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Break/Pause key on PC keyboard

The break key on computer keyboards 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 a physical break in 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 used to generate an interrupt, allow login, or stop a running program.

On personal computers, the Break key can be used by software in several different ways, such as to switch between multiple login sessions, to terminate a program, or to interrupt a modem connection. On a BBC Micro computer, the Break key generates an interrupt which would normally cause a warm restart of the computer. A cold restart is triggered by typing Ctrl+break. If a DFS ROM is installed, Shift+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 and ZX81 computers, the Break is accessed by pressing Space. On the ZX Spectrum it is accessed by Shift and Space. The Spectrum+ and later computers have a dedicated Break 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 computer, without a break key, maps the function to Ctrl+Space. 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. On early keyboards without a Pause key (before the introduction of 101-key keyboards) this function was assigned to Ctrl+NumLock, and this key-combination still works, even on modern PCs with modern keyboards.

In programming, especially old DOS-style BASIC, Pascal and C, Break is used (in conjunction with Ctrl) 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.

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: for example Ctrl+Fn+F11.

IBM/Wintel Keyboard:
Esc	F1	F2	F3	F4	F5	F6	F7	F8	F9	F10	F11	F12	PtSc	ScLk	Brk

<imagemap> Image:KB United States.svg\|450px desc none # Row are 60 pixel tall in reference to the unsized original # rect left top right bottom [[link]] # Top row rect 0 0 60 60 Tilde rect 60 0 120 60 1 (number) rect 120 0 180 60 2 (number) rect 180 0 240 60 3 (number) rect 240 0 300 60 4 (number) rect 300 0 360 60 5 (number) rect 360 0 420 60 6 (number) rect 420 0 480 60 7 (number) rect 480 0 540 60 8 (number) rect 540 0 600 60 9 (number) rect 600 0 660 60 0 (number) rect 660 0 720 60 Hyphen rect 720 0 780 60 Equals sign rect 780 0 900 60 Backspace Second rowrect rect 0 61 90 120 Tab key rect 90 61 150 120 Q rect 150 61 210 120 W rect 210 61 270 120 E rect 270 61 330 120 R rect 330 61 390 120 T rect 390 61 450 120 Y rect 450 61 510 120 U rect 510 61 570 120 I rect 570 61 630 120 O rect 630 61 690 120 P rect 690 61 750 120 Bracket rect 750 61 810 120 Bracket rect 810 61 900 120 Vertical bar Third row rect 0 121 105 180 Caps lock rect 105 121 165 180 A rect 165 121 225 180 S rect 225 121 285 180 D rect 285 121 345 180 F rect 345 121 405 180 G rect 405 121 465 180 H rect 465 121 525 180 J rect 525 121 585 180 K rect 585 121 645 180 L rect 645 121 705 180 Semicolon rect 705 121 765 180 Apostrophe rect 765 121 900 180 Carriage return Fourth row rect 0 181 135 240 Shift key rect 135 181 195 240 Z rect 195 181 255 240 X rect 255 181 315 240 C rect 315 181 375 240 V rect 375 181 435 240 B rect 435 181 495 240 N rect 495 181 555 240 M rect 555 181 615 240 Comma (punctuation) rect 615 181 675 240 Full stop rect 675 181 735 240 Question mark rect 735 181 900 240 Shift key Bottom row rect 0 241 90 300 Control key rect 90 241 150 300 Windows key rect 150 241 240 300 Alt key rect 240 241 600 300 Space bar rect 600 241 690 300 AltGr key rect 690 241 750 300 Windows key rect 750 241 810 300 Menu key rect 810 241 900 300 Control key </imagemap>													Ins	Home	PgUp	Num	/	*	-
													Del	End	PgDn	7	8	9	+
																4	5	6	+
														↑		1	2	3	Ent
													←	↓	→	0		.	Ent

Keyboard keys
Dead keys	Compose
Modifier keys	Control · Shift · Alt/Option (Apple) · AltGr · Command/Meta (Apple/MIT/Sun keyboards) · Windows/Super · Fn (compact keyboards)
Lock keys	Scroll lock · Num lock · Caps lock
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 · Any key

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		Sci-Tech Dictionary. McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved. Read more
		Computer Desktop Encyclopedia. THIS COPYRIGHTED DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher. © 1981-2008 Computer Language Company Inc. All rights reserved. Read more
		Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Break key". Read more