$DF11/57105/MMC+1:      Control Register

   +-------+---------------------------------------------------------+
   | Bit 7 |   Unused                                                |
   | Bit 6 |   0 = SPI write trigger mode, 1 = SPI read trigger mode |
   | Bit 5 |   0 = allow external ROM when MMC64-BIOS is disabled,   |
   |       |   1 = disable external ROM                              |
   | Bit 4 |   0 = Normal Operation, 1 = Flash Mode (*)              |
   | Bit 3 |   0 = clock port at $DE00, 1 = clock port at $DF20      |
   |       |   Shadow: 0 = MMC64 active, 1 = MMC64 disabled (**)     |
   | Bit 2 |   0 = 250KHz transfer, 1 = 8MHz transfer                |
   | Bit 1 |   0 = Card selected, 1 = Card not selected              |
   | Bit 0 |   0 = MMC64-BIOS active, 1 = external ROM active        |
   +-------+---------------------------------------------------------+
   Notes: (*)  Bit 4 can only be written when flash jumper is set
          (**) The shadow of Bit 3 can only be modified after
               unlocking at $DF13