---------+----------+
  * Add 1 when page boundary is crossed.


  SEC                        SEC Set carry flag                         SEC

  Operation:  1 -> C                                    N Z C I D V
                                                        _ _ 1 _ _ _
                                (Ref: 3.0.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   SEC                 |    38   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  SED                       SED Set decimal mode                        SED
                                                        N Z C I D V
  Operation:  1 -> D                                    _ _ _ _ 1 _
                                (Ref: 3.3.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   SED                 |    F8   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  SEI                 SEI Set interrupt disable status                  SED
                                                        N Z C I D V
  Operation:  1 -> I                                    _ _ _ 1 _ _
                                (Ref: 3.2.1)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   SEI                 |    78   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  STA                  STA Store accumulator in memory                  STA

  Operation:  A -> M                                    N Z C I D V
                                                        _ _ _ _ _ _
                                (Ref: 2.1.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Zero Page     |   STA Oper            |    85   |    2    |    3     |
  |  Zero Page,X   |   STA Oper,X          |    95   |    2    |    4     |
  |  Absolute      |   STA Oper            |    80   |    3    |    4     |
  |  Absolute,X    |   STA Oper,X          |    90   |    3    |    5     |
  |  Absolute,Y    |   STA Oper, Y         |    99   |    3    |    5     |
  |  (Indirect,X)  |   STA (Oper,X)        |    81   |    2    |    6     |
  |  (Indirect),Y  |   STA (Oper),Y        |    91   |    2    |    6     |
  +----------------+-----------------------+---------+---------+----------+


  STX                    STX Store index X in memory                    STX

  Operation: X -> M                                     N Z C I D V
                                                        _ _ _ _ _ _
                                 (Ref: 7.2)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Zero Page     |   STX Oper            |    86   |    2    |    3     |
  |  Zero Page,Y   |   STX Oper,Y          |    96   |    2    |    4     |
  |  Absolute      |   STX Oper            |    8E   |    3    |    4     |
  +----------------+-----------------------+---------+---------+----------+


  STY                    STY Store index Y in memory                    STY

  Operation: Y -> M                                     N Z C I D V
                                                        _ _ _ _ _ _
                                 (Ref: 7.3)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Zero Page     |   STY Oper            |    84   |    2    |    3     |
  |  Zero Page,X   |   STY Oper,X          |    94   |    2    |    4     |
  |  Absolute      |   STY Oper            |    8C   |    3    |    4     |
  +----------------+-----------------------+---------+---------+----------+


  TAX                TAX Transfer accumulator to index X                TAX

  Operation:  A -> X                                    N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 7.11)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   TAX                 |    AA   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  TAY                TAY Transfer accumulator to index Y                TAY

  Operation:  A -> Y                                    N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 7.13)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   TAY                 |    A8   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+


  TSX              TSX Transfer stack pointer to index X                TSX

  Operation:  S -> X                                    N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 8.9)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   TSX                 |    BA   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+

  TXA                TXA Transfer index X to accumulator                TXA
                                                        N Z C I D V
  Operation:  X -> A                                    / / _ _ _ _
                                 (Ref: 7.12)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   TXA                 |    8A   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+

  TXS              TXS Transfer index X to stack pointer                TXS
                                                        N Z C I D V
  Operation:  X -> S                                    _ _ _ _ _ _
                                 (Ref: 8.8)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   TXS                 |    9A   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+

  TYA                TYA Transfer index Y to accumulator                TYA

  Operation:  Y -> A                                    N Z C I D V
                                                        / / _ _ _ _
                                 (Ref: 7.14)
  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   TYA                 |    98   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+



  +------------------------------------------------------------------------
  | INSTRUCTION ADDRESSING MODES AND RELATED EXECUTION TIMES
  | (in clock cycles)
  +------------------------------------------------------------------------

