+------------------------------------------------------------------------
  |
  |      WD 177X-Disc-Controller
  |
  +------------------------------------------------------------------------
  |
  |   $6000/24576/WD+0       Command- / Status Register
  |   $6001/24577/WD+1       Track Register
  |   $6002/24578/WD+2       Sector Register
  |   $6003/24579/WD+3       Data Register
  |
  +------------------------------------------------------------------------