To improve a display processing performance by providing a bus control means with a mapping state holding means which holds a copy of the contents held by a state holding means and shorten the time of response to a CPU and the time of window switching.

The bus control means 2 is equipped with address holding means 12 and 13 and mapping state holding means 14 and 15. Through a timer means 16, the bus control means 2 reads the contents of state holding means 18 and 19 out to a bus 7 according to the address holding means 12 and 13 and periodically updates the contents of the mapping state holding means 14 and 15. A mapping display storage means 17 in the bus control means 2 temporarily stores part of contents of image display storage means 5 and 6. The mapping display storage means 17 servers as a buffer and when data stored in the image display storage means 5 and 6 are needed, access to the image display storage means may be only started through a drawing means 3 in parallel to data processing of part of the data stored in the mapping display storage means 17, thereby making the processing fast on the whole.