The controller for a monitor also works as a bit serial device at an equally low level. It reads bytes containing the characters to be displayed from memory and generates the signals used to modulate the CRT beam. The controller also gen- erates the signals for making a CRT beam do a horizontal retrace after it has fin- ished a scan line, as well as the signals for making it do a vertical retrace after the entire screen has been scanned. On an LCD screen these signals select individual pixels and control their brightness, simulating the effect of the electron beam in a CRT. If it were not for the video controller, the operating system programmer