$DD0D/56589/CIA2+13:  Interrupt (NMI) Control Register

   +-------+------------------------------------------------------+
   | Bit 7 |   On Read:  1 = Interrupt occured                    |
   |       |   On Write: 1 = Set Int.-Flags, 0 = Clear Int.-Flags |
   | Bit 4 |   FLAG1 NMI (User/RS232 Received Data Input)         |
   | Bit 3 |   Serial Port Interrupt ($DD0C full/empty)           |
   | Bit 2 |   Time-of-Day Clock Alarm Interrupt                  |
   | Bit 1 |   Timer B Interrupt (RS232)                          |
   | Bit 0 |   Timer A Interrupt (RS232)                          |
   +-------+------------------------------------------------------+

   Your CIA does NOT clear this register! You have to do this by
   simply reading it.

   Default Value: $7F/127 (%01111111)

Kernal-Reference:

 LDY $DD0D   : $FE51
 STA $DD0D   : $EF3B $EF46 $EF80 $F0B3 $F485 $FDA8 $FE4E $FE88 $FEB9 $FEF9