Recursive Factorial (x86_64 ASM + C)

callfactorial.c

/*
 * An application that illustrates calling the factorial function defined in 'factorial.asm'
 * https://cs.lmu.edu/~ray/notes/nasmtutorial/
 *
 * To build & run:
 * nasm -f elf64 factorial.asm && gcc -std=c99 -o factorial2 factorial.o callfactorial.c
 */

#include <stdio.h>
#include <inttypes.h>

uint64_t factorial (uint64_t);

int main() {
	for (uint64_t i = 0; i < 20; i++) {
		printf("factorial(%2lu) = %lu\n", i, factorial(i));
	}
	return 0;
}

factorial.asm

; An implementation of the recursive function:
;
; uint64_t factorial(uint64_t n) {
;	return (n <= 1) ? 1 : n * factorial(n-1);
; }
;
; C is used to provide a wrapper and make calling this function in a loop a bit less painful.
; This requires GCC, NASM, and a 64-bit processor. This will most likely *only* work on Linux 
; https://cs.lmu.edu/~ray/notes/nasmtutorial/
;
; To build & run:
; nasm -f elf64 factorial.asm && gcc -std=c99 -o factorial2 factorial.o callfactorial.c

	GLOBAL	factorial

	section	.text
factorial:
	cmp	rdi, 1		; n <= 1?
	jnbe	L1		; if not, do a recursive call
	mov	rax, 1		; otherwise return 1
	ret
L1:
	push	rdi		; save n on stack (also aligns %rsp)
	dec	rdi		; n - 1
	call	factorial	; factorial(n-1), result goes in %rax
	pop	rdi		; restore n
	imul	rax, rdi	; n * factorial(n-1), stored in %rax
	ret