$DD00/56576/CIA2+0:   Data Port A (Serial Bus, RS-232, VIC Memory Control)

   +----------+---------------------------------------------------+
   | Bit  7   |  Serial Bus Data Input                            |
   | Bit  6   |  Serial Bus Clock Pulse Input                     |
   | Bit  5   |  Serial Bus Data Output                           |
   | Bit  4   |  Serial Bus Clock Pulse Output                    |
   | Bit  3   |  Serial Bus ATN Signal Output                     |
   | Bit  2   |  RS-232 Data Output (User Port)                   |
   | Bit  1-0 |  VIC Chip System Memory Bank Select (Default = 11)|
   +----------+---------------------------------------------------+