Instruction Reference

Click on any of following links to go straight to the information for that instruction.

ADC - Add with Carry

A,Z,C,N = A+M+C

This instruction adds the contents of a memory location to the accumulator together with the carry bit. If overflow occurs the carry bit is set, this enables multiple byte addition to be performed.

Processor Status after use:

See also: SBC

AND - Logical AND

A,Z,N = A&M

A logical AND is performed, bit by bit, on the accumulator contents using the contents of a byte of memory.

Processor Status after use:

See also: EOR, ORA

ASL - Arithmetic Shift Left

A,Z,C,N = M*2 or M,Z,C,N = M*2

This operation shifts all the bits of the accumulator or memory contents one bit left. Bit 0 is set to 0 and bit 7 is placed in the carry flag. The effect of this operation is to multiply the memory contents by 2 (ignoring 2's complement considerations), setting the carry if the result will not fit in 8 bits.

Processor Status after use:

See also: LSR, ROL, ROR

BCC - Branch if Carry Clear

If the carry flag is clear then add the relative displacement to the program counter to cause a branch to a new location.

Processor Status after use:

See also: BCS

BCS - Branch if Carry Set

If the carry flag is set then add the relative displacement to the program counter to cause a branch to a new location.

Processor Status after use:

See also: BCC

BEQ - Branch if Equal

If the zero flag is set then add the relative displacement to the program counter to cause a branch to a new location.

Processor Status after use:

See also: BNE

BIT - Bit Test

A & M, N = M7, V = M6

This instructions is used to test if one or more bits are set in a target memory location. The mask pattern in A is ANDed with the value in memory to set or clear the zero flag, but the result is not kept. Bits 7 and 6 of the value from memory are copied into the N and V flags.

Processor Status after use:

BMI - Branch if Minus

If the negative flag is set then add the relative displacement to the program counter to cause a branch to a new location.

Processor Status after use:

See also: BPL

BNE - Branch if Not Equal

If the zero flag is clear then add the relative displacement to the program counter to cause a branch to a new location.

Processor Status after use:

See also: BEQ

BPL - Branch if Positive

If the negative flag is clear then add the relative displacement to the program counter to cause a branch to a new location.

Processor Status after use:

See also: BMI

BRK - Force Interrupt

The BRK instruction forces the generation of an interrupt request. The program counter and processor status are pushed on the stack then the IRQ interrupt vector at $FFFE/F is loaded into the PC and the break flag in the status set to one.

The interpretation of a BRK depends on the operating system. On the BBC Microcomputer it is used by language ROMs to signal run time errors but it could be used for other purposes (e.g. calling operating system functions, etc.).

BVC - Branch if Overflow Clear

If the overflow flag is clear then add the relative displacement to the program counter to cause a branch to a new location.

Processor Status after use:

See also: BVS

BVS - Branch if Overflow Set

If the overflow flag is set then add the relative displacement to the program counter to cause a branch to a new location.

Processor Status after use:

See also: BVC

CLC - Clear Carry Flag

C = 0

Set the carry flag to zero.

See also: SEC

CLD - Clear Decimal Mode

D = 0

Sets the decimal mode flag to zero.

NB:
The state of the decimal flag is uncertain when the CPU is powered up and it is not reset when an interrupt is generated. In both cases you should include an explicit CLD to ensure that the flag is cleared before performing addition or subtraction.

See also: SED

CLI - Clear Interrupt Disable

I = 0

Clears the interrupt disable flag allowing normal interrupt requests to be serviced.

See also: SEI

CLV - Clear Overflow Flag

V = 0

Clears the overflow flag.

CMP - Compare

Z,C,N = A-M

This instruction compares the contents of the accumulator with another memory held value and sets the zero and carry flags as appropriate.

Processor Status after use:

See also: CPX, CPY

CPX - Compare X Register

Z,C,N = X-M

This instruction compares the contents of the X register with another memory held value and sets the zero and carry flags as appropriate.

Processor Status after use:

See also: CMP, CPY

CPY - Compare Y Register

Z,C,N = Y-M

This instruction compares the contents of the Y register with another memory held value and sets the zero and carry flags as appropriate.

Processor Status after use:

See also: CMP, CPX

DEC - Decrement Memory

M,Z,N = M-1

Subtracts one from the value held at a specified memory location setting the zero and negative flags as appropriate.

Processor Status after use:

See also: DEX, DEY

DEX - Decrement X Register

X,Z,N = X-1

Subtracts one from the X register setting the zero and negative flags as appropriate.

Processor Status after use:

See also: DEC, DEY

DEY - Decrement Y Register

Y,Z,N = Y-1

Subtracts one from the Y register setting the zero and negative flags as appropriate.

Processor Status after use:

See also: DEC, DEX

EOR - Exclusive OR

A,Z,N = A^M

An exclusive OR is performed, bit by bit, on the accumulator contents using the contents of a byte of memory.

Processor Status after use:

See also: AND, ORA

INC - Increment Memory

M,Z,N = M+1

Adds one to the value held at a specified memory location setting the zero and negative flags as appropriate.

Processor Status after use:

See also: INX, INY

INX - Increment X Register

X,Z,N = X+1

Adds one to the X register setting the zero and negative flags as appropriate.

Processor Status after use:

See also: INC, INY

INY - Increment Y Register

Y,Z,N = Y+1

Adds one to the Y register setting the zero and negative flags as appropriate.

Processor Status after use:

See also: INC, INX

JMP - Jump

Sets the program counter to the address specified by the operand.

