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The Break key (or the symbol ) of a
computer keyboard A computer keyboard is a peripheral input device modeled after the typewriter keyboard which uses an arrangement of buttons or keys to act as mechanical levers or electronic switches. Replacing early punched cards and paper tape technology ...
refers to breaking a
telegraph Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas p ...
circuit and originated with 19th century practice. In modern usage, the key has no well-defined purpose, but while this is the case, it can be used by software for miscellaneous tasks, such as to switch between multiple login sessions, to terminate a program, or to interrupt a modem connection. Because the break function is usually combined with the pause function on one key since the introduction of the
IBM Model M Model M designates a group of computer keyboards designed and manufactured by IBM starting in 1985, and later by Lexmark International, Maxi Switch, and Unicomp. The keyboard's many variations have their own distinct characteristics, with ...
101-key keyboard in 1985, the Break key is also called the Pause key. It can be used to pause some
computer game Video games, also known as computer games, are electronic games that involves interaction with a user interface or input device such as a joystick, game controller, controller, computer keyboard, keyboard, or motion sensing device to gener ...
s.


History

A standard telegraph circuit connects all the keys, sounders and batteries in a single series loop. Thus the sounders actuate only when both keys are down (closed, also known as " marking" — after the ink marks made on paper tape by early printing telegraphs). So the receiving operator has to hold their key down or close a built-in shorting switch in order to let the other operator send. As a consequence, the receiving operator could interrupt the sending operator by opening their key, breaking the circuit and forcing it into a "
spacing Spacing may refer to: * ''Spacing'' (magazine), a Canadian magazine * Spacing effect in psychology; the opposite of cramming * The usage of spaces in typography ** Letter-spacing, the amount of space between a group of letters ** Line spacing or ...
" condition. Both sounders stop responding to the sender's keying, alerting the sender. (A physical break in the telegraph line would have the same effect.) The
teleprinter A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Initia ...
operated in a very similar fashion except that the sending station kept the loop closed (logic 1, or "marking") even during short pauses between characters. Holding down a special "break" key opened the loop, forcing it into a continuous logic 0, or "spacing", condition. When this occurred, the teleprinter mechanisms continually actuated without printing anything, as the all-0s character is the non-printing ''NUL'' in both Baudot and
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
. The resulting noise got the sending operator's attention. This practice carried over to teleprinter use on
time-sharing In computing, time-sharing is the sharing of a computing resource among many users at the same time by means of multiprogramming and multi-tasking.DEC Timesharing (1965), by Peter Clark, The DEC Professional, Volume 1, Number 1 Its emergence a ...
computers. A continuous spacing (logical 0) condition violates the rule that every valid character has to end with one or more logic 1 (marking) "stop" bits. The computer (specifically the
UART A universal asynchronous receiver-transmitter (UART ) is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significan ...
) recognized this as a special "break" condition and generated an
interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...
that typically stopped a running program or forced the operating system to prompt for a login. Although asynchronous serial telegraphy is now rare, the key once used with terminal emulators can still be used by software for similar purposes.


Sinclair

On the
Sinclair ZX80 The Sinclair ZX80 is a home computer launched on 29 January 1980 by Science of Cambridge Ltd. (later to be better known as Sinclair Research). It is notable for being one of the first computers available in the United Kingdom for less than a h ...
and
ZX81 The ZX81 is a home computer that was produced by Sinclair Research and manufactured in Dundee, Scotland, by Timex Corporation. It was launched in the United Kingdom in March 1981 as the successor to Sinclair's ZX80 and designed to be a low-cost ...
computers, the Break is accessed by pressing . On the
Sinclair ZX Spectrum The ZX Spectrum () is an 8-bit home computer that was developed by Sinclair Research. It was released in the United Kingdom on 23 April 1982, and became Britain's best-selling microcomputer. Referred to during development as the ''ZX81 Colour ...
it is accessed by . The Spectrum+ and later computers have a dedicated key. It does not trigger an interrupt but will halt any running BASIC program, or terminate the loading or saving of data to cassette tape. An interrupted BASIC program can usually be resumed with the CONTINUE command. The
Sinclair QL The Sinclair QL (for ''Quantum Leap'') is a personal computer launched by Sinclair Research in 1984, as an upper-end counterpart to the ZX Spectrum. The QL was aimed at the serious home user and professional and executive users markets from small ...
computer, without a key, maps the function to .


