Last updated April 10, 2022

Status: Tags: #cards/cmpt295/assembly Links: x86-64 Assembly

Memory adressing modes

Principles

Help specify instruction operands

Use cases for q type ?

• Push/pop
• Adding displacement to address

Use cases for l type ?

• Moving values to registers

Use cases for b type

• Adding integers

Dereferencing is for ?

• Getting the adress

Variables

Immediate

• Constant int used as operand in instruction
• Operand syntax: Imm
• ex) movq $0x4 %rax, movb $-17. %al

Register

• used as operand in instruction
• Operand value: R[$r_a$]
• Operand syntax: %$r_a$
• ex) movq, %rax, %rdx

Memory adressing mode (expression) used as operand in an instruction

Types

?

• Absolute
• Indirect
• “Base + displacement”
• 2 indexed
• 4 scaled indexed

Absolute memory adressing mode

?

• Use memory adress as operand directly in instruction
  • Operand called immediate
• Operand syntax: Imm
• Effect: M[Imm]
• ex) call plus where plus is a function

Indirect memory adressing mode

?

• When a register contains an adress
  • Similar to pointer in C
• To access, use parentheses
  • ex) ($r_b$)
    • b is base,
• Effect: M[R[$r_b$]]
  • R is register

“Base + displacement” memory adressing mode

?

• General syntax: Imm($r_b$)
  • Call adress, then add value
    • Base is adress, displacement is value
• Effect: M[Imm + R[$r_b$]]
• ex)movq %rax, -8(%rsp)
  • leaq 7(%rdi), %rax
  • Why 8? alignment, make space onto stack

Indexed memory adressing mode

?

• Adding offset to a base adresss
  • Help visit every value
  • ideal to store and access values stored in arrays
• General syntax 1: ($r_b$, $r_1$)
  • Effect: M[R[$r_b$] + R[$r_i$]]
  • Example: movb (%rdi, %rcx), %al
  • r_i is an iteration index
• General syntax 2: Imm($r_b$, $r_i$)
  • Effect: M[Imm + R[$r_b$] + R[$r_i$]]
  • Example: movw 0xA(%rdi, %rcx), %r11w

Scaled indexed memory adressing mode

?

• s can only be 1,2,4,8

1. General Syntax: (, $r_i$, s)
   • Effect: M[R[$r_i$] * s]
   • ex) (, %rdi, 2)
2. General Syntax: Imm(,$r_i$, s)
   • Effect: M[Imm + R[$r_i$] * s]
   • ex) 3(, %rcx, 8)
3. General Syntax: ($r_b$, $r_i$, s)
   • Effect: M[R[$r_b$] + R[$r_i$] * s]
   • ex) (%rdi, %rsi, 4)
4. General Syntax: Imm($r_b$, $r_i$, s)
   • Effect: M[Imm + R[$r_b$] + R[$r_i$] * s]
   • ex) 8(%rdi, %rsi, 4)

• Leaq’s effect is to not reference any memory !!!

Example

Backlinks

1

list from Memory adressing modes AND !outgoing(Memory adressing modes)

References:

Created:: 2022-02-04 12:50