Altair 8800 at the Computer History Museum, cropped

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 Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered, ...

. The front panel usually consisted of arrays of

indicator lamp A check engine light or malfunction indicator lamp (MIL), is a tell-tale that a computerized engine-management system uses to indicate a malfunction. Found on the instrument panel of most automobiles, it usually bears the legend , , , , , o ...

s, digit and symbol displays,

toggle switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...

es, 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 console One meaning of system console, computer console, root console, operator's console, or simply console is the text entry and display device for system administration messages, particularly those from the BIOS or boot loader, the kernel, from the ...

s, many computers such as the

IBM 1620 The IBM 1620 was announced by IBM on October 21, 1959, and marketed as an inexpensive scientific computer. After a total production of about two thousand machines, it was withdrawn on November 19, 1970. Modified versions of the 1620 were used as ...

had console typewriters. Usually the contents of one or more

hardware register In digital electronics, especially computing, hardware registers are circuits typically composed of flip flops, often with many characteristics similar to memory, such as: * The ability to read or write multiple bits at a time, and * Using an ...

s 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 switch A sense switch, or program switch, is a switch on the front panel of a computer whose state can be tested by conditional branch instructions in software.Rudolf F. Graf ''Modern dictionary of electronics seventh edition'', Newnes, 1999 Most early ...

es''. 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 Blinkenlights is a neologism for Blinkenlights#Actual blinkenlights, diagnostic lights usually on the front panels on old mainframe computers, minicomputers, many early microcomputers, and modern network hardware. It has been seen as a ...

'', for example,

RSTS/E RSTS () is a multi-user time-sharing operating system developed by Digital Equipment Corporation (DEC, now part of Hewlett-Packard) for the PDP-11 series of 16-bit minicomputers. The first version of RSTS (RSTS-11, Version 1) was implemented in 1 ...

and

RSX-11 RSX-11 is a discontinued family of multi-user real-time operating systems for PDP-11 computers created by Digital Equipment Corporation. In widespread use through the late 1970s and early 1980s, RSX-11 was influential in the development of later ...

, would frequently have an

idle Idle generally refers to idleness, a lack of motion or energy. Idle or ''idling'', may also refer to: Technology * Idle (engine), engine running without load ** Idle speed * Idle (CPU), CPU non-utilisation or low-priority mode ** Synchronous ...

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 The MCP (Master Control Program) is the operating system of the Burroughs small, medium and large systems, including the Unisys Clearpath/MCP systems. MCP was originally written in 1961 in ESPOL (Executive Systems Problem Oriented Language). In ...

for the

Burroughs Corporation The Burroughs Corporation was a major American manufacturer of business equipment. The company was founded in 1886 as the American Arithmometer Company. In 1986, it merged with Sperry UNIVAC to form Unisys. The company's history paralleled many ...

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 Seymour Roger Cray (September 28, 1925 – October 5, 1996
) was an American

CDC 6600 supercomputer with a very simple and elegant display console containing only 2 CRT displays and a

keyboard Keyboard may refer to: Text input * Keyboard, part of a typewriter * Computer keyboard ** Keyboard layout, the software control of computer keyboards and their mapping ** Keyboard technology, computer keyboard hardware and firmware Music * Musi ...

, 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 A microcomputer is a small, relatively inexpensive computer having a central processing unit (CPU) made out of a microprocessor. The computer also includes memory and input/output (I/O) circuitry together mounted on a printed circuit board (PC ...

such as the 1975 Altair 8800 also relied on front panels, but since the introduction of the

Apple II The Apple II (stylized as ) is an 8-bit home computer and one of the world's first highly successful mass-produced microcomputer products. It was designed primarily by Steve Wozniak; Jerry Manock developed the design of Apple II's foam-m ...

TRS-80 The TRS-80 Micro Computer System (TRS-80, later renamed the Model I to distinguish it from successors) is a desktop microcomputer launched in 1977 and sold by Tandy Corporation through their Radio Shack stores. The name is an abbreviation of '' ...

, and

Commodore PET The Commodore PET is a line of personal computers produced starting in 1977 by Commodore International. A single all-in-one case combines a MOS Technology 6502 microprocessor, Commodore BASIC in read-only memory, keyboard, monochrome monitor, ...

during the home computer boom of 1977, the vast majority of microcomputers came with keyboards and connections for TV screens or other monitors.

