$4000/16384/CIA+0:       Data Port Register A

   +----------+---------------------------------------------------+
   | Bit  7   |   0 = Disk Change (will be reset after the        |
   |          |       next move of the head)                      |
   | Bit  6   |   1 = Drive LED on                                |
   | Bit  5   |   1 = Error LED bright                            |
   | Bits 4+3 |   Position of DIP Switches for Device Number      |
   | Bit  2   |   0 = Motor on                                    |
   | Bit  1   |   1 = no Disk inserted                            |
   | Bit  0   |   Side Select                                     |
   +----------+---------------------------------------------------+

ROM-Reference:

 LDA $4000  : $AF0A $AFE9 $C57C $CBB1 $CBBA $CBC3 $CBCC $CD83 $CDA1 $CDBE
              $CE1F $CE2F $CE39 $CFC6
 ORA $4000  : $AEFB
 STA $4000  : $AEFE $AF0F $AF28 $C583 $CBB6 $CBBF $CBC8 $CBD1 $CE24 $CE36
              $CFCD