@opt stack_pointer RG
@opt csr_scratch RF

@use ext_std:*

@label program
  ; call fibo subroutine with parameter 10
  ldi RA 10
  csi fibo
  hlt

@label fibo
  ; function fibo
  ; parameters:
  ;  RA   n : u16
  ; return value:
  ;  RA   fibo(n) : u16

  spu RB                 ; prelude
  spu RC

  subi RA 2 RA           ; if n < 2, return n
  bri f_sign fibo_early

  spu RD                 ; prelude

  ldi RB 0               ; a = 0
  ldi RC 1               ; b = 1
@label fibo_loop
  add RB RC RD           ; v = a + b
  mov RC RB              ; a = b
  mov RD RC              ; b = v
  dec RA                 ; n -= 1
  bri f_zero fibo_done   ; loop while n > 0
  jpi fibo_loop

@label fibo_early
  spo RC
  spo RB
  rsr
@label fibo_done
  mov RD RA
  spo RD                 ; restore registers
  spo RC
  spo RB
  rsr