- 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. (0xFE on all commands)
- optimizing the code move the kernel entry 'kernel_main'
  away from it's expected location at 0x8800. How can we pin
  it there? And not use ELF. :-)
- boot loader doesn't work beyond 64k limit