$DF01/57089/REU+1:   Command Register

   +----------+---------------------------------------------------+
   | Bit  7   |   Execute: 1 = Transfer current Configuration     |
   | Bit  6   |   Reserved (normally 0)                           |
   | Bit  5   |   Load: 1 = enable autoload option                |
   | Bit  4   |   FF00: 1 = start immediately                     |
   |          |         0 = wait for write access to $FF00        |
   | Bits 3-2 |   Reserved (normally 0)                           |
   | Bits 1-0 |   Transfer Type:  00 = transfer C64 -> REU        |
   |          |                   01 = transfer REU -> C64        |
   |          |                   10 = swap C64 <-> REU           |
   |          |                   11 = compare C64 - REU          |
   +----------+---------------------------------------------------+