Video controller: A Fixed area of the system memory controller for the frame buffer, and the video controller is given direct access to the frame – buffer memory, frame – buffer locations, and the corresponding screen positions are referenced in Cartesian coordinates for many graphic computer, monitors, the coordinates origin is defined at the lower left screen corner. Figure. The screen surface is then represented as the first quadrant of a two – dimensional system with positive x value, increasing to the right and positive y values increasing from bottom to top. on some personal computer, the coordinate origin is referred as the upper left corner of the screen, so the y values are invented. Scan line are then labeled from y max at the top of the screen to 0 at the bottom. Along each scan line screen pixel positions are labeled from 0 to ax max, figure. The basic refresh operations of the video controller are shown in diagram. Two registers are used to store the coordinates of the screen pixels. Initially the x register is set to x and y register is set to y m frame buffer for this pixel position is then retrieved and used to set the intensity of the CRT beam. Since the screen must be referred at the rate of 60 frames per second, the simple procedure illustrated in figure cannot be accommodated by typical RAM chips. The cycle times is too slow.