Translate NASM program into GNU Assembler one

Sunless · 09-20-2020, 09:09 AM

Hi all!
There are some code in C and Assembly here https://montcs.bloomu.edu/Informatio...asm-stack.html See examples add2.asm in NASM.
I didn't understand square brackets operation. How to translate

Quote:

mov eax, [ ebp + 8 ]

into GNU Assembler call? What does 8 mean in square brackets? I assume that sizeof(int) has 4 value. Thank you.

SoftSprocket · 09-20-2020, 09:30 AM

Code:

mov eax, [ebp + 8]

Square brackets dereference and intel syntax the parameters are reversed, so move the value at ebp+8 to eax.

GazL · 09-21-2020, 07:13 AM

Intel syntax addressing: [BASE + INDEX * SCALE + DISPLACEMENT]
AT&T syntax addressing: DISPLACEMENT(BASE, INDEX, SCALE)

So, essentially, in AT&T (gas) syntax it would be:
mov 8(%ebp), %eax

Yes, sizeof(int) will be 4. The reason it's ebp + 8 and not ebp + 4 is because of the function preamble ( push ebp ; mov ebp, esp ) which has already pushed ebp to the stack and reloaded ebp from the new stack pointer, which is now pointing 4 bytes lower.

The stack will look like this:
+12 second int arg
+8 first int arg
+4 return address
SP original ebp value (which the function preamble just pushed)