mov %eax,(%esp)

Question

What is the difference between the following statements?

mov %eax,%esp
mov %eax,(%esp)

I'm working on diffusing a binary bomb and am having trouble with a few of the mov and leal commands early on in the assembly.

One copies the register. The other loads from the memory it points to. — Mysticial, Oct 19 '12 at 15:58
@Mysticial: This is AT&T syntax, so everything is backwards -- %esp and (%esp) are the destinations, not the sources. — Jerry Coffin, Oct 19 '12 at 16:00
Other memory addressing-modes are possible, not just register-indirect. See [A couple of questions about \[base + index\*scale + disp\] and AT&T disp(base, index, scale)](https://stackoverflow.com/q/27936196) — Peter Cordes, Aug 07 '22 at 10:09

score 12 · Accepted Answer · answered Oct 19 '12 at 15:59

12

This copies the value in %eax into %esp.

mov %eax,%esp

This copies the value from %eax to the location in memory that %esp points to.

mov %eax,(%esp)

answered Oct 19 '12 at 15:59

Dirk Holsopple

The latter is essentially pop eax without the decrement of esp – Goaler444 Jul 11 '16 at 13:40
@Goaler444: No, it's storing to memory, so it's like a push, except actually overwriting the last dword pushed. – Peter Cordes Aug 07 '22 at 10:08

1 Answers1