li one, 1
  li f, fib
  li two, 2
  li sp, 0
  li zero 0 

  read n
  li cont, write-result

fib:
  ;; computes f(n) into return and jumps to cont, leaving sp as it found it,
  ;; without any guarantee what will be in the n register afterwards

  slt test, n, two
  jeqz test, main

  add return n zero
  j cont

main-label:
  st n, sp
  add sp sp one
  sub n n one
  st cont sp
  add sp sp one
  li cont fbranch
  j f

fbranch:
  sub sp sp one
  ld cont sp
  sub sp sp one
  ld n sp

  sub n n two
  st return sp
  add sp sp one
  st cont sp
  add sp sp one
  li cont adding
  j f

adding:
  ;; at this point, return contains f(n-2) and on top of the stack we
  ;;  have the continuation and under that the value of f(n-1)

write-result:
  write return
  halt