Addressing modes in `mov`, and their C version

asm-addr-mode.md

Addressing modes in mov, and their C version

Below are addressing modes (immediate, register, memory) on the examples of the mov assembly instruction. Here we use movl ("move long") for a 32-bit word.

Note: cannot move memory-to-memory, need two instructions.

movl	Src	Dest	Src, Dest	C version
	Imm	Reg	movl $0x4,%eax	x = 0x4;
		Mem	movl $-5,(%eax)	*px = -5;
	Reg	Reg	movl %eax,%edx	x = y;
		Mem	movl %eax,(%edx)	*px = y;
	Mem	Reg	movl (%eax),%edx	y = *px;

General form of memory access

Also, the most general form of memory access is:

D(Rb, Ri, S)

That corresponds to the following memory address calculation:

Mem[Reg[Rb] + S * Reg[Ri] + D]

Where:

• D: Constant "displacement" (offset), 1, 2, or 4 bytes
• Rb: Base register: Any of the 8/16 integer registers
• Ri: Index register: Any, except for %esp/%rsp
• S: Scale: 1, 2, 4, 8 (numbers are to support arrays of values of these sizes)

Besides, special cases can use any combination of D, Rb, Ri, and S:

(Rb, Ri)        Mem[Reg[Rb] + Reg[Ri]]
D(Rb, Ri)       Mem[Reg[Rb] + Reg[Ri] + D]
(Rb, Ri, S)     Mem[Reg[Rb] + S * Reg[Ri]]

Examples:

• Parameters access on the stack:

movl 8(%ebp), %eax       ; parameter with offset 8 from the base pointer
movl 12(%ebp), %edx      ; parameter with offset 12 from the base pointer

• Address computation instruction: leal Src, Dest, "load effective address", where Src is address mode expression. The result is stored in the Dest.

leal (%edx, %ecx, 4), %eax

The leal can be used for:

• Computing addresses without a memory reference. E.g. translation of p = &x[i];
• Computing arithmetic expressions of the form x + k * i, where k is 1, 2, 4, or 8.