CODEWITHSUNDEEP
functionassemblyx86callstackcpu-registers
MasterMastic
MasterMastic

Reputation: 21306

Why should you keep ESP in EBP inside a call?

I'm reading in Professional Assembly Language by Richard Blum that when you enter a call you should copy the value of the ESP register to EBP, and he also provided the following template:

function_label:
    pushl %ebp
    movl %esp, %ebp
    < normal function code goes here>
    movl %ebp, %esp
    popl %ebp
    ret

I don't understand why this is necessary. When you push something inside the function, you obviously intend to pop it back, thus restoring ESP to it's original value.

So why have this template?
And what's the use of the EBP register anyway?

I'm obviously missing something, but what is it?

Upvotes: 0

Views: 772

Answers (1)

Hans Passant
Hans Passant

Reputation: 942518

When you push something inside the function, you obviously intend to pop it back

That's just part of the reason for using stack. The far more common usage is the one that's missing from your snippet, storing local variables. The next common code you see after setting up EBP is a substraction on ESP, equivalent to the amount of space required for local variable storage. That's of course easy to balance as well, just add the same amount back at the function epilogue. It gets more difficult when the code is also using things like C99 variable length arrays or the non-standard but commonly available _alloca() function. Being able to restore ESP from EBP makes this simple.

More to the point perhaps, it is not necessary to setup the stack frame like this. Most any x86 compiler supports an optimization option called "frame pointer omission". Turned on with GCC's -fomit-frame-pointer, /Oy on MSVC. Which makes the EBP register available for general usage, that can be very helpful on x86 with its dearth of cpu registers.

That optimization has a very grave disadvantage though. Without the EBP register pointing at the start of a stack frame, it gets very difficult to perform stack walks. That matters when you need to debug your code. A stack trace can be very important to find out how your code ended up crashing. Invaluable when you get a "core dump" of a crash from your customer. So valuable that Microsoft agreed to turn off the optimization on Windows binaries to give their customers a shot at diagnosing crashes.

Upvotes: 2

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