articles/assembly.md: add x86-64 factorial assembly

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