Break key

"Pause button" redirects here. For the media control, see Media controls. For other uses, see Pause.

Break/Pause key on PC keyboard

The Break key (or the symbol ⎉) of a computer keyboard refers to breaking a telegraph 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 101-key keyboard in 1985, the Break key is also called the Pause key. It can be used to pause some computer games.

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" 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 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. The resulting noise got the sending operator's attention.

This practice carried over to teleprinter use on time-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 the UART) recognized this as a special "break" condition and generated an interrupt that typically stopped a running program or forced the operating system to prompt for a login. Although asynchronous serial telegraphy is now rare, the Break key once used with terminal emulators can still be used by software for similar purposes.

Sinclair

On the ZX80 and ZX81 computers, the Break is accessed by pressing Space. On the ZX Spectrum it is accessed by Caps Shift+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.

BBC Micro

On a BBC Micro computer, the Break key generates a hardware reset which would normally cause a warm restart of the computer. A cold restart is triggered by pressing Ctrl+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 to Acorn MOS, RISC OS. These behaviours could be changed or exchanged in software, and were often used in rudimentary anti-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.

Modern keyboards

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 101/102-key keyboards) the Pause function was assigned to Ctrl+NumLock, and the Break function to Ctrl+ScrLock; these key-combinations still work with most programs, even on modern PCs with modern keyboards. Pressing the dedicated Pause key on 101/102-key keyboards sends the same scancodes as pressing Ctrl, then NumLock, then releasing them in the reverse order would do; additionally, an E1_hex prefix is sent, which enables 101/102-key-aware software to discern the two situations, while older software usually just ignores the prefix. The Pause 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 Break key is usually labeled Pause with Break below, sometimes separated by a line: ⁠Pause/Break⁠, 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 ⊞ Win+Pause brings up the system properties.

Keyboards without Break key

Compact and notebook keyboards often do not have a dedicated Pause/Break key.

Substitutes for Break:

• Ctrl+Fn+F11 or Fn+B or Fn+Ctrl+B on certain Lenovo laptops.
• Ctrl+Fn+B or Fn+B on certain Dell laptops.
• Ctrl+Fn+S on some other Dell laptops.
• Fn+Esc on Samsung.
• Ctrl+Fn+⇧ Shift on certain HP laptops.
• Fn+R on certain HP laptops.
• Fn+Ctrl+B on certain Logitech (LOGI) keyboards.

Substitutes for Pause:

• Fn+P or Fn+Ctrl+P or Fn+Alt+P on certain Lenovo laptops.
• Fn+B on certain Dell laptops.
• Fn+⇧ Shift on certain HP laptops.
• Fn+W on certain HP laptops.

For some Dell laptops, without a Break key, press the Alt+Space and select "Interrupt".

Usage for breaking the program's execution

While both Ctrl+Break and Ctrl+C 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 environments. 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) 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, while Ctrl+Break is often translated instantly (e.g. by INT 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 the BREAK=ON CONFIG.SYS statement.

See also

• System request
• Scroll lock
• Num lock

External links

• Media related to Break key at Wikimedia Commons

IBM PC keyboard (Windows, ANSI US layout)
Esc		F1	F2	F3	F4		F5	F6	F7	F8		F9	F10	F11	F12		PrtScn/ SysRq	Scroll Lock	Pause/ Break
<imagemap> File:KB United States-NoAltGr.svg|450px desc none # Row are 60 pixel tall in reference to the unsized original # rect left top right bottom [[link]] # Top row shift rect 0 0 60 30 Tilde rect 60 0 120 30 Exclamation mark rect 120 0 180 30 At sign rect 180 0 240 30 Number sign rect 240 0 300 30 Dollar sign rect 300 0 360 30 Percent sign rect 360 0 420 30 Caret rect 420 0 480 30 Ampersand rect 480 0 540 30 Asterisk rect 540 0 600 30 Parenthesis rect 600 0 660 30 Parenthesis rect 660 0 720 30 Underscore rect 720 0 780 30 Plus sign rect 780 0 900 30 Backspace Top row rect 0 0 60 60 backtick 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 rect 660 0 720 60 Hyphen-minus rect 720 0 780 60 Equals sign rect 780 0 900 60 Backspace Second row shift rect 0 61 90 90 Tab key rect 90 61 150 90 Q rect 150 61 210 90 W rect 210 61 270 90 E rect 270 61 330 90 R rect 330 61 390 90 T rect 390 61 450 90 Y rect 450 61 510 90 U rect 510 61 570 90 I rect 570 61 630 90 O rect 630 61 690 90 P rect 690 61 750 90 Curly bracket rect 750 61 810 90 Curly bracket rect 810 61 900 90 Vertical bar Second row 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 Square bracket rect 750 61 810 120 Square bracket rect 810 61 900 120 Backslash Third row shift rect 0 121 105 150 Caps lock rect 105 121 165 150 A rect 165 121 225 150 S rect 225 121 285 150 D rect 285 121 345 150 F rect 345 121 405 150 G rect 405 121 465 150 H rect 465 121 525 150 J rect 525 121 585 150 K rect 585 121 645 150 L rect 645 121 705 150 Colon (punctuation) rect 705 121 765 150 Quotation mark rect 765 121 900 150 Enter key 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 Enter key Fourth row shift rect 0 181 135 210 Shift key rect 135 181 195 210 Z rect 195 181 255 210 X rect 255 181 315 210 C rect 315 181 375 210 V rect 375 181 435 210 B rect 435 181 495 210 N rect 495 181 555 210 M rect 555 181 615 210 Bracket rect 615 181 675 210 Bracket rect 675 181 735 210 Question mark rect 735 181 900 210 Shift key 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 Slash (punctuation) 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 Alt key rect 690 241 750 300 Windows key rect 750 241 810 300 Menu key rect 810 241 900 300 Control key </imagemap>																	Insert	Home	PgUp		Num Lock	∕	∗	−
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