A front panel was used on early electronic computers to display and allow the alteration of the state of the machine's internal registers and memory. The front panel usually consisted of arrays of indicator lamps, digit and symbol displays, toggle switches, dials, and push buttons mounted on a sheet metal face plate. In early machines, CRTs might also be present (as an oscilloscope, or, for example, to mirror the contents of Williams–Kilburn tube memory). Prior to the development of CRT system consoles, many computers such as the IBM 1620 had console typewriters. Usually the contents of one or more hardware registers would be represented by a row of lights, allowing the contents to be read directly when the machine was stopped. The switches allowed direct entry of data and address values into registers or memory. Details On some machines, certain lights and switches were reserved for use under program control. These were often referred to as sense indicators, sense lights and sense switches. For example, the original Fortran compiler for the IBM 704 contained specific statements for testing and manipulation of the 704's sense lights and switches. These switches were often used by the program to control optional behavior, for example information might be printed only if a particular sense switch was set. Operating systems made for computers with blinkenlights, for example, RSTS/E and RSX-11, would frequently have an idle task blink the panel lights in some recognizable fashion. System programmers often became very familiar with these light patterns and could tell from them how busy the system was and, sometimes, exactly what it was doing at the moment. The Master Control Program for the Burroughs Corporation B6700 mainframe would display a large block-letter "B" when the system was idle. Switches and lights required little additional logic circuitry and usually no software support, important when logic hardware components were costly and software often limited. This baroque style of front panels began to die out in 1964 when Seymour Cray designed his CDC 6600 supercomputer with a very simple and elegant display console containing only 2 CRT displays and a keyboard, replacing all the hundreds of switches, buttons, and blinking lights. The 6600 had support from ten supporting "peripheral processors" whose duties included reading the keyboard and driving the graphics displays. Early microcomputers such as the 1975 Altair 8800 also relied on front panels, but since the introduction of the Apple