PURPOSE: To improve the working rate of a computer system by saving the contents of a main storage working at abnormality of the system into an auxiliary storage, etc., in a non-working state of the main storage and using these saved contents of the main storage as the abnormality analysis data.

CONSTITUTION: A working main storage address converter 3 sets one of both main storages 1 and 2 in a working state and the other in a non-working state respectively and controls the accesses of both storages. An arithmetic controller 4, an auxiliary storage 5, and an input/output device 6 have accesses to the storages 1 and 2 via the converter 3. Thus the accesses are possible to both working and non-working main storages with no consciousness of the physical addresses of both main storages. Therefore a program which analyzes the contents of the main storages obtained at discontinuation of a computer system and is executed by the controller 4 is able to output the contents of the main storages to the storage 5 or the device 6. Thus the working and non-working main storages can be alternately used every time the system is started.