global fact
section .text

; Input and output in EAX register
fact:
  cmp    eax, 1
  je    .done   ; if eax == 1 goto done

  ; inductive case
  push  eax  ; save n (ie. what EAX is)
  dec   eax  ; n - 1
  call  fact ; fact(n - 1)
  pop   ebx  ; fetch old n
  mul   ebx  ; multiplies EAX with EBX, ie. n * fac(n - 1)
  ret

.done:
  ; base case: return 1
  mov   eax, 1
  ret