TheBreak key (or the symbol⎉) of acomputer keyboard refers to breaking atelegraph 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 theIBM Model M 101-key keyboard in 1985, the Break key is also called thePause key. It can be used to pause somecomputer games.
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" 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).
Theteleprinter 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-printingNUL in bothBaudot andASCII. The resulting noise got the sending operator's attention.
This practice carried over to teleprinter use ontime-sharing 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 theUART) recognized this as a special "break" condition and generated aninterrupt that typically stopped a running program or forced the operating system to prompt for a login. Although asynchronous serial telegraphy is now rare, theBreak key once used with terminal emulators can still be used by software for similar purposes.
On theZX80 andZX81 computers, the Break is accessed by pressingSpace. On theZX Spectrum it is accessed byCaps Shift+Space. The Spectrum+ and later computers have a dedicatedBreak 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 theCONTINUE command. TheSinclair QL computer, without aBreak key, maps the function toCtrl+Space.
On aBBC Micro computer, theBreak key generates a hardware reset which would normally cause awarm restart of the computer. Acold restart is triggered by pressingCtrl+Break. If a filing system is installed,⇧ Shift+Break 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 toAcorn MOS,RISC OS. These behaviours could be changed or exchanged in software, and were often used in rudimentaryanti-piracy 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.
On many modernPCs,Pause interrupts screen output byBIOS until another key is pressed. This is effective duringboot intext mode and in aDOS box inWindows safe mode with 50 lines. On early keyboards without aPause key (before the introduction of 101/102-key keyboards) the Pause function was assigned toCtrl+NumLock, and the Break function toCtrl+ScrLock; these key-combinations still work with most programs, even on modern PCs with modern keyboards. Pressing the dedicatedPause key on 101/102-key keyboards sends the samescancodes as pressingCtrl, thenNumLock, 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. ThePause 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.[1]
On modern keyboards, theBreak key is usually labeledPause withBreak below, sometimes separated by a line:Pause/Break, orPause on the top of the keycap andBreak on the front, or onlyPause withoutBreak at all. In most Windows environments, the key combination⊞ Win+Pause brings up the system properties.
Compact and notebook keyboards often do not have a dedicatedPause/Break key.
Substitutes forBreak:
Substitutes forPause:
For some[clarification needed] Dell laptops, without aBreak key, press theAlt+Space and select "Interrupt".[8][6]
While bothCtrl+Break andCtrl+C combination are commonly implemented as a way of breaking the execution of a console application, they are also used for similar effect inintegrated development environments.[9][10] Although these two are often considered interchangeable,[11] compilers and execution environments usually assign different signals to these.[12] Additionally, in some kernels (e.g. miscellaneous DOS variants)Ctrl+C 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,[13][14] whileCtrl+Break is often translated instantly (e.g. byINT 1Bh under DOS). Because of this,Ctrl+Break is usually a more effective choice under these operating systems; sensitivity for these two combinations can be enhanced by theBREAK=ONCONFIG.SYS statement.
Except for the Pause key, all keys are make/break.
Media related toBreak key at Wikimedia Commons| Esc | F1 | F2 | F3 | F4 | F5 | F6 | F7 | F8 | F9 | F10 | F11 | F12 | PrtScn/ SysRq | Scroll Lock | Pause/ Break | |||||||||
 | Insert | Home | PgUp | Num Lock | ∕ | ∗ | − | |||||||||||||||||
| Delete | End | PgDn | 7 | 8 | 9 | + | ||||||||||||||||||
| 4 | 5 | 6 | ||||||||||||||||||||||
| ↑ | 1 | 2 | 3 | Enter | ||||||||||||||||||||
| ← | ↓ | → | 0 Ins | . Del | ||||||||||||||||||||