Question

I’m performing a couple of processes that already require the use of a source index. If I need to use a source index that’s already being used for another item, how can I do that? Is there a way to save edi as data and then retrieve it when necessary?

Thanks.

Answer 1

The stack is for that:

push %esi
push %edi
...
...
...
pop %edi
pop %esi

Always pay attention to the order. Also, use the appropiate version of push/pop.

If you need to save and access different values at the same time, making your own stackframe is a better solution.

Example (at the beginning of the function, AT&T syntax):

push %ebp  // save the old ebp
mov %esp, %ebp // ebp now points to the top of the stack
sub X, %ebp // reserve X bytes for the current function

Now you can reference memory locations in reference to %ebp, example:

movl $1, -4(%ebp) // moves constant 1 to the first free location
movl $1, -8(%ebp) // moves constant 1 to the next free location
addl -4(%ebp), -8(%ebp) // add -4(%ebp) and -8(%ebp) together and write in -8(%ebp)

Similiary, you could save your first variable in -4(%ebp) and your second in -8(%ebp).

To the destroy the stackframe and return:

movl %ebp, %esp
popl %ebp
ret

You don’t need to clear the memory, as it’ll be overwritten once that space is used again.