@opt stack_pointer RF
@opt csr_scratch RH
@use ext_std:*

; jump over data segment
  jpi program

; data segment
@label ram_offset
  0x8000

@label program
; set up stack pointer to point to beginning of RAM
  ldi ram_offset RA
  ldr RA RF

; call fibo subroutine with parameter 10
  ldi 10 RA
  csi fibo
  hlt

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

; store RB, RC on stack
  spu RB
  spu RC
; if n < 2, return n
  subi RA 2 RA
  bri sign fibo_early
; store also RD on stack
  spu RD
; a = 0
  ldi 0 RB
; b = 1
  ldi 1 RC
@label fibo_loop
; v = a + b
  add RB RC RD
; a = b
  mov RC RB
; b = v
  mov RD RC
; n -= 1
  dec RA
; loop while n > 0
  bri zero fibo_done
  jpi fibo_loop
@label fibo_early
; restore used registers
  spo RC
  spo RB
; return from subroutine
  rsr
@label fibo_done
  mov RD RA
; restore used registers
  spo RD
  spo RC
  spo RB
; return from subroutine
  rsr