$DC0D/56333/CIA1+13:  CIA Interrupt Control Register (IRQ)

   +-------+------------------------------------------------------+
   | Bit 7 |   IRQ Flag (1 = IRQ Occurred) / Set-Clear Flag       |
   | Bit 4 |   FLAG1 IRQ (Cassette Read / Serial Bus SRQ Input)   |
   | Bit 3 |   Serial Port Interrupt ($DC0C full/empty)           |
   | Bit 2 |   Time-of-Day Clock Alarm Interrupt                  |
   | 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.