Addressing Modes

• Relative Addressing and Branches
• Indirect Addressing
• Indexed Addressing
• 6502 addressing modes
• 6800 addressing modes
• 8085 addressing modes
• x86 addressing modes

Relative Addressing

• A compiler or assembler produces code
• sometimes (especially without virtual memory) we want to be able to put this code at different places in memory
• sometimes we even want to move it around at runtime, to make room for other things (overlays)
• this is easiest if all references to labels in the code are relative to the PC/IP
• this also tends to save memory, since most jumps are to nearby locations and the offset can be represented with one byte

Relative Branches

• a relative branch always counts from the instruction following the branch
• that is because the PC/IP is incremented before the instruction execution

0123          BRA 02
0124                   ! offset byte
0125          BRA FE  ! infinite loop
0126                   ! offset byte
0127 label:   INC      ! target of first branch

Indirect Addressing

• in direct addressing, the memory location of the operand is in the two/four bytes following the opcode
• in indirect addressing, the memory location of a pointer to the operand is in the two/four bytes following the opcode
• for example, the following code jumps to the location pointed to by the two bytes at location 1000₁₆:

0123          JMP (1000)

• in register indirect addressing, a register is used as a pointer

Indexed Addressing

• Index register
• constant (immediate) offset
• the two are added together to give the effective address
• the operand is found at the effective address
• 6800 example using index register X with offset 19:

LDAB 19,X

6502 Addressing Modes

• relative addressing only available for branches
• indirect addressing only available for JMP instruction (JMP has absolute and indirect modes)
• in-class exercise: what is the indirect jump useful for?

More 6502 Addressing Modes

• zero page, X (Y): 8-bit offset added to 8-bit X (Y) register, truncated to 8 bits
• absolute, X (Y): 16-bit offset added to 8-bit X (Y) register truncated to 16 bits
• indirect indexed LDA (aa), Y: aa and aa+1 contain a 16-bit pointer which is added to Y to give the effective address
• indexed indirect LDA (aa,X): aa+X and aa+X+1 contain a 16-bit pointer to the effective address
• in-class exercise: find a use for indirect indexed and a use for indexed indirect

6800 and 8085 Addressing Modes

• 6800 relative addressing: jumps and subroutine calls
• 6800 indexed addressing LDAA aa, X: the effective address is the 16-bit sum aa + X (index registers on 6800 are 16 bits)
• 8085 register indirect: 16-bit register pair, usually HL, holds the effective address

x86 Addressing Modes

• register indirect addressing MOV AL, [BX]: registers BP (uses stack segment), BX, SI, DI (data segment) EA is the contents of the register (plus segment)
• register relative addressing MOV AL, [BX + 89AB]: EA is the 16- (32-) bit sum of the contents of BX and the constant offset (plus segment)
• program relative addressing: JMP 333 at location 300 has offset 31
• program indirect addressing: JMP [BX+100] jumps to the address found at memory location pointed to by BX
• base plus index addressing: MOV AX, [BX+DI]
• base relative plus index addressing: MOV AX, [BX+SI+10]