Break key
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The break key is a special key on computer keyboards that no longer has a well-defined purpose. Its origins go back to telegraph practices.
A standard telegraph key has a built-in knife switch that can be used to short the key's contacts. When the key was not in use, that switch was kept closed, so that a signal was continually sent. If that steady-state signal was interrupted, it indicated one of two things: either the operator was about to start transmitting, or something else had happened to break the connection — such as hostiles cutting the telegraph line.
As the teletype came into use, a key was created that would temporarily interrupt the communications line, allowing this practice to continue. When this occurred, the receiving teletype mechanism would activate but not print characters (it was typing the non-printing "DEL" character repeatedly). The noise got the operator's attention.
Time-sharing computers connected to modems could recognize the break because the "DEL" characters received had "bad parity." This was usually used to generate an interrupt and allow login or stop a running program.
On personal computers, the Break key is used in different ways by different programs, but usually involves some sense of changing the interaction with the computer, such as switching between multiple login sessions, terminating a program, or interrupting a modem connection.
On a BBC Micro computer, the Break key generated an interrupt which would normally cause a warm restart of the computer. By holding Control and pressing break, a cold restart is triggered. If a DFS ROM is installed, holding Shift and pressing break will cause the computer to search for and load a file called !boot on the floppy disk in drive 0.
On the Sinclair ZX80, ZX81 and ZX Spectrum computers, the Break key is accessed by pressing Shift and Space.[dubious — see talk page] The Spectrum+ and later computers have a dedicated Break key that emulates this keypress. 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 computer does not have a break key, but pressing Control and Space has the same effect.
On many modern PCs, Pause interrupts screen output by BIOS until another key is pressed. This is effective during boot in text mode and in a DOS box in Windows safe mode with 50 lines. This functionality had been there with the very first PCs, except that before the introduction of 101 keys keyboards (previous keyboards did not have a pause key), one had to press Ctrl + Num Lock instead, a combination that even on modern PCs with modern keyboards still does exactly the same as the 'Pause' key.
In programming, especially old DOS-style BASIC, Pascal and C++, Break is used (in conjunction with Control key) to stop program execution. In addition to this, Linux and variants, as well as many DOS programs, treat this combination the same as Ctrl+C. Finally, the combination can be used to stop a batch file, which is highly useful on older DOS machines that ran batch scripts on boot up.
On modern keyboards, the break key is usually labeled Pause/Break. On many games this was the key to pause the game. In most Windows environments, the key combination Windows key+Pause brings up the system properties.
Compact and notebook keyboards often have the Pause/Break key mapped to a function command. As a result, pressing Ctrl+Pause in a notebook computer may require the user to press something else, such as Ctrl+Fn+F11.
[edit] See also
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|---|---|
| Dead keys | Compose |
| Modifier keys | Control · Shift · Alt/Option (Apple) · AltGr · Command/Meta (MIT keyboards) · Windows · Fn (compact keyboards) |
| Lock keys | Scroll lock · Num lock · Caps lock · Shift key |
| Navigation | Arrow · Page scrolling (Page up/Page down) · Home/End |
| Editing | Return/Enter · Backspace · Insert · Delete · Tab · Space bar |
| Misc. | SysRq/Print screen · Break/Pause · Escape · Menu · Numeric keypad · Function · Power management (Power, Sleep, Wake) · Language input |
