NMOS 6502 Opcodes

by John Pickens, Updated by Bruce Clark and by Ed Spittles

INDEX

ADC (ADd with Carry)

Affects Flags: N V Z C

MODE SYNTAX HEX LEN TIM Immediate ADC #$44 $69 2 2 Zero Page ADC $44 $65 2 3 Zero Page,X ADC $44,X $75 2 4 Absolute ADC $4400 $6D 3 4 Absolute,X ADC $4400,X $7D 3 4+ Absolute,Y ADC $4400,Y $79 3 4+ Indirect,X ADC ($44,X) $61 2 6 Indirect,Y ADC ($44),Y $71 2 5+ + add 1 cycle if page boundary crossed

There is no way to add without carry.

AND (bitwise AND with accumulator)

Affects Flags: N Z

MODE SYNTAX HEX LEN TIM Immediate AND #$44 $29 2 2 Zero Page AND $44 $25 2 3 Zero Page,X AND $44,X $35 2 4 Absolute AND $4400 $2D 3 4 Absolute,X AND $4400,X $3D 3 4+ Absolute,Y AND $4400,Y $39 3 4+ Indirect,X AND ($44,X) $21 2 6 Indirect,Y AND ($44),Y $31 2 5+ + add 1 cycle if page boundary crossed

ASL (Arithmetic Shift Left)

Affects Flags: N Z C

MODE SYNTAX HEX LEN TIM Accumulator ASL A $0A 1 2 Zero Page ASL $44 $06 2 5 Zero Page,X ASL $44,X $16 2 6 Absolute ASL $4400 $0E 3 6 Absolute,X ASL $4400,X $1E 3 7

BIT (test BITs)

Affects Flags: N V Z

MODE SYNTAX HEX LEN TIM Zero Page BIT $44 $24 2 3 Absolute BIT $4400 $2C 3 4

BIT is often used to skip one or two following bytes as in:

CLOSE1 LDX #$10 If entered here, we .BYTE $2C effectively perform CLOSE2 LDX #$20 a BIT test on $20A2, .BYTE $2C another one on $30A2, CLOSE3 LDX #$30 and end up with the X CLOSEX LDA #12 register still at $10 STA ICCOM,X upon arrival here.

Branch Instructions

Affect Flags: none

All branches are relative mode and have a length of two bytes. Syntax is "Bxx Displacement" or (better) "Bxx Label". See the notes on the Program Counter for more on displacements.

Branches are dependant on the status of the flag bits when the op code is encountered. A branch not taken requires two machine cycles. Add one if the branch is taken and add one more if the branch crosses a page boundary.

MNEMONIC HEX BPL (Branch on PLus) $10 BMI (Branch on MInus) $30 BVC (Branch on oVerflow Clear) $50 BVS (Branch on oVerflow Set) $70 BCC (Branch on Carry Clear) $90 BCS (Branch on Carry Set) $B0 BNE (Branch on Not Equal) $D0 BEQ (Branch on EQual) $F0

A page boundary crossing occurs when the branch destination is on a different page than the instruction AFTER the branch instruction. For example:

SEC BCS LABEL NOP

CLV BVC LABEL LABEL NOP

BRK (BReaK)

Affects Flags: B

MODE SYNTAX HEX LEN TIM Implied BRK $00 1 7

CMP (CoMPare accumulator)

Affects Flags: N Z C

MODE SYNTAX HEX LEN TIM Immediate CMP #$44 $C9 2 2 Zero Page CMP $44 $C5 2 3 Zero Page,X CMP $44,X $D5 2 4 Absolute CMP $4400 $CD 3 4 Absolute,X CMP $4400,X $DD 3 4+ Absolute,Y CMP $4400,Y $D9 3 4+ Indirect,X CMP ($44,X) $C1 2 6 Indirect,Y CMP ($44),Y $D1 2 5+ + add 1 cycle if page boundary crossed

CPX (ComPare X register)

Affects Flags: N Z C

MODE SYNTAX HEX LEN TIM Immediate CPX #$44 $E0 2 2 Zero Page CPX $44 $E4 2 3 Absolute CPX $4400 $EC 3 4

