To unnecessitate a dedicated video memory in a computer system.

DRAM being a system memory is made segments to divide into a video memory segment 14x+1 controlled by a video controller 20 and a system memory segment 14x controlled by a system memory controller 15. When the controller 15 outputs a request for accessing the segment 14x+1 and the controller 20 permits it, the controller 15 can access the segment 14x+1 through an address buffer 33 and a data buffer 45. After finish, the controller 20 can use the segment 14x+1. Therefore, a video memory segment in the system memory is shared to unnecessitate the dedicated video memory.