PURPOSE: To increase a processing speed by providing a mode switching register for switching between the read modify write and normal read/write and a write data register for read modify write.

CONSTITUTION: A write data register 34 where write data for read modify write from a CPU (central processing unit) 21 is temporarily stored, a logical operation part 23 which performs the logical operation between contents of the write data register 34 and picture data in a picture memory 26, a select register 37 which selects read modify write or read/write by the CPU 21, and a multiplexer 25 which inputs the output of the select register 37 and switches the connection destination of the picture memory 26 to the logical operation part 23, the CPU 21, or a display control part 24 are provided. Thus, the processing is performed at a higher speed than a conventional device capable of read modify write.