$DC0D/56333/CIA1+13: CIA Interrupt Control Register (IRQ) +-------+------------------------------------------------------+ | Bit 7 | IRQ Flag (1 = IRQ Occurred) / Set-Clear Flag | | Bit 4 | FLAG1 IRQ (Cassette Read / Serial Bus SRQ Input) | | Bit 3 | Serial Port Interrupt ($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.