8271 Command Set Summary
                              ========================
    
                               8271 Status and Errors
                               ----------------------

  &00 Successful completion                                              -+
  &02 Success, scan met equal                                             | Action
  &04 Success, scan met not equal                                         | Completed
  &06                                                                    -+

  &08 Clock error         Bad block read    Bad/unrecognised disk format -+
  &0A Late DMA/Late ISR   Abort                                           | Data
  &0C ID CRC error        Bad sector ID                                   | error
  &0E Data CRC error                                                     -+

  &10 Drive not ready                                                    -+
  &11                                                                     |
  &12 Disk write protected                                                | Drive
  &13                                                                     |
  &14 Track 0 not found                                                   | error
  &15                                                                     |
  &16 Write error                                                         |
  &17                                Bad address/memory access fault     -+

  &18 Sector not found                                                   -+
  &19                                Bad drive number                     |
  &1A Media changed                                                       |  Data
  &1B                                Duplicate disk/already mounted       |
  &1C Media change request                                                | absent
  &1D                                Bad image file name                  |
  &1E Drive not present/drive empty                                       |
  &1F                                Disk not mounted on drive           -+
  &FE Unsupported/bad disk command

8271 error/status values are even numbers. Odd values are additions used when
implementing a disk access emulation layer. Clearing bit 0 with AND &1E results
in them being a matching 8271 native value.

References
----------
* Intel 8271 Floppy Disk Controller datasheet
* mdfs.net/Apps/Emulators/Tube/cZ80Tube.zip
* mdfs.net/Docs/Comp/Disk/Errors