- on AMD-K5 machine, bootloader results in DISK ERROR 0x0004
  (too many sectors read in one row?)
- PS/2 answers with ERROR if PS/2 mouse is not connected,
  read_ack should handle error cases correctly. We should
  also probe correctly for the mouse.
- floppy drive doesn't spin down after reading stage2 and
  kernel from floppy. Do we have to call 0x13, 0x00 reset
  drive?