                  A   A   A   B   B   B   B   B   B   B   B   B   B   C
                  D   N   S   C   C   E   I   M   N   P   R   V   V   L
                  C   D   L   C   S   Q   T   I   E   L   K   C   S   C
  Accumulator  |  .   .   2   .   .   .   .   .   .   .   .   .   .   .
  Immediate    |  2   2       .   .   .   .   .   .   .   .   .   .   .
  Zero Page    |  3   3   5   .   .   .   3   .   .   .   .   .   .   .
  Zero Page,X  |  4   4   6   .   .   .   .   .   .   .   .   .   .   .
  Zero Page,Y  |  .   .   .   .   .   .   .   .   .   .   .   .   .   .
  Absolute     |  4   4   6   .   .   .   4   .   .   .   .   .   .   .
  Absolute,X   |  4*  4*  7   .   .   .   .   .   .   .   .   .   .   .
  Absolute,Y   |  4*  4*  .   .   .   .   .   .   .   .   .   .   .   .
  Implied      |  .   .   .   .   .   .   .   .   .   .   .   .   .   2
  Relative     |  .   .   .   2** 2** 2** .   2** 2** 2** 7   2** 2** .
  (Indirect,X) |  6   6   .   .   .   .   .   .   .   .   .   .   .   .
  (Indirect),Y |  5*  5*  .   .   .   .   .   .   .   .   .   .   .   .
  Abs. Indirect|  .   .   .   .   .   .   .   .   .   .   .   .   .   .
               +-----------------------------------------------------------
                  C   C   C   C   C   C   D   D   D   E   I   I   I   J
                  L   L   L   M   P   P   E   E   E   O   N   N   N   M
                  D   I   V   P   X   Y   C   X   Y   R   C   X   Y   P
  Accumulator  |  .   .   .   .   .   .   .   .   .   .   .   .   .   .
  Immediate    |  .   .   .   2   2   2   .   .   .   2   .   .   .   .
  Zero Page    |  .   .   .   3   3   3   5   .   .   3   5   .   .   .
  Zero Page,X  |  .   .   .   4   .   .   6   .   .   4   6   .   .   .
  Zero Page,Y  |  .   .   .   .   .   .   .   .   .   .   .   .   .   .
  Absolute     |  .   .   .   4   4   4   6   .   .   4   6   .   .   3
  Absolute,X   |  .   .   .   4*  .   .   7   .   .   4*  7   .   .   .
  Absolute,Y   |  .   .   .   4*  .   .   .   .   .   4*  .   .   .   .
  Implied      |  2   2   2   .   .   .   .   2   2   .   .   2   2   .
  Relative     |  .   .   .   .   .   .   .   .   .   .   .   .   .   .
  (Indirect,X) |  .   .   .   6   .   .   .   .   .   6   .   .   .   .
  (Indirect),Y |  .   .   .   5*  .   .   .   .   .   5*  .   .   .   .
  Abs. Indirect|  .   .   .   .   .   .   .   .   .   .   .   .   .   5
               +-----------------------------------------------------------
     *  Add one cycle if indexing across page boundary
     ** Add one cycle if branch is taken, Add one additional if branching
        operation crosses page boundary


  ------------------------------------------------------------------------+
    INSTRUCTION ADDRESSING MODES AND RELATED EXECUTION TIMES              |
    (in clock cycles)                                                     |
  ------------------------------------------------------------------------+

