Assembly MOV Instruction

Question

Hi guys im working on a CTF challenge and don't quite understand this asm command:

mov    %edx,0x20(%esp,%eax,4)

Im assuming its: move $edx into (0x20 + $esp + $eax + 4)

However when I checked this with gdb it isn't correct. Anyone mind explaining how this instruction works?

Thanks for your help!

Answer 1

AT&T syntax for x86 memory references goes like this:

displacement(base,index,scale)

giving the address of displacement+base+(index*scale).

That is, destination memory address for your instuction is %ESP+0x20+(4*%EAX).