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System/360 Model 91 front panel

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, toggle switches, 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).

On some machines, certain lights and switches were reserved for use under program control. These were often referred to as 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.

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.

Common usage

An operator would stand at the front panel to bootstrap the computer, to debug running programs, and to find hardware faults. Typically, the operator would read from a scrap of paper containing a short series of bootstrap instructions that would be hand-entered using the toggle switches. First, the operator would set the "address" switch, and enter the address in binary using the switches. Then the operator would set the "value" switch, and then enter the value intended for that address. After punching in a dozen or so of these instructions (most computers had a "deposit next" button, which would deposit subsequent values in subsequent addresses, relieving the operator of needing to toggle in addresses), the operator would then set the starting address of the bootstrap program and press the "run" switch to begin the execution of the program. Often, the bootstrap would turn on the punched tape reader, which would load a somewhat longer program, which in turn would load the operating system from disk.

Some machines accelerated the bootstrap process by allowing the operator to set the switches to one or two machine language instructions and then directly executing those instructions. Other machines allowed I/O devices to be explicitly commanded from the front panel (for example, "Read-In Preset" on the PDP-10 or the accessing of memory-mapped I/O devices on a PDP-11). Some machines also contained various bootstrap programs in ROM and all that was required to boot the system was to start it executing at the address of the correct ROM program.

Entertainment

For fun, bored programmers would create programs to display animated light shows. Front panels in the late 60s and early 70s were quite brightly colored. When bootstrap ROMs enabled computers to start themselves without operator intervention in the late 1970s and early 1980s, most computers were built without a front switch panel. High-powered calculators, such as the HP 9830 based on ROM, were among the first computers to do away with front panels, and operators.

Huge banks of "blinkenlights" and "blowenfuzen" were featured on TV and movies as the popular image of the "computer" during the 1950s to 1970. (A Burroughs B205 was used as a Hollywood prop for many of these shows.)

Example

PDP-8/e front panel.

The following procedure would bootstrap an RK05 moving-head magnetic disk on a PDP-8 system:

  1. Ensure that the machine is halted by lowering and raising the Halt switch; the front panel "RUN" light should then be off.
  2. Set the 12 data switches to 0030 (Octal address 30), depress the Load Address ("ADDR LOAD") switch. The address lights will change to "0030".
  3. Set the switches to 6743, raise the Deposit switch. The data lights will show this instruction.
  4. Set the switches to 5031, raise the Deposit switch. The data lights will show this instruction.
  5. Set the switches to 0030 (Octal address 30), depress the Load Address switch. The address lights will change back to "0030".
  6. Depress the Clear switch.
  7. Depress the Continue switch. The "RUN" light will illuminate and the operating system on the disk will be bootstrapped.

This process works by depositing a simple, two-instruction program in memory and executing it. The first instruction commands the disk controller to begin reading the disk from the current disk address into the current memory address. The second instruction is a JMP instruction that jumps to itself endlessly. When "Clear" is pressed, the disk controller's current disk address is set to sector 0 and its current memory address is set to memory location 0000. When the read is commanded, the program stored in disk sector 0 overlays the bootstrap program and, once the JMP instruction is overlayed, the disk program takes control of the machine.

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