                  J   L   L   L   L   N   O   P   P   P   P   R   R   R
                  S   D   D   D   S   O   R   H   H   L   L   O   O   T
                  R   A   X   Y   R   P   A   A   P   A   P   L   R   I
  Accumulator  |  .   .   .   .   2   .   .   .   .   .   .   2   2   .
  Immediate    |  .   2   2   2   .   .   2   .   .   .   .   .   .   .
  Zero Page    |  .   3   3   3   5   .   3   .   .   .   .   5   5   .
  Zero Page,X  |  .   4   .   4   6   .   4   .   .   .   .   6   6   .
  Zero Page,Y  |  .   .   4   .   .   .   .   .   .   .   .   .   .   .
  Absolute     |  6   4   4   4   6   .   4   .   .   .   .   6   6   .
  Absolute,X   |  .   4*  .   4*  7   .   4*  .   .   .   .   7   7   .
  Absolute,Y   |  .   4*  4*  .   .   .   4*  .   .   .   .   .   .   .
  Implied      |  .   .   .   .   .   2   .   3   3   4   4   .   .   6
  Relative     |  .   .   .   .   .   .   .   .   .   .   .   .   .   .
  (Indirect,X) |  .   6   .   .   .   .   6   .   .   .   .   .   .   .
  (Indirect),Y |  .   5*  .   .   .   .   5*  .   .   .   .   .   .   .
  Abs. Indirect|  .   .   .   .   .   .   .   .   .   .   .   .   .   .
               +-----------------------------------------------------------
                  R   S   S   S   S   S   S   S   T   T   T   T   T   T
                  T   B   E   E   E   T   T   T   A   A   S   X   X   Y
                  S   C   C   D   I   A   X   Y   X   Y   X   A   S   A
  Accumulator  |  .   .   .   .   .   .   .   .   .   .   .   .   .   .
  Immediate    |  .   2   .   .   .   .   .   .   .   .   .   .   .   .
  Zero Page    |  .   3   .   .   .   3   3   3   .   .   .   .   .   .
  Zero Page,X  |  .   4   .   .   .   4   .   4   .   .   .   .   .   .
  Zero Page,Y  |  .   .   .   .   .   .   4   .   .   .   .   .   .   .
  Absolute     |  .   4   .   .   .   4   4   4   .   .   .   .   .   .
  Absolute,X   |  .   4*  .   .   .   5   .   .   .   .   .   .   .   .
  Absolute,Y   |  .   4*  .   .   .   5   .   .   .   .   .   .   .   .
  Implied      |  6   .   2   2   2   .   .   .   2   2   2   2   2   2
  Relative     |  .   .   .   .   .   .   .   .   .   .   .   .   .   .
  (Indirect,X) |  .   6   .   .   .   6   .   .   .   .   .   .   .   .
  (Indirect),Y |  .   5*  .   .   .   6   .   .   .   .   .   .   .   .
  Abs. Indirect|  .   .   .   .   .   .   .   .   .   .   .   .   .   .
               +-----------------------------------------------------------
     *  Add one cycle if indexing across page boundary
     ** Add one cycle if branch is taken, Add one additional if branching
        operation crosses page boundary



        00 - BRK                        20 - JSR
        01 - ORA - (Indirect,X)         21 - AND - (Indirect,X)
        02 - Future Expansion           22 - Future Expansion
        03 - Future Expansion           23 - Future Expansion
        04 - Future Expansion           24 - BIT - Zero Page
        05 - ORA - Zero Page            25 - AND - Zero Page
        06 - ASL - Zero Page            26 - ROL - Zero Page
        07 - Future Expansion           27 - Future Expansion
        08 - PHP                        28 - PLP
        09 - ORA - Immediate            29 - AND - Immediate
        0A - ASL - Accumulator          2A - ROL - Accumulator
        0B - Future Expansion           2B - Future Expansion
        0C - Future Expansion           2C - BIT - Absolute
        0D - ORA - Absolute             2D - AND - Absolute
        0E - ASL - Absolute             2E - ROL - Absolute
        0F - Future Expansion           2F - Future Expansion
        10 - BPL                        30 - BMI
        11 - ORA - (Indirect),Y         31 - AND - (Indirect),Y
        12 - Future Expansion           32 - Future Expansion
        13 - Future Expansion           33 - Future Expansion
        14 - Future Expansion           34 - Future Expansion
        15 - ORA - Zero Page,X          35 - AND - Zero Page,X
        16 - ASL - Zero Page,X          36 - ROL - Zero Page,X
        17 - Future Expansion           37 - Future Expansion
        18 - CLC                        38 - SEC
        19 - ORA - Absolute,Y           39 - AND - Absolute,Y
        1A - Future Expansion           3A - Future Expansion
        1B - Future Expansion           3B - Future Expansion
        1C - Future Expansion           3C - Future Expansion
        1D - ORA - Absolute,X           3D - AND - Absolute,X
        1E - ASL - Absolute,X           3E - ROL - Absolute,X
        1F - Future Expansion           3F - Future Expansion

