CODEWITHSUNDEEP
assembly
chitech
chitech

Reputation: 109

push local variables

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:

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

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

Upvotes: 8

Views: 13223

Answers (2)

Flinsch
Flinsch

Reputation: 4341

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).

Upvotes: 2

Brian Knoblauch
Brian Knoblauch

Reputation: 21369

Practically speaking, if you know how much stack space you need and reserve it in a single operation you can then use the mov instruction which executes faster than a push immed (especially now that the offset calculation has dedicated hardware). There's also perhaps a legacy component having to do with the fact that push immed didn't become available on the x86 until the release of the 80186/80188 processors. By that time, the sub/mov convention had become a well established pattern.

Upvotes: 3

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