Open main menu

CDOT Wiki β

6502 Addressing Modes

The 6502 processor has 13 Addressing Modes. Not all addressing modes are valid for every instruction.

Accumulator

The operation works on the accumulator. No additional data is required. Examples:

ASL A
LSR A
ROL A
ROR A

Note that the explicit A is not required by most assemblers, so you could write:

ASL
LSR
ROL
ROR

Absolute

Data is accessed using 16-bit address specified as a constant.

LDA $06d3
STX $0200

Absolute, X

Data is accessed using a 16-bit address specified as a constant, to which the value of the X register is added (with carry).

LDA $8000,x
STA $8000,x

Absolute, Y

Data is accessed using a 16-bit address specified as a constant, to which the value of the X register is added (with carry).

LDA $8000,y
STA $8000,y

Immediate

Data is taken from the byte following the opcode.

LDA #$05
ORA #$80

Implied

The data is implied by the operation. Examples:

CLC
RTS

Indirect

Data is accessed using a pointer. The 16-bit address of the pointer is given in the two bytes following the opcode.

JMP ($9000) ; Jumps to the location pointed to by addresses $9000 (low) and $9001 (high)

X, Indirect

An 8-bit zero-page address and the X register are added, without carry (if the addition overflows, the address wraps around within page 0). The resulting address is used as a pointer to the data being accessed. Note that, effectively, this makes the X register an index into a list of pointers.

LDA ($05,x)

Indirect, Y

An 8-bit address identifies a pointer. The value of the Y register is added to the address contained in the pointer. Effectively, the pointer is the base address and the Y register is an index past that base address.

LDA ($10),y

Relative

An 8-bit signed offset is provided. This value is added to the program counter (PC) to find the effective address.

BNE $0600 ; The value "$0600" is assembled into a signed offset. If the target must be in the range of (-128:127) bytes of the current PC value.

Zero page

An 8-bit address is provided within the zero page.

LDX $13
AND $07

Zero page, X

An 8-bit address is provided, to which the X register is added (without carry - if the addition overflows, the address wraps around within the zero page).

STA $00,x
LDA $00,x

Zero page, Y

An 8-bit address is provided, to which the Y register is added (without carry - if the addition overflows, the address wraps around within the zero page).

STA $00,y
LDA $00,y