        40 - RTI                        60 - RTS
        41 - EOR - (Indirect,X)         61 - ADC - (Indirect,X)
        42 - Future Expansion           62 - Future Expansion
        43 - Future Expansion           63 - Future Expansion
        44 - Future Expansion           64 - Future Expansion
        45 - EOR - Zero Page            65 - ADC - Zero Page
        46 - LSR - Zero Page            66 - ROR - Zero Page
        47 - Future Expansion           67 - Future Expansion
        48 - PHA                        68 - PLA
        49 - EOR - Immediate            69 - ADC - Immediate
        4A - LSR - Accumulator          6A - ROR - Accumulator
        4B - Future Expansion           6B - Future Expansion
        4C - JMP - Absolute             6C - JMP - Indirect
        4D - EOR - Absolute             6D - ADC - Absolute
        4E - LSR - Absolute             6E - ROR - Absolute
        4F - Future Expansion           6F - Future Expansion
        50 - BVC                        70 - BVS
        51 - EOR - (Indirect),Y         71 - ADC - (Indirect),Y
        52 - Future Expansion           72 - Future Expansion
        53 - Future Expansion           73 - Future Expansion
        54 - Future Expansion           74 - Future Expansion
        55 - EOR - Zero Page,X          75 - ADC - Zero Page,X
        56 - LSR - Zero Page,X          76 - ROR - Zero Page,X
        57 - Future Expansion           77 - Future Expansion
        58 - CLI                        78 - SEI
        59 - EOR - Absolute,Y           79 - ADC - Absolute,Y
        5A - Future Expansion           7A - Future Expansion
        5B - Future Expansion           7B - Future Expansion
        5C - Future Expansion           7C - Future Expansion
        50 - EOR - Absolute,X           70 - ADC - Absolute,X
        5E - LSR - Absolute,X           7E - ROR - Absolute,X
        5F - Future Expansion           7F - Future Expansion

        80 - Future Expansion           A0 - LDY - Immediate
        81 - STA - (Indirect,X)         A1 - LDA - (Indirect,X)
        82 - Future Expansion           A2 - LDX - Immediate
        83 - Future Expansion           A3 - Future Expansion
        84 - STY - Zero Page            A4 - LDY - Zero Page
        85 - STA - Zero Page            A5 - LDA - Zero Page
        86 - STX - Zero Page            A6 - LDX - Zero Page
        87 - Future Expansion           A7 - Future Expansion
        88 - DEY                        A8 - TAY
        89 - Future Expansion           A9 - LDA - Immediate
        8A - TXA                        AA - TAX
        8B - Future Expansion           AB - Future Expansion
        8C - STY - Absolute             AC - LDY - Absolute
        80 - STA - Absolute             AD - LDA - Absolute
        8E - STX - Absolute             AE - LDX - Absolute
        8F - Future Expansion           AF - Future Expansion
        90 - BCC                        B0 - BCS
        91 - STA - (Indirect),Y         B1 - LDA - (Indirect),Y
        92 - Future Expansion           B2 - Future Expansion
        93 - Future Expansion           B3 - Future Expansion
        94 - STY - Zero Page,X          B4 - LDY - Zero Page,X
        95 - STA - Zero Page,X          BS - LDA - Zero Page,X
        96 - STX - Zero Page,Y          B6 - LDX - Zero Page,Y
        97 - Future Expansion           B7 - Future Expansion
        98 - TYA                        B8 - CLV
        99 - STA - Absolute,Y           B9 - LDA - Absolute,Y
        9A - TXS                        BA - TSX
        9B - Future Expansion           BB - Future Expansion
        9C - Future Expansion           BC - LDY - Absolute,X
        90 - STA - Absolute,X           BD - LDA - Absolute,X
        9E - Future Expansion           BE - LDX - Absolute,Y
        9F - Future Expansion           BF - Future Expansion

