PURPOSE: To shorten the total starting time of a computer system by ordering a CPU to set only a check start address, an end address, the write data, and a check mode respectively when the system starts.

CONSTITUTION: A write data control means 14 outputs the checking write data WD into a picture data storage 18 based on a write control signal Sc and a write grant signal Se. A read data control means 16 inputs the read data RD received from the storage 18 based on a read control signal Sd and a read grant signal Sf. Thus, the storage 18 can be checked in a short time in terms of hardware despite a large capacity is given to the storage device (video RAM) 18. Thus, the control means (CPU) 16 does not need to exclusively check the storage 18 and can process other data. Then the total starting time is shortened for a computer system.