PURPOSE: To improve the retry rate of a service processor by carrying out a diagnosis program via a fixed address of a microprogram when a monitor timer has an overflow through its executing process and jumping the operation to the executing process again when no fault occurs.

CONSTITUTION: The contents of a monitor timer 3 are replaced every fixed time via signal lines 14 and 15. An overflow signal is outputted onto a signal line 16 when the contents of the timer 3 exceed a fixed level. An execution/ interruption state FF 4 is set at logic '1' via a signal line 17 in the execution processing time and then at logic '0' at the time of the interruption processing. Therefore the output signal outputted onto a signal line 18 is applied to an AND gate 5 at the time of the execution processing together with the overflow signal applied onto the line 16. The output of the gate 5 resets a microprogram address register (MPADR) 2 via a signal line 19. As a result, the control jumps to a fixed address 0 of the MPADR 2 and a diagnosis program runs from the address 0.