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

   +-------+------------------------------------------------------+
   | Bit 7 |   NMI Flag (1 = NMI Occurred) / Set-Clear Flag       |
   | Bit 4 |   FLAG1 NMI (User/RS-232 Received Data Input)        |
   | Bit 3 |   Serial Port Interrupt ($DD0C full/empty)           |
   | Bit 1 |   Timer B Interrupt                                  |
   | Bit 0 |   Timer A Interrupt                                  |
   +-------+------------------------------------------------------+

   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