CPY (ComPare Y register)

Affects Flags: N Z C

MODE SYNTAX HEX LEN TIM Immediate CPY #$44 $C0 2 2 Zero Page CPY $44 $C4 2 3 Absolute CPY $4400 $CC 3 4

DEC (DECrement memory)

Affects Flags: N Z

MODE SYNTAX HEX LEN TIM Zero Page DEC $44 $C6 2 5 Zero Page,X DEC $44,X $D6 2 6 Absolute DEC $4400 $CE 3 6 Absolute,X DEC $4400,X $DE 3 7

EOR (bitwise Exclusive OR)

Affects Flags: N Z

MODE SYNTAX HEX LEN TIM Immediate EOR #$44 $49 2 2 Zero Page EOR $44 $45 2 3 Zero Page,X EOR $44,X $55 2 4 Absolute EOR $4400 $4D 3 4 Absolute,X EOR $4400,X $5D 3 4+ Absolute,Y EOR $4400,Y $59 3 4+ Indirect,X EOR ($44,X) $41 2 6 Indirect,Y EOR ($44),Y $51 2 5+ + add 1 cycle if page boundary crossed

Flag (Processor Status) Instructions

Affect Flags: as noted

These instructions are implied mode, have a length of one byte and require two machine cycles.

MNEMONIC HEX CLC (CLear Carry) $18 SEC (SEt Carry) $38 CLI (CLear Interrupt) $58 SEI (SEt Interrupt) $78 CLV (CLear oVerflow) $B8 CLD (CLear Decimal) $D8 SED (SEt Decimal) $F8

The Interrupt flag is used to prevent (SEI) or enable (CLI) maskable interrupts (aka IRQ's). It does not signal the presence or absence of an interrupt condition. The 6502 will set this flag automatically in response to an interrupt and restore it to its prior status on completion of the interrupt service routine. If you want your interrupt service routine to permit other maskable interrupts, you must clear the I flag in your code.

The Decimal flag controls how the 6502 adds and subtracts. If set, arithmetic is carried out in packed binary coded decimal. This flag is unchanged by interrupts and is unknown on power-up. The implication is that a CLD should be included in boot or interrupt coding.

The Overflow flag is generally misunderstood and therefore under-utilised. After an ADC or SBC instruction, the overflow flag will be set if the twos complement result is less than -128 or greater than +127, and it will cleared otherwise. In twos complement, $80 through $FF represents -128 through -1, and $00 through $7F represents 0 through +127. Thus, after:

CLC LDA #$7F ; +127 ADC #$01 ; + +1

CLC LDA #$81 ; -127 ADC #$FF ; + -1

INC (INCrement memory)

Affects Flags: N Z

MODE SYNTAX HEX LEN TIM Zero Page INC $44 $E6 2 5 Zero Page,X INC $44,X $F6 2 6 Absolute INC $4400 $EE 3 6 Absolute,X INC $4400,X $FE 3 7

JMP (JuMP)

Affects Flags: none

MODE SYNTAX HEX LEN TIM Absolute JMP $5597 $4C 3 3 Indirect JMP ($5597) $6C 3 5

AN INDIRECT JUMP MUST NEVER USE A VECTOR BEGINNING ON THE LAST BYTE OF A PAGE

JSR (Jump to SubRoutine)

Affects Flags: none

MODE SYNTAX HEX LEN TIM Absolute JSR $5597 $20 3 6

LDA (LoaD Accumulator)

Affects Flags: N Z

MODE SYNTAX HEX LEN TIM Immediate LDA #$44 $A9 2 2 Zero Page LDA $44 $A5 2 3 Zero Page,X LDA $44,X $B5 2 4 Absolute LDA $4400 $AD 3 4 Absolute,X LDA $4400,X $BD 3 4+ Absolute,Y LDA $4400,Y $B9 3 4+ Indirect,X LDA ($44,X) $A1 2 6 Indirect,Y LDA ($44),Y $B1 2 5+ + add 1 cycle if page boundary crossed

