push local variables

Question

Alo

After I have read about function and stack from http://en.wikibooks.org/wiki/X86_Disassembly/Functions_and_Stack_Frames I have a question about local variables.

Snapshot from the article:

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push ebp     ; save the value of ebp
mov ebp, esp ; ebp now points to the top of the stack
sub esp, 12  ; space allocated on the stack for the local variables

This means local variables can be accessed by referencing ebp. Consider the following C code fragment and corresponding assembly code:

a = 10;
b = 5;
c = 2;

mov [ebp -  4], 10  ; location of variable a
mov [ebp -  8], 5   ; location of b
mov [ebp - 12], 2   ; location of c

Remember that pushing basically does this:

sub esp, 4   ; "allocate" space for the new stack item
mov [esp], X ; put new stack item value X in

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Why are local variables not push into the stack like this:

push 10
push 5
push 2

instead of

sub esp, 12

mov [ebp -  4], 10  ; location of variable a
mov [ebp -  8], 5   ; location of b
mov [ebp - 12], 2   ; location of c

Answer 1

It's more a matter of semantics rather than of technical correctness: push and pop are used to save and restore registers or values; but providing local variables for a function does not correspond to this regular purpose of push/pop. So, the stack is managed manually here (except of push ebp and pop ebp, because here we actually want to save and restore ebp in the true sense of push/pop).