$DE01/56833/RR+1:   Extended Control-Register

  On read:

   +-------+--------------------------------------------------------+
   | Bit 7 |   ROM bank selector (A15)                              |
   | Bit 6 |   1 = REU compatible memory map active                 |
   | Bit 5 |   ROM bank selector (A16)                              |
   | Bit 4 |   ROM bank selector (A14)                              |
   | Bit 3 |   ROM bank selector (A13)                              |
   | Bit 2 |   1 = Freeze button pressed                            |
   | Bit 1 |   AllowBank bit (see Write)                            |
   | Bit 0 |   1 = Flashmode active (jumper set)                    |
   +-------+--------------------------------------------------------+

  On write:

   +-------+--------------------------------------------------------+
   | Bit 7 |   ROM bank selector (A15) - mirror of $DE00            |
   | Bit 6 |   1 = REU compatible memory map                        |
   | Bit 5 |   ROM bank selector (A16) - only in Flashmode          |
   | Bit 4 |   ROM bank selector (A14) - mirror of $DE00            |
   | Bit 3 |   ROM bank selector (A13) - mirror of $DE00            |
   | Bit 2 |   1 = disables Freeze function                         |
   | Bit 1 |   AllowBank bit (1 = allows banking of RAM in          |
   |       |   $DE02/$DF00 areas)                                   |
   | Bit 0 |   1 = enable accessory connector (SilverSurfer)        |
   +-------+--------------------------------------------------------+

   If not in Flash mode: Bits 1, 2, 6 can only be written once.
                         Bit 5 is always 0.

   If in Flash mode:     Bit 6 cannot be set.
                         Register will not be disabled by the first write.