A controller / driver for driving main and sub display panels is composed of first and second memory sections, a color reduction circuit, and a data line driver circuit. The first and second memory sections are used as multipurpose display memories. When the controller / driver is placed in a first mode, the first and second memory sections store therein a pair of image data color-reduced under different conditions for achieving frame rate control. When the controller / driver is placed in a second mode, on the other hand, the first and second memory sections store sub and main image data respectively associated with images to be displayed on main and sub display panels.