$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 (low active!) |
   +----------+---------------------------------------------------+

   Default Value: $17/23 (%00010111)

Kernal-Reference:

 CMP $DD00   : $ED69 $EE5D $EE6A $EEAC
 LDA $DD00   : $ED2E $ED66 $ED84 $EDBE $EDF3 $EE5A $EE67 $EE85 $EE8E $EE97
               $EEA0 $EEA9 $FE7B
 ORA $DD00   : $F492
 STA $DD00   : $ED33 $ED8B $EDC3 $EDF8 $EE8A $EE93 $EE9C $EEA5 $F495 $FDCD
               $FE82