     $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