        C0 - Cpy - Immediate            E0 - CPX - Immediate
        C1 - CMP - (Indirect,X)         E1 - SBC - (Indirect,X)
        C2 - Future Expansion           E2 - Future Expansion
        C3 - Future Expansion           E3 - Future Expansion
        C4 - CPY - Zero Page            E4 - CPX - Zero Page
        C5 - CMP - Zero Page            E5 - SBC - Zero Page
        C6 - DEC - Zero Page            E6 - INC - Zero Page
        C7 - Future Expansion           E7 - Future Expansion
        C8 - INY                        E8 - INX
        C9 - CMP - Immediate            E9 - SBC - Immediate
        CA - DEX                        EA - NOP
        CB - Future Expansion           EB - Future Expansion
        CC - CPY - Absolute             EC - CPX - Absolute
        CD - CMP - Absolute             ED - SBC - Absolute
        CE - DEC - Absolute             EE - INC - Absolute
        CF - Future Expansion           EF - Future Expansion
        D0 - BNE                        F0 - BEQ
        D1 - CMP   (Indirect@,Y         F1 - SBC - (Indirect),Y
        D2 - Future Expansion           F2 - Future Expansion
        D3 - Future Expansion           F3 - Future Expansion
        D4 - Future Expansion           F4 - Future Expansion
        D5 - CMP - Zero Page,X          F5 - SBC - Zero Page,X
        D6 - DEC - Zero Page,X          F6 - INC - Zero Page,X
        D7 - Future Expansion           F7 - Future Expansion
        D8 - CLD                        F8 - SED
        D9 - CMP - Absolute,Y           F9 - SBC - Absolute,Y
        DA - Future Expansion           FA - Future Expansion
        DB - Future Expansion           FB - Future Expansion
        DC - Future Expansion           FC - Future Expansion
        DD - CMP - Absolute,X           FD - SBC - Absolute,X
        DE - DEC - Absolute,X           FE - INC - Absolute,X
        DF - Future Expansion           FF - Future Expansion


INSTRUCTION OPERATION

 The following code has been taken from VICE for the purposes of showing
how each instruction operates. No particular addressing mode is used since
we only wish to see the operation of the instruction itself. 

     src : the byte of data that is being addressed.
     SET_SIGN : sets\resets the sign flag depending on bit 7.
     SET_ZERO : sets\resets the zero flag depending on whether the result
                is zero or not.
     SET_CARRY(condition) : if the condition has a non-zero value then the
                carry flag is set, else it is reset.
     SET_OVERFLOW(condition) : if the condition is true then the overflow
                flag is set, else it is reset.
     SET_INTERRUPT :  }
     SET_BREAK :      }  As for SET_CARRY and SET_OVERFLOW.
     SET_DECIMAL :    }
     REL_ADDR(PC, src) : returns the relative address obtained by adding
                the displacement src to the PC.
     SET_SR : set the Program Status Register to the value given.
     GET_SR : get the value of the Program Status Register.
     PULL : Pull a byte off the stack.
     PUSH : Push a byte onto the stack.
     LOAD : Get a byte from the memory address.
     STORE : Store a byte in a memory address.
     IF_CARRY, IF_OVERFLOW, IF_SIGN, IF_ZERO etc : Returns true if the
                relevant flag is set, otherwise returns false.
     clk : the number of cycles an instruction takes. This is shown below
                in situations where the number of cycles changes depending
                on the result of the instruction (eg. Branching instructions).

     AC = Accumulator
     XR = X register
     YR = Y register
     PC = Program Counter
     SP = Stack Pointer


/* ADC */
    unsigned int temp = src + AC + (IF_CARRY() ? 1 : 0);
    SET_ZERO(temp & 0xff);	/* This is not valid in decimal mode */
    if (IF_DECIMAL()) {
        if (((AC & 0xf) + (src & 0xf) + (IF_CARRY() ? 1 : 0)) > 9) temp += 6;
	SET_SIGN(temp);
	SET_OVERFLOW(!((AC ^ src) & 0x80) && ((AC ^ temp) & 0x80));
	if (temp > 0x99) temp += 96;
	SET_CARRY(temp > 0x99);
    } else {
	SET_SIGN(temp);
	SET_OVERFLOW(!((AC ^ src) & 0x80) && ((AC ^ temp) & 0x80));
	SET_CARRY(temp > 0xff);
    }
    AC = ((BYTE) temp);

/* AND */
    src &= AC;
    SET_SIGN(src);
    SET_ZERO(src);
    AC = src;

/* ASL */
    SET_CARRY(src & 0x80);
    src <<= 1;
    src &= 0xff;
    SET_SIGN(src);
    SET_ZERO(src);
    STORE src in memory or accumulator depending on addressing mode.

