PURPOSE: To prevent a CPU from stalling repetitively the same instruction by informing a unit of a diagnostic system of the abnormality of an input/output device when no input/output instruction is accepted from a CPU and the wait time is not changed even with the repetitive retries.
CONSTITUTION: A CPU 1 recognizes the waiting time held in a register 6 and performs a retry when the time equivalent to the waiting time passed. If a wait answer is returned again under such conditions, the CPU 1 reads the precedent wait time out of a register 5 before the second retry. This wait time is compared with the returned wait answer by a comparator 12. If the value of the returned wait answer is not updated yet, the abnormality of an input/output device 2 is decided. This abnormality of the device 2 is reported to a unit of a diagnostic system via a signal line 11. Thus it is possible to prevent the CPU 1 from stalling repetitively the same instruction.