;; fac n :
;;  if n <= 0 return 1
;;  else return n * fac n - 1

;; main :  a = fac 3

;; let r1 = a
;; let r2 = input n, r3 = output retvalue
;; let r4 = link, r5 = sp

.s
  sp 5
  link 4
  a 100
.a 0
.c 
:main
  ld sp #200  ;; initialise sp
  ld r2 #3
  jal link fac
  st a r3
  trap stop r0

:fac
  ;; let t1 = n to do 
  ;; n * fac n - 1  is
  ;; t1 = n
  ;; n = n - 1
  ;; retvalue = call fac
  ;; retvalue = t1 * retvalue
  ;; save locals : r6 = t1
  st @1 sp link  ;; push link
  st @2 sp r6  ;; push t1
  add sp sp #2
  sub r0 r2 r0 ;; n <= 0 ?
  jmp GT else
  ld r3 #1
  jmp always ret
:else
  or r6 r2 r0 ;; t1 = n
  sub r2 r2 #1 ;; n - 1
  jal link fac
  mul r3 r6 r3 ;; n * fac n - 1
:ret
  ;; restore
  ld r6 @0 sp  ;; pop t1
  ld link @-1 sp ;; pop link
  sub sp sp #2
  jr link
.e