1. An image processor which is connected to a system bus that connects a processor for forming a graphic command related to image processing to a main memory, stores said graphic command and original image data in a graphic data memory, and draws an image in a frame buffer based upon said graphic command stored in said graphic data memory, wherein said processor has a data bus change-over unit which connects said system bus to a first data bus that is connected to said graphic data memory which holds said graphic command and said original image data, or connects said first data bus to the frame buffer which holds the data to be displayed.