LDX (LoaD X register)

Affects Flags: N Z

MODE SYNTAX HEX LEN TIM Immediate LDX #$44 $A2 2 2 Zero Page LDX $44 $A6 2 3 Zero Page,Y LDX $44,Y $B6 2 4 Absolute LDX $4400 $AE 3 4 Absolute,Y LDX $4400,Y $BE 3 4+ + add 1 cycle if page boundary crossed

LDY (LoaD Y register)

Affects Flags: N Z

MODE SYNTAX HEX LEN TIM Immediate LDY #$44 $A0 2 2 Zero Page LDY $44 $A4 2 3 Zero Page,X LDY $44,X $B4 2 4 Absolute LDY $4400 $AC 3 4 Absolute,X LDY $4400,X $BC 3 4+ + add 1 cycle if page boundary crossed

LSR (Logical Shift Right)

Affects Flags: N Z C

MODE SYNTAX HEX LEN TIM Accumulator LSR A $4A 1 2 Zero Page LSR $44 $46 2 5 Zero Page,X LSR $44,X $56 2 6 Absolute LSR $4400 $4E 3 6 Absolute,X LSR $4400,X $5E 3 7

Wrap-Around

Use caution with indexed zero page operations as they are subject to wrap-around. For example, if the X register holds $FF and you execute LDA $80,X you will not access $017F as you might expect; instead you access $7F i.e. $80-1. This characteristic can be used to advantage but make sure your code is well commented.

It is possible, however, to access $017F when X = $FF by using the Absolute,X addressing mode of LDA $80,X. That is, instead of:

LDA $80,X ; ZeroPage,X - the resulting object code is: B5 80

LDA $0080,X ; Absolute,X - the resulting object code is: BD 80 00

.BYTE $BD,$80,$00 ; LDA $0080,X (absolute,X addressing mode)

In cases where you are writing code that will be relocated you must consider wrap-around when assigning dummy values for addresses that will be adjusted. Both zero and the semi-standard $FFFF should be avoided for dummy labels. The use of zero or zero page values will result in assembled code with zero page opcodes when you wanted absolute codes. With $FFFF, the problem is in addresses+1 as you wrap around to page 0.

Program Counter

When the 6502 is ready for the next instruction it increments the program counter before fetching the instruction. Once it has the op code, it increments the program counter by the length of the operand, if any. This must be accounted for when calculating branches or when pushing bytes to create a false return address (i.e. jump table addresses are made up of addresses-1 when it is intended to use an RTS rather than a JMP).

The program counter is loaded least signifigant byte first. Therefore the most signifigant byte must be pushed first when creating a false return address.

When calculating branches a forward branch of 6 skips the following 6 bytes so, effectively the program counter points to the address that is 8 bytes beyond the address of the branch opcode; and a backward branch of $FA (256-6) goes to an address 4 bytes before the branch instruction.

Execution Times

Op code execution times are measured in machine cycles; one machine cycle equals one clock cycle. Many instructions require one extra cycle for execution if a page boundary is crossed; these are indicated by a + following the time values shown.

NOP (No OPeration)

Affects Flags: none

MODE SYNTAX HEX LEN TIM Implied NOP $EA 1 2

ORA (bitwise OR with Accumulator)

Affects Flags: N Z

MODE SYNTAX HEX LEN TIM Immediate ORA #$44 $09 2 2 Zero Page ORA $44 $05 2 3 Zero Page,X ORA $44,X $15 2 4 Absolute ORA $4400 $0D 3 4 Absolute,X ORA $4400,X $1D 3 4+ Absolute,Y ORA $4400,Y $19 3 4+ Indirect,X ORA ($44,X) $01 2 6 Indirect,Y ORA ($44),Y $11 2 5+ + add 1 cycle if page boundary crossed

Register Instructions

Affect Flags: N Z

These instructions are implied mode, have a length of one byte and require two machine cycles.

