.symbol
  link 3
  x 2
  tmp 1
  sp 4
  retv 5
.code 0
:main
  mov sp #100
  mov tmp #1
  push sp tmp
  jal link double
  trap retv #1  ; print retv
  trap r0 #0    ; stop
:double
  pop sp x
  add retv x x     ; double x
  ret link 
.end