/* BCC */
    if (!IF_CARRY()) {
	clk += ((PC & 0xFF00) != (REL_ADDR(PC, src) & 0xFF00) ? 2 : 1);
	PC = REL_ADDR(PC, src);
    }

/* BCS */
    if (IF_CARRY()) {
	clk += ((PC & 0xFF00) != (REL_ADDR(PC, src) & 0xFF00) ? 2 : 1);
	PC = REL_ADDR(PC, src);
    }

/* BEQ */
    if (IF_ZERO()) {
	clk += ((PC & 0xFF00) != (REL_ADDR(PC, src) & 0xFF00) ? 2 : 1);
	PC = REL_ADDR(PC, src);
    }

/* BIT */
    SET_SIGN(src);
    SET_OVERFLOW(0x40 & src);	/* Copy bit 6 to OVERFLOW flag. */
    SET_ZERO(src & AC);

/* BMI */
    if (IF_SIGN()) {
	clk += ((PC & 0xFF00) != (REL_ADDR(PC, src) & 0xFF00) ? 2 : 1);
	PC = REL_ADDR(PC, src);
    }

/* BNE */
    if (!IF_ZERO()) {
	clk += ((PC & 0xFF00) != (REL_ADDR(PC, src) & 0xFF00) ? 2 : 1);
	PC = REL_ADDR(PC, src);
    }

/* BPL */
    if (!IF_SIGN()) {
	clk += ((PC & 0xFF00) != (REL_ADDR(PC, src) & 0xFF00) ? 2 : 1);
	PC = REL_ADDR(PC, src);
    }

/* BRK */
    PC++;
    PUSH((PC >> 8) & 0xff);	/* Push return address onto the stack. */
    PUSH(PC & 0xff);
    SET_BREAK((1));             /* Set BFlag before pushing */
    PUSH(SR);
    SET_INTERRUPT((1));
    PC = (LOAD(0xFFFE) | (LOAD(0xFFFF) << 8));

/* BVC */
    if (!IF_OVERFLOW()) {
	clk += ((PC & 0xFF00) != (REL_ADDR(PC, src) & 0xFF00) ? 2 : 1);
	PC = REL_ADDR(PC, src);
    }

/* BVS */
    if (IF_OVERFLOW()) {
	clk += ((PC & 0xFF00) != (REL_ADDR(PC, src) & 0xFF00) ? 2 : 1);
	PC = REL_ADDR(PC, src);
    }

/* CLC */
    SET_CARRY((0));

/* CLD */
    SET_DECIMAL((0));

/* CLI */
    SET_INTERRUPT((0));

/* CLV */
    SET_OVERFLOW((0));

/* CMP */
    src = AC - src;
    SET_CARRY(src < 0x100);
    SET_SIGN(src);
    SET_ZERO(src &= 0xff);

/* CPX */
    src = XR - src;
    SET_CARRY(src < 0x100);
    SET_SIGN(src);
    SET_ZERO(src &= 0xff);

/* CPY */
    src = YR - src;
    SET_CARRY(src < 0x100);
    SET_SIGN(src);
    SET_ZERO(src &= 0xff);

/* DEC */
    src = (src - 1) & 0xff;
    SET_SIGN(src);
    SET_ZERO(src);
    STORE(address, (src));

/* DEX */
    unsigned src = XR;
    src = (src - 1) & 0xff;
    SET_SIGN(src);
    SET_ZERO(src);
    XR = (src);

/* DEY */
    unsigned src = YR;
    src = (src - 1) & 0xff;
    SET_SIGN(src);
    SET_ZERO(src);
    YR = (src);

/* EOR */
    src ^= AC;
    SET_SIGN(src);
    SET_ZERO(src);
    AC = src;

