From 08e7f3361d845604af658a9b66e0121eaceabf24 Mon Sep 17 00:00:00 2001
From: tocariimaa <tocariimaa@firemail.cc>
Date: Fri, 14 Mar 2025 13:31:28 -0300
Subject: [PATCH] articles/assembly.md: add x86-64 factorial assembly

---
 articles/assembly.md | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/articles/assembly.md b/articles/assembly.md
index 77e342b..0aeffa3 100644
--- a/articles/assembly.md
+++ b/articles/assembly.md
@@ -65,8 +65,33 @@ cc -nostdlib -o hello hello.o
 ```
 
 ### Factorial (x86-64, Linux)
+Returns the calculated factorial as exit code (which is 255 max.)
+
 ```nasm
+default rel
+
+section .text
+global _start
+
+; Compute the factorial of `rdi`, saving the result in `rax`
+fact:
+	mov eax, 1			; register holding result
+.l:
+	mul rdi				; rax *= rdi
+	dec rdi				; decrement rdi
+	test rdi, rdi		; check if rdi = 0
+	jne .l				; loop again if rdi != 0
+	ret
+	
+_start:
+	; x86 registers are sliced, here we assign to edi (32 bit slice)
+	; which gets extended to rdi later...
+	mov edi, 5
+	call fact
+	; exit syscall
+	mov rdi, rax
+	mov eax, 0x3c
+	syscall
 ```
-TODO
 
 TODO examples for other architectures