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, digit[a] and symbol displays, toggle switches, dials, and push buttons mounted on a sheet metal face plate.

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.

An operator would use the front panel to bootstrap the computer, to debug running programs, and to find hardware faults.

Typically, the operator would have a written procedure containing a short series of bootstrap instructions to be hand-entered using, e.g., dials, keyboard, toggle switches.

The programmer could read and alter register contents, change program instructions or data in memory or force a branch to another section of code.

Programs called debuggers were written which provided the programmer with the equivalent of the front-panel functions without requiring the entire machine.

The following procedure would bootstrap a PDP-8 system from an RK05 moving-head magnetic disk: This process works by depositing a simple, two-instruction program in memory and executing it.