PURPOSE: To eliminate the need of a switch operation, and also, to eliminate the need of conditioning to an extension of a memory by checking automatically a type of a RAM and its existence.

CONSTITUTION: The title system is provided with a ROM 2 in which a program of an initializing routine of the system has been written and a microprocessor 1 for executing an instruction which has been written in this ROM 2, and by executing the instruction by the microprocessor 1, a RAM is brought to access. To put it concretely, this system has a register for storing the information for showing a kind of the RAM and its existence and controlling the correspond ing RAS line. Accordingly, mixture of that of a different type is allowed as an option RAM, and also, it is unnecessary to limit the switch setting from the outside and the mounting order of the RAM of a different type. In such a way, the RAS can be controlled by discriminating automatically a type of the RAM and its existence against a prescribed RAS line without executing switch setting from the outside.