BBC Micro

On a
BBC Micro The British Broadcasting Corporation Microcomputer System, or BBC Micro, is a series of microcomputers and associated peripherals designed and built by Acorn Computers in the 1980s for the BBC Computer Literacy Project. Designed with an emphas ...
computer, the key generates a hardware reset which would normally cause a warm restart of the computer. A
cold restart In computing, rebooting is the process by which a running computer system is restarted, either intentionally or unintentionally. Reboots can be either a cold reboot (alternatively known as a hard reboot) in which the power to the system is physi ...
is triggered by pressing . If a filing system is installed, will cause the computer to search for and load or run a file called !Boot on the filing system's default device (e.g. floppy disk 0, network user BOOT). The latter two behaviours were inherited by the successor to
Acorn MOS The Machine Operating System (MOS) or OS is a discontinued computer operating system (OS) used in Acorn Computers' BBC computer range. It included support for four-channel sound, graphics, file system abstraction, and digital and analogue inp ...
,
RISC OS RISC OS is a computer operating system originally designed by Acorn Computers Ltd in Cambridge, England. First released in 1987, it was designed to run on the ARM chipset, which Acorn had designed concurrently for use in its new line of Archim ...
. These behaviours could be changed or exchanged in software, and were often used in rudimentary
anti-piracy Anti-piracy may refer to: * Anti-piracy, protection against copying of computer software. * Piracy#Anti-piracy measures anti-piracy measures, measures to counter maritime pirates. See also * Pirate (disambiguation) A pirate is a person who comm ...
techniques. Because of the BBC Micro's near universal usage in British schools, later versions of the machine incorporated a physical lock on the Break key to stop children from intentionally resetting the computer.


Modern keyboards

On many modern PCs, interrupts screen output by
BIOS In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
until another key is pressed. This is effective during
boot A boot is a type of footwear. Most boots mainly cover the foot and the ankle, while some also cover some part of the lower calf. Some boots extend up the leg, sometimes as far as the knee or even the hip. Most boots have a heel that is cle ...
in
text mode Text mode is a computer display mode in which content is internally represented on a computer screen in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of ''character cells'', each ...
and in a
DOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicatio ...
box in
Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
safe mode with 50 lines. On early keyboards without a key (before the introduction of 101/102-key keyboards) the Pause function was assigned to , and the Break function to ; these key-combinations still work with most programs, even on modern PCs with modern keyboards. Pressing the dedicated key on 101/102-key keyboards sends the same
scancode A scancode (or scan code) is the data that most computer keyboards send to a computer to report which keys have been pressed. A number, or sequence of numbers, is assigned to each key on the keyboard. Variants Mapping key positions by row an ...
s as pressing , then , then releasing them in the reverse order would do; additionally, an E1hex prefix is sent, which enables 101/102-key-aware software to discern the two situations, while older software usually just ignores the prefix. The key is different from all other keys in that it sends no scancodes at all on release in PS/2 modes 1 or 2, so it is impossible to determine whether this key is being held down with older devices. In PS/2 mode 3 or USB HID mode, there is a release scancode, so it is possible to determine whether this key is being held down on modern computers. On modern keyboards, the key is usually labeled ''Pause'' with ''Break'' below, sometimes separated by a line: , or ''Pause'' on the top of the keycap and ''Break'' on the front, or only ''Pause'' without ''Break'' at all. In most Windows environments, the key combination brings up the system properties.


Keyboards without Break key

Compact and notebook keyboards often do not have a dedicated key. Substitutes for : * or or on certain
Lenovo Lenovo Group Limited, often shortened to Lenovo ( , ), is a Chinese Multinational corporation, multinational technology company specializing in designing, manufacturing, and marketing consumer electronics, Personal computer, personal computers, ...
laptops. * or on certain Dell laptops. * on some other Dell laptops. * on Samsung. * on certain HP laptops. * on certain HP laptops. * on certain Logitech (LOGI) keyboards. Substitutes for : * or or on certain Lenovo laptops. * on certain Dell laptops. * on certain HP laptops. * on certain HP laptops. For some Dell laptops, without a key, press the and select "Interrupt".


Usage for breaking the program's execution

While both and combination are commonly implemented as a way of breaking the execution of a console application, they are also used for similar effect in
integrated development environment An integrated development environment (IDE) is a software application that provides comprehensive facilities to computer programmers for software development. An IDE normally consists of at least a source code editor, build automation tools a ...
s. Although these two are often considered interchangeable, compilers and execution environments usually assign different signals to these. Additionally, in some kernels (e.g. miscellaneous DOS variants) is detected only at the time OS tries reading from a keyboard buffer and only if it's the only key sequence in the buffer, while is often translated instantly (e.g. by INT 1Bh under DOS). Because of this, is usually a more effective choice under these operating systems; sensitivity for these two combinations can be enhanced by the
BREAK Break or Breaks or The Break may refer to: Time off from duties * Recess (break), time in which a group of people is temporarily dismissed from its duties * Break (work), time off during a shift/recess ** Coffee break, a short mid-morning rest ...
=ON
CONFIG.SYS CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system's DOS BIOS (typically residing ...
statement.


See also

*
System request System Request (SysRq or Sys Req) is a key on personal computer keyboards that has no standard use. Introduced by IBM with the PC/AT, it was intended to be available as a special key to directly invoke low-level operating system functions with no ...
* Scroll lock * Num lock


References


External links

* {{DEFAULTSORT:Break Key Computer keys Out-of-band management