PURPOSE: To obtain a real or absolute address and to give an access to a memory by performing an address converting process in response to the attribute of a virtual computer under the control of a microprogram when an access is given to the memory with a logical address.

CONSTITUTION: When an access request is received to a memory 80, the deciding result of an address conversion mode is checked. If the checking contents are turned off, a memory control unit 60 uses a logical address directly as a real address of the memory 80 to give an access to the memory 80. If the contents of the deciding result of the address conversion mode are turned on a signal line 30, it is checked whether to logical address on a signal line 31 is registered in an address conversion buffer 70 or not. If so, the buffer 70 outputs a real address corresponding to the registered logical address. Then the unit 60 outputs the real address and has an access to the memory 80. If the logical address is not registered in the buffer 70, the unit 60 starts a microprogram 50 and carries out an address converting process in accordance with the attribute of a virtual computer.