Processor Status after use:

NB:
An original 6502 has does not correctly fetch the target address if the indirect vector falls on a page boundary (e.g. $xxFF where xx is any value from $00 to $FF). In this case fetches the LSB from $xxFF as expected but takes the MSB from $xx00. This is fixed in some later chips like the 65SC02 so for compatibility always ensure the indirect vector is not at the end of the page.

JSR - Jump to Subroutine

The JSR instruction pushes the address (minus one) of the return point on to the stack and then sets the program counter to the target memory address.

Processor Status after use:

See also: RTS

LDA - Load Accumulator

A,Z,N = M

Loads a byte of memory into the accumulator setting the zero and negative flags as appropriate.

See also: LDX, LDY

LDX - Load X Register

X,Z,N = M

Loads a byte of memory into the X register setting the zero and negative flags as appropriate.

See also: LDA, LDY

LDY - Load Y Register

Y,Z,N = M

Loads a byte of memory into the Y register setting the zero and negative flags as appropriate.

See also: LDA, LDX

LSR - Logical Shift Right

A,C,Z,N = A/2 or M,C,Z,N = M/2

Each of the bits in A or M is shift one place to the right. The bit that was in bit 0 is shifted into the carry flag. Bit 7 is set to zero.

Processor Status after use:

See also: ASL, ROL, ROR

NOP - No Operation

The NOP instruction causes no changes to the processor other than the normal incrementing of the program counter to the next instruction.

Processor Status after use:

ORA - Logical Inclusive OR

A,Z,N = A|M

An inclusive OR is performed, bit by bit, on the accumulator contents using the contents of a byte of memory.

Processor Status after use:

See also: AND, EOR

PHA - Push Accumulator

Pushes a copy of the accumulator on to the stack.

Processor Status after use:

See also: PLA

PHP - Push Processor Status

Pushes a copy of the status flags on to the stack.

Processor Status after use:

See also: PLP

PLA - Pull Accumulator

Pulls an 8 bit value from the stack and into the accumulator. The zero and negative flags are set as appropriate.

See also: PHA

PLP - Pull Processor Status

Pulls an 8 bit value from the stack and into the processor flags. The flags will take on new states as determined by the value pulled.

Processor Status after use:

See also: PHP

ROL - Rotate Left

Move each of the bits in either A or M one place to the left. Bit 0 is filled with the current value of the carry flag whilst the old bit 7 becomes the new carry flag value.

Processor Status after use:

See also: ASL, LSR, ROR

ROR - Rotate Right

Move each of the bits in either A or M one place to the right. Bit 7 is filled with the current value of the carry flag whilst the old bit 0 becomes the new carry flag value.

Processor Status after use:

See also ASL, LSR, ROL

RTI - Return from Interrupt

The RTI instruction is used at the end of an interrupt processing routine. It pulls the processor flags from the stack followed by the program counter.

Processor Status after use:

RTS - Return from Subroutine

The RTS instruction is used at the end of a subroutine to return to the calling routine. It pulls the program counter (minus one) from the stack.

Processor Status after use:

See also: JSR

SBC - Subtract with Carry

A,Z,C,N = A-M-(1-C)

This instruction subtracts the contents of a memory location to the accumulator together with the not of the carry bit. If overflow occurs the carry bit is clear, this enables multiple byte subtraction to be performed.

Processor Status after use:

See also: ADC

SEC - Set Carry Flag

C = 1

Set the carry flag to one.

See also: CLC

SED - Set Decimal Flag

D = 1

Set the decimal mode flag to one.

See also: CLD

SEI - Set Interrupt Disable

I = 1

Set the interrupt disable flag to one.

See also: CLI

STA - Store Accumulator

M = A

Stores the contents of the accumulator into memory.

Processor Status after use:

See also: STX, STY

STX - Store X Register

M = X

Stores the contents of the X register into memory.

Processor Status after use:

See also: STA, STY

STY - Store Y Register

M = Y

Stores the contents of the Y register into memory.

Processor Status after use:

See also: STA, STX

TAX - Transfer Accumulator to X

X = A

Copies the current contents of the accumulator into the X register and sets the zero and negative flags as appropriate.

Processor Status after use:

See also: TXA

TAY - Transfer Accumulator to Y

Y = A

Copies the current contents of the accumulator into the Y register and sets the zero and negative flags as appropriate.

Processor Status after use:

See also: TYA

TSX - Transfer Stack Pointer to X

X = S

Copies the current contents of the stack register into the X register and sets the zero and negative flags as appropriate.

Processor Status after use:

See also: TXS

TXA - Transfer X to Accumulator

A = X

Copies the current contents of the X register into the accumulator and sets the zero and negative flags as appropriate.

Processor Status after use:

See also: TAX

TXS - Transfer X to Stack Pointer

S = X

Copies the current contents of the X register into the stack register.

Processor Status after use:

See also: TSX

TYA - Transfer Y to Accumulator

A = Y

Copies the current contents of the Y register into the accumulator and sets the zero and negative flags as appropriate.

Processor Status after use:

See also: TAY

Contents

Originally this was all in one HTML file but it grew too big for any of my editors to handle without crashing so I had to cut it up into 8 sections.

• Architecture describes the few basic details of the processor.
• Registers goes over each of the internal registers and their use.
• Instructions gives a summary of whole instruction set.
• Addressing describes each of the 6502 memory addressing modes.
• Algorithms contains examples of basic 6502 coding.
• Reference describes the complete instruction set in detail.
• Links contains some links to other sites worth visiting.

Click on any of the above links or the links at the bottom of each page to move between the sections.