CODEWITHSUNDEEP
assemblystack
francisaugusto
francisaugusto

Reputation: 1197

What happens when I move %esp to %ebp?

I know this is a pretty basic question, but here it goes:

I am starting to learn Assembler, and I'm trying to understand how the stack works.

At first, when I pass a value to an assembler function, I'd access it like this:

movl 4(%esp),%eax  # first parameter
movl 8(%esp),%ebx  # second parameter

But I then was told that it was better to do this:

push %ebp
movl %esp,%ebp
# and then I'd access the values on %ebp:
movl 8(%ebp),%eax
movl 12(%ebp),%eax
pop %ebp

Ok, what is the difference here? When I accessed the values directly from %esp, weren't they already on the stack? What's the advantage of using push to do it again?

Does anyone have a tip on a good learning tool (for dummies-type) on how to learn how stack works so one can learn on these stack pointers, return addresses and so on? I haven't found any good visual demonstration on how it works. Thanks!

Upvotes: 1

Views: 2099

Answers (2)

Martin Rosenau
Martin Rosenau

Reputation: 18503

In most cases using %ebp has historical reasons: In 16-bit programs instructions like "movw 2(%sp), %ax" are not supported by x86 CPUs because x86 CPUs only support %bx, %si, %di and %bp registers for memory addressing. So in 16-bit programs you cannot use %sp so you use %bp instead.

When using a well-optimizing modern compiler you will not see the "push" and "mov %esp, %ebp" instructions any more in 32-bit code but the code will look like your first example ("4(%esp)" instead of "8(%ebp)").

Such compilers sometimes use the %ebp register for different purposes.

There is one use case where %ebp still is necessary: The alloca() function: This function reserves memory on the stack by doing a "sub %eax, %esp" (or similar) instruction. After this instruction you do not know any longer where your arguments are.

However in 32-bit code with "flat" memory layout (%ds=%ss) you may also use any other register instead of %ebp.

Upvotes: 2

Sparky
Sparky

Reputation: 14057

Both the difference and the why has to do with stack frames. The following link has a fairly good summary (if I do say so myself).

What is stack frame in assembly?

Upvotes: 1

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