$DD0D/56589/CIA2+13: CIA Interrupt Control Register (NMI) +-------+------------------------------------------------------+ | Bit 7 | NMI Flag (1 = NMI Occurred) / Set-Clear Flag | | Bit 4 | FLAG1 NMI (User/RS-232 Received Data Input) | | Bit 3 | Serial Port Interrupt ($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.