assemblybuffer-overflow.net-security

Assembly MOV Instruction

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!

Solution

  • 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).