Base pointer and stack pointer

Question

Given this piece of code:

       swap:              push ebp ; back up the base pointer,             mov ebp, esp             ; push the context of the registers on the stack              push eax             push ebx             push ecx             push edx              mov eax, [ebp+8] ; address of the first parameter             mov ebx, [ebp+12] ; address of the second parameter             mov dl, [eax]             mov cl, [ebx]              mov [eax], cl              mov [ebx], dl              ; restore the context of the registers from the stack              pop edx             pop ecx               pop ebx             pop eax             ; restore the ebp             pop ebp             ret 

(This is just the method. Previously we pushed the first and the second parameter on the stack.)

My question is: why do we add 8 to the Base Pointer to get to the address of the first parameter and then 12 ?

I get the fact that they are dword so each of them are 4 bytes..so from ebp + 8 to ebp + 12 it makes sens to make. But why the first one is ebp + 8 ? Because if ESP points to the TOP of the stack, mov ebp, esp means that EBP points to the TOP of the stack. Then we push 4 values on the stack : eax, ebx, ecx and edx. Why is EBP + 8 pointing on the first parameter ?

Answer 1

When the function is called, the stack looks like:

+-------------+ | Parameter 2 | +-------------+ | Parameter 1 | +-------------+ | Return Addr |  <-- esp +-------------+     

then after the "stack frame" is set up:

+-------------+ | Parameter 2 | <-- [ebp + 12] +-------------+ | Parameter 1 | <-- [ebp + 8] +-------------+ | Return Addr |   +-------------+     | saved ebp   | <-- ebp +-------------+ <-- esp 

Now the context is saved:

+-------------+ | Parameter 2 | <-- [ebp + 12] +-------------+ | Parameter 1 | <-- [ebp + 8] +-------------+ | Return Addr |   +-------------+     | saved ebp   | <-- ebp +-------------+  | saved eax   |   +-------------+     | saved ebx   |   +-------------+     | saved ecx   |   +-------------+     | saved edx   | <-- esp +-------------+     

Don't forget that on many systems the stack grows downward (and that is definitely true of the x86 family), so the top of the stack will have the lowest memory address.