MNEMONIC HEX TAX (Transfer A to X) $AA TXA (Transfer X to A) $8A DEX (DEcrement X) $CA INX (INcrement X) $E8 TAY (Transfer A to Y) $A8 TYA (Transfer Y to A) $98 DEY (DEcrement Y) $88 INY (INcrement Y) $C8

ROL (ROtate Left)

Affects Flags: N Z C

MODE SYNTAX HEX LEN TIM Accumulator ROL A $2A 1 2 Zero Page ROL $44 $26 2 5 Zero Page,X ROL $44,X $36 2 6 Absolute ROL $4400 $2E 3 6 Absolute,X ROL $4400,X $3E 3 7

ROR (ROtate Right)

Affects Flags: N Z C

MODE SYNTAX HEX LEN TIM Accumulator ROR A $6A 1 2 Zero Page ROR $44 $66 2 5 Zero Page,X ROR $44,X $76 2 6 Absolute ROR $4400 $6E 3 6 Absolute,X ROR $4400,X $7E 3 7

RTI (ReTurn from Interrupt)

Affects Flags: all

MODE SYNTAX HEX LEN TIM Implied RTI $40 1 6

Note that unlike RTS, the return address on the stack is the actual address rather than the address-1.

RTS (ReTurn from Subroutine)

Affects Flags: none

MODE SYNTAX HEX LEN TIM Implied RTS $60 1 6

RTS is frequently used to implement a jump table where addresses-1 are pushed onto the stack and accessed via RTS eg. to access the second of four routines:

LDX #1 JSR EXEC JMP SOMEWHERE LOBYTE .BYTE <ROUTINE0-1,<ROUTINE1-1 .BYTE <ROUTINE2-1,<ROUTINE3-1 HIBYTE .BYTE >ROUTINE0-1,>ROUTINE1-1 .BYTE >ROUTINE2-1,>ROUTINE3-1 EXEC LDA HIBYTE,X PHA LDA LOBYTE,X PHA RTS

SBC (SuBtract with Carry)

Affects Flags: N V Z C

MODE SYNTAX HEX LEN TIM Immediate SBC #$44 $E9 2 2 Zero Page SBC $44 $E5 2 3 Zero Page,X SBC $44,X $F5 2 4 Absolute SBC $4400 $ED 3 4 Absolute,X SBC $4400,X $FD 3 4+ Absolute,Y SBC $4400,Y $F9 3 4+ Indirect,X SBC ($44,X) $E1 2 6 Indirect,Y SBC ($44),Y $F1 2 5+ + add 1 cycle if page boundary crossed

There is no way to subtract without the carry which works as an inverse borrow. i.e, to subtract you set the carry before the operation. If the carry is cleared by the operation, it indicates a borrow occurred.

STA (STore Accumulator)

Affects Flags: none

MODE SYNTAX HEX LEN TIM Zero Page STA $44 $85 2 3 Zero Page,X STA $44,X $95 2 4 Absolute STA $4400 $8D 3 4 Absolute,X STA $4400,X $9D 3 5 Absolute,Y STA $4400,Y $99 3 5 Indirect,X STA ($44,X) $81 2 6 Indirect,Y STA ($44),Y $91 2 6

Stack Instructions

These instructions are implied mode, have a length of one byte and require machine cycles as indicated. The "PuLl" operations are known as "POP" on most other microprocessors. With the 6502, the stack is always on page one ($100-$1FF) and works top down.

MNEMONIC HEX TIM TXS (Transfer X to Stack ptr) $9A 2 TSX (Transfer Stack ptr to X) $BA 2 PHA (PusH Accumulator) $48 3 PLA (PuLl Accumulator) $68 4 PHP (PusH Processor status) $08 3 PLP (PuLl Processor status) $28 4

STX (STore X register)

Affects Flags: none

MODE SYNTAX HEX LEN TIM Zero Page STX $44 $86 2 3 Zero Page,Y STX $44,Y $96 2 4 Absolute STX $4400 $8E 3 4

STY (STore Y register)

Affects Flags: none

MODE SYNTAX HEX LEN TIM Zero Page STY $44 $84 2 3 Zero Page,X STY $44,X $94 2 4 Absolute STY $4400 $8C 3 4