Calling Conventions

There are certain rules to be followed to call the assembly routines from higher level languages like C.
These rules are called calling conventions.

What is a Naming Convention

C puts an underscore before the function or variable names meanwhile PASCAL translates all characters to upper case.
C++ has weird naming mangling which is compiler dependent so to avoid it, we need to use extern "C" directive to force C++ to use C naming style.

How Are Parameters Passed to Routines

In PASCAL, the first argument is passed from left.
In C, the first argument is passed from right.

Which Registers Must Be Preserved

Both standards preserve: - EBX - ESI
- EDI - EBP - ESP - DS - ES - SS

Which Registers Are Used as Scratch

Both standards do not guarantee values for: - EAX - ECX - EDX - FS - GS - EFLAGS and other registers

Which Registers Holds the return Value

In both standards, the 32-bit values are returned in EAX and 64-bit ones in EDX:EAX

Who is Responsible for Removing the Parameters

In C, the caller removes the parameters.
In PASCAL, the callee removes the parameters.

Calling C From Assembly

Imagine we have a function in C:

int divide(int dividend, int divisor);

To call it from assembly:

push dword [mydivisor]
push dword [mydividend]
call _divide
add esp, 8
; EAX holds the answer

Calling Assembly from C

Imagine we got an assembly program

[section .text]
global _swap

_swap:
    ; Copy parameter p1 to ecx
    mov ecx, [esp + 4]

    ; Copy parameter p2 to edx
    mov edx, [esp + 8]

    ; Copy *p1 into eax
    mov eax, [ecx]

    ; Exchange eax with *p2
    xchg eax, [edx]

    ; Copy eax into *p1
    mov [ecx], eax

    ; Return from this function
    ret

Then we use

nasm –f win32 swap.asm

and get swap.obj file.

Then in C

#include <stdio.h>
void swap(int* p1, int* p2);

int main() {
    int a = 10, b = 20;
    printf("a=%d b=%d\n", a, b); 
    swap(&a, &b); 
    printf("a=%d b=%d\n", a, b); 

    system("PAUSE");
    return 0;
}