factorial

factorial.s

@@ -0,0 +1,72 @@
+.intel_syntax noprefix # allow mov rax, 1 syntax in GAS (GNU Assembly)
+.global _start
+
+# factorial
+# - rdi as input number
+# - rax as result
+factorial:
+    push rbp
+    mov rbp, rsp
+
+    cmp rdi, 1
+    jle base_case      # if n <= 1 return 1
+
+    push rdi           # save n
+    dec rdi            # rdi = n-1
+    call factorial     # rax = factorial(n-1)
+
+    pop rdx            # rdx = original n
+    imul rax, rdx      # rax = factorial(n-1) * n
+    jmp done
+
+base_case:
+    mov rax, 1
+
+done:
+    mov rsp, rbp
+    pop rbp
+    ret
+
+print_rax_digit:
+    sub rsp, 16
+
+    mov rbx, 10
+
+    # ones digit
+    xor rdx, rdx
+    mov rcx, rax
+    div rbx
+    add dl, '0'
+    mov [rsp+2], dl
+
+    # tens digit
+    xor rdx, rdx
+    mov rcx, rax
+    div rbx
+    add dl, '0'
+    mov [rsp+1], dl
+
+    # hundreds digit
+    add al, '0'
+    mov [rsp], al
+
+    mov rax, 1
+    mov rdi, 1
+    mov rsi, rsp
+    mov rdx, 3
+    syscall
+
+    add rsp, 16
+    ret
+
+_start:
+    mov rdi, 5 # Use 5 for our example input
+    mov rax, 1 # Set starting result to 1
+
+    call factorial
+
+    call print_rax_digit
+
+    mov rax, 60 # syscall exit - 64 bit
+    mov rdi, 0  # exit code
+    syscall