MOV Assembly Instruction

Question

What does this instruction do?

mov (%r11,%r12,1), %edx

Thanks.

Answer 1

Look here. It says

In the AT&T Syntax, memory is referenced in the following way,

segment-override:signed-offset(base,index,scale)

Down on the page there are some examples. I find this the best:

GAS memory operand   NASM memory operand
------------------   -------------------
(%ecx,%ebx,2)    [ecx+ebx*2]

mov source, destination in AT&T syntax copies the value from source to destination. Also consider the size of edx. How many bytes (4) do you think mov will copy ?

Answer 2

mov (%r11,%r12,1), %edx this instruction is use to calculate the address (indexed addressing mode).

• %r11 is a base adress
• %r12 is a index
• and 1 is a multipler

It's work like this:

adres = base adres + index * multipler

base adres and multipler must be constants (base adress can be register), index must be register.