IA32 Assembly clarification

Question

I am working on an assignment that requires debugging AT&T assembly. I am having trouble with the following two lines, and just wanted to ask for a little clarification.

add    -0x8(%esi,%ebx,4),%eax
cmp    %eax,-0x4(%esi,%ebx,4)

I know that %esi holds the value input by the user, and both %eax and %ebx are equal to 2.

For the first line, I am interpreting this as "-8 + %esi + (%ebx*4) + %eax". Since I know %ebx is 2, this evaluates to %esi + %eax. So essentially, the add line just adds 2 to the input value.

The second line I know is comparing %eax to -0x4(%esi,%ebx,4). At this point %eax equals %esi + 2, and following the previous logic the right side value equals -4 + %esi + (%ebx*4) which would evaluate to %esi + 4 since %ebx is still equal to 2.

If I am correct, the that means the second line is comparing %esi + 2 to %esi + 4 which can never be true for any input value. Yet, it has to be true as per the assignment. Where am I going wrong in my translation?

Answer 1

Those are memory operands.

add    -0x8(%esi,%ebx,4),%eax

Adds the value stored at address esi + ebx*4 - 8 to eax.

cmp    %eax,-0x4(%esi,%ebx,4)

Compares the value stored at address esi + ebx*4 - 4 with eax.