PURPOSE: To surely display the storage state of an extended storage device by storing information expressing whether maintaining the extended storage device is possible or not without regard to the presence/absence of a non-volatile mechanism in a flag.

CONSTITUTION: In the maintenance control of the extended storage device provided in a data processing system independently from a main storage device, the flag 7 stores information expressing whether maintenance is possible or not without regard to the presence/absence of the non-volatile mechanism. A control means 8 is made on/off by control from CPU 1 through a storage control device 4. The flag 7 is read/written specially by the reading/writing instructions of battery invalidation BI, a battery failure BF and a data invalidation DI flag. The flag 7 can be made on/off by a control instruction instructing the control of the state of the device 5 so that the present state of the device 5 can accurately be judged at the time of maintenance, etc., by looking at the flag 7.