/* INC */
    src = (src + 1) & 0xff;
    SET_SIGN(src);
    SET_ZERO(src);
    STORE(address, (src));

/* INX */
    unsigned src = XR;
    src = (src + 1) & 0xff;
    SET_SIGN(src);
    SET_ZERO(src); 
    XR = (src);

/* INY */
    unsigned src = YR;
    src = (src + 1) & 0xff;
    SET_SIGN(src);
    SET_ZERO(src);
    YR = (src);

/* JMP */
    PC = (src);

/* JSR */
    PC--;
    PUSH((PC >> 8) & 0xff);	/* Push return address onto the stack. */
    PUSH(PC & 0xff);
    PC = (src);

/* LDA */
    SET_SIGN(src);
    SET_ZERO(src);
    AC = (src);

/* LDX */
    SET_SIGN(src);
    SET_ZERO(src);
    XR = (src);

/* LDY */
    SET_SIGN(src);
    SET_ZERO(src);
    YR = (src);

/* LSR */
    SET_CARRY(src & 0x01);
    src >>= 1;
    SET_SIGN(src);
    SET_ZERO(src);
    STORE src in memory or accumulator depending on addressing mode.

/* NOP */
    Nothing.

/* ORA */
    src |= AC;
    SET_SIGN(src);
    SET_ZERO(src);
    AC = src;

/* PHA */
    src = AC;
    PUSH(src);

/* PHP */
    src = GET_SR;
    PUSH(src);

/* PLA */
    src = PULL();
    SET_SIGN(src);	/* Change sign and zero flag accordingly. */
    SET_ZERO(src);

/* PLP */
    src = PULL();
    SET_SR((src));

/* ROL */
    src <<= 1;
    if (IF_CARRY()) src |= 0x1;
    SET_CARRY(src > 0xff);
    src &= 0xff;
    SET_SIGN(src);
    SET_ZERO(src);
    STORE src in memory or accumulator depending on addressing mode.

/* ROR */
    if (IF_CARRY()) src |= 0x100;
    SET_CARRY(src & 0x01);
    src >>= 1;
    SET_SIGN(src);
    SET_ZERO(src);
    STORE src in memory or accumulator depending on addressing mode.

/* RTI */
    src = PULL();
    SET_SR(src);
    src = PULL();
    src |= (PULL() << 8);	/* Load return address from stack. */
    PC = (src);

/* RTS */
    src = PULL();
    src += ((PULL()) << 8) + 1;	/* Load return address from stack and add 1. */
    PC = (src);

/* SBC */
    unsigned int temp = AC - src - (IF_CARRY() ? 0 : 1);
    SET_SIGN(temp);
    SET_ZERO(temp & 0xff);	/* Sign and Zero are invalid in decimal mode */
    SET_OVERFLOW(((AC ^ temp) & 0x80) && ((AC ^ src) & 0x80));
    if (IF_DECIMAL()) {
	if ( ((AC & 0xf) - (IF_CARRY() ? 0 : 1)) < (src & 0xf)) /* EP */ temp -= 6;
	if (temp > 0x99) temp -= 0x60;
    }
    SET_CARRY(temp < 0x100);
    AC = (temp & 0xff);

/* SEC */
    SET_CARRY((1));

/* SED */
    SET_DECIMAL((1));

/* SEI */
    SET_INTERRUPT((1));

/* STA */
    STORE(address, (src));

/* STX */
    STORE(address, (src));

/* STY */
    STORE(address, (src));

/* TAX */
    unsigned src = AC;
    SET_SIGN(src);
    SET_ZERO(src);
    XR = (src);

/* TAY */
    unsigned src = AC;
    SET_SIGN(src);
    SET_ZERO(src);
    YR = (src);

/* TSX */
    unsigned src = SP;
    SET_SIGN(src);
    SET_ZERO(src);
    XR = (src);

/* TXA */
    unsigned src = XR;
    SET_SIGN(src);
    SET_ZERO(src);
    AC = (src);

/* TXS */
    unsigned src = XR;
    SP = (src);

/* TYA */
    unsigned src = YR;
    SET_SIGN(src);
    SET_ZERO(src);
    AC = (src);