Common usage

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

Storage alteration and display

Many computers had controls, e.g., buttons, dials, keyboards, toggle switches, for entering addresses, controls for displaying the data at an address and controls for altering storage contents. These were typically used for booting and debugging.

Booting

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. First, the operator would, e.g., press the address switch and enter the address. For easier entry and readout, on some computers (such as the DEC

PDP-8 The PDP-8 is a 12-bit computing, 12-bit minicomputer that was produced by Digital Equipment Corporation, Digital Equipment Corporation (DEC). It was the first commercially successful minicomputer, with over 50,000 units being sold over the model's ...

or MITS Altair 8800) binary digits were grouped into threes or fours on the front panel, with each group of lights or switches representing a single

octal The octal numeral system, or oct for short, is the radix, base-8 number system, and uses the Numerical digit, digits 0 to 7. This is to say that 10octal represents eight and 100octal represents sixty-four. However, English, like most languages, ...

(between 0 and 7) or

hexadecimal In mathematics and computing, the hexadecimal (also base-16 or simply hex) numeral system is a positional numeral system that represents numbers using a radix (base) of 16. Unlike the decimal system representing numbers using 10 symbols, hexa ...

(between 0 and F) digit. Some decimal computers, e.g.,

, used binary-coded decimal for memory addresses. Next the operator would enter the value intended for that address. After entering several of these instructions (some computers had a deposit next button, which would deposit subsequent values in subsequent addresses, relieving the operator of needing to enter subsequent 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. The bootstrap program usually read a somewhat longer program from punched paper-tape, punched cards, magnetic tape, drum or disk which in turn would load the operating system from disk. Some machines accelerated the bootstrap process by allowing the operator to set the controls to contain one or two

machine language In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...

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 Digital Equipment Corporation (DEC)'s PDP-10, later marketed as the DECsystem-10, is a mainframe computer family manufactured beginning in 1966 and discontinued in 1983. 1970s models and beyond were marketed under the DECsystem-10 name, espec ...

or the accessing of

memory-mapped I/O Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O pr ...

devices on a PDP-11). Some machines also contained various bootstrap programs in

ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * ...

and all that was required to ''boot'' the system was to start it executing at the address of the correct ROM program.

Debugging

Front panels were often used to debug programs when operating support was limited and most programs were run in stand-alone mode. Typically a front-panel switch could cause the computer to ''single-step'', that is run a single instruction and stop until the programmer pressed a button to execute the next instruction. An ''address stop'' could be set to stop a running program when it attempted to execute an instruction or access data at a specified address. The contents of registers and memory would be displayed in the front-panel lights. The programmer could read and alter register contents, change program instructions or data in memory or force a branch to another section of code. When multiprogramming became the norm it was no longer acceptable to tie up an entire machine for debugging, except for special situations. Programs called ''

debugger A debugger or debugging tool is a computer program used to test and debug other programs (the "target" program). The main use of a debugger is to run the target program under controlled conditions that permit the programmer to track its executi ...

s'' were written which provided the programmer with the equivalent of the front-panel functions without requiring the entire machine.

Entertainment

For fun, bored programmers would create programs to display animated light shows. Front panels in the late 1960s and early 1970s were quite brightly colored. When bootstrap

s 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 The HP 9800 is a family of what were initially called programmable calculators and later desktop computers that were made by Hewlett-Packard, replacing their first HP 9100 calculator. It is also named "98 line". The 9830 and its successors wer ...

based on

, 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

The following procedure would bootstrap a

system from an

RK05 Digital Equipment Corporation's RK05 is a disk drive whose removable disk pack can hold about 2.5 megabytes of data. Introduced 1972, it is similar to IBM's 1964-introduced 2310, and uses a disk pack similar to IBM's 2315 disk pack, although ...

moving-head

magnetic disk Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is ac ...

: # Ensure that the machine is halted by lowering and raising the Halt switch; the front panel "RUN" light should then be off. # Set the 12 data switches to 0030 (Octal address 30), depress the Load Address ("ADDR LOAD") switch. The address lights will change to "0030". # Set the switches to 6743, raise the Deposit switch. The data lights will show this instruction. # Set the switches to 5031, raise the Deposit switch. The data lights will show this instruction. # Set the switches to 0030 (Octal address 30), depress the Load Address switch. The address lights will change back to "0030". # Depress the Clear switch. # 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 overlaid, the disk program takes control of the machine.

Notes

References

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