PURPOSE: To reduce the cost a controller by using an external storage device as page memory and ensuring that the device is accessed quite in the same manner as is possible using a conventional memory element.

CONSTITUTION: A memory address which is output is output to an address conversion circuit 6 using a bus control circuit 5b, and RD is output to an external storage device controller 7. The address conversion circuit 6 determines whether a memory space is one assigned to an external storage device 8 for memory address to be entered. If this is true, the circuit 6 outputs the address in the form of converted data which is accessible to an external storage device such as a track number or a sector number to the external storage device controller. The controller 7 reads font data stored on the external storage device 8 using the converted data, or reads bit map data and writes the data using a control signal from a bus control circuit 5b. In addition, the memory address space assigned by CPU 1 allows a memory space of the external storage device 8 to be occupied on the most significant address side.