Recursive Faculty Function

This shows a recursive faculty implementation in both C and thenafter in ASM. The C example is pretty easy, but doing the same in ASM gets a bit tricky, so it took me some time to figure out a simple way to represent that C function.


The whole program can be easily implemented with a recursion as in following in C:


#include "stdio.h"

int fac(unsigned int a){
    int b = a;
    if (a <= 1) 
        return b;
    return b * fac(--a);

int main(){
    unsigned int a;
    int res;
    a = 5;
    res = fac(a);
    printf("%d\n", res);



#include "cdecl.h"
#include "stdio.h"

int PRE_CDECL asm_main( size_t ) POST_CDECL;
// The param must be size_t to ensure a 32-bit unsigned integer. Negative faculty not allowed

int main() {
    size_t a;
    int result;
    a = 5;
    result = asm_main(a);
    printf("%d! = %d\n", a, result);
    return 0;

ASM Code

segment .data
buffer: times 1 dd 0    ; define a 32-bit buffer

segment .text
global asm_main

fac: ; fac: int -> int, fac(a) = a!, REQ: a in EAX, Ret EAX
    CMP eax, 1  ; for EAX <= 1, return EAX
    JLE return
    mov ebx, eax
    DEC ebx ; EBX--
    mul ebx     ; EDX:EAX = EAX * EBX
rec_fac:    ; The main (recursive) faculty function. Never call standalone without fac
    CMP ebx, 1
    JLE return
    DEC ebx
    mul ebx
    call rec_fac        ; recursion

    enter 0, 0
    push ebp
    mov ebp, esp    ; save the stack pointer on ebp

    mov eax, [ebp + 12]    ; move first int argument to eax

    call fac                ; result is saved in eax now

    mov ecx, buffer        ; get buffer's address into the register
    mov [ecx], eax           ; save the modulo result into the buffer

    pop ebp

    mov eax, [buffer]        ; move the saved result back into eax

NOTE: The EDX:EAX register stores the multiplication results when multiplying EBX with EAX using mul ebx while EBX keeps its value. This fact is used within the fac and especially in the rec_fac parts. So just EBX has to be decremented and looked at whether it reached 1. So fac calculates faculty(EAX) while rec_fac actually calculates EAX * faculty(EBX) and saves its result in EAX recursively.


$ nasm -f elf faculty.asm
$ gcc -m32 -c facultydriver.c
$ gcc -m32 -o faculty facultydriver.o faculty.o


$ ./faculty
5! = 120

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