PURPOSE: To read storage contents after recovery from the fault of a host device and to investigate whether or not writing operation is interrupted and its address by storing the address and sate of an area where an external memory device performs writing operation.

CONSTITUTION: If a fault which disables the execution of a program occurs to the host device 10 for some reason, the host device 10 is recovered in a suitable way. Then, the host device 10 executes an instruction for reading a memory device number and the contents of a write start indicator and an end indicator to know whether or not incomplete writing is performed. Further, when the incomplete writing is performed, its address is known immediately and the area corresponding to the fault is easily recovered.