putc:
  set  R8 3       # Point R8 towards argument c
  add  R8 SP R8   # ... located as SP+3
  ld   R8 R8      # Load argument c
  set  R9 1012    # Put command for screen
  bus  R9 R8 R8   # Write char to screen
  set  R0 1       # Set return value
  ret