     $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.