Break key

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Break/Pause key on PC keyboard

The Break key (or the symbol ⎊[1]) 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 and 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).

Break key on Teletype Model 33

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.[2][3] When this occurred, the teleprinter mechanisms continually actuated without printing anything,[3] 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 as a terminal 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[4] or forced the operating system to prompt for a login. Similarly to teleprinters, video terminals kept the Script error: No such module "key". key[5][6] which put transmission line to zero state.[7]

With its function of interrupting terminal communication, the Script error: No such module "key". key was adopted by software terminal emulators and in applications which use serial protocols such as RS-232. This concept of interruption also extends to the boot process of computers, network devices and to halting operations in computer applications like command-line interfaces, debuggers and spreadsheets.[8]

Sinclair

On the ZX80 and ZX81 computers, the Break is accessed by pressing Script error: No such module "key".. On the ZX Spectrum it is accessed by Script error: No such module "key".. The Spectrum+ and later computers have a dedicated Script error: No such module "key". 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 Script error: No such module "key". key, maps the function to Script error: No such module "key"..

BBC Micro

On a BBC Micro computer, the Script error: No such module "key". key generates a hardware reset which would normally cause a warm restart of the computer. A cold restart is triggered by pressing Script error: No such module "key".. If a filing system is installed, Script error: No such module "key". 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.

Modern keyboards

On many modern PCs, Script error: No such module "key". 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 Script error: No such module "key". key (before the introduction of 101/102-key keyboards) the Pause function was assigned to Script error: No such module "key"., and the Break function to Script error: No such module "key".; these key-combinations still work with most programs, even on modern PCs with modern keyboards. Pressing the dedicated Script error: No such module "key". key on 101/102-key keyboards sends the same scancodes as pressing Script error: No such module "key"., then Script error: No such module "key"., 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 Script error: No such module "key". 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. Also, it is not "typematic"— that is, unlike the other keys, the key sends repeat scancodes while being held. In PS/2 mode 3 or USB HID mode, there is a release scancode, so it is possible to determine whether the key is being held down on modern computers.[9]

On modern keyboards, the Script error: No such module "key". key is usually labeled Pause with Break below, sometimes separated by a line: Script error: No such module "key"., or Pause on the top of the keycap and Break on the front, or only Pause without Break at all. Keyboards with ISO/IEC 9995-7 markings including Canadian CSA keyboard use ⎊ symbol for Break and ⎉ for Pause.[1] In most Windows environments, the key combination Script error: No such module "key". brings up the system properties.

Keyboards without Break key

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Compact and notebook keyboards often do not have a dedicated Script error: No such module "key". key.

Substitutes for Script error: No such module "key".:

  • Script error: No such module "key". or Script error: No such module "key". or Script error: No such module "key". on certain Lenovo laptops.[10][11][12]
  • Script error: No such module "key". or Script error: No such module "key". on certain Dell laptops.[13][14]
  • Script error: No such module "key". on some other Dell laptops.[15][14]
  • Script error: No such module "key". on Samsung.
  • Script error: No such module "key". on certain HP laptops.
  • Script error: No such module "key". on certain HP laptops.
  • Script error: No such module "key". on certain Logitech (LOGI) keyboards.

Substitutes for Script error: No such module "key".:

  • Script error: No such module "key". or Script error: No such module "key". or Script error: No such module "key". on certain Lenovo laptops.[12]
  • Script error: No such module "key". on certain Dell laptops.[12][14]
  • Script error: No such module "key". on certain HP laptops.
  • Script error: No such module "key". on certain HP laptops.

For some Dell laptops, without a Script error: No such module "key". key, press the Script error: No such module "key". and select "Interrupt".[16][14]

Usage for breaking the program's execution

While both Script error: No such module "key". and Script error: No such module "key". 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.[17][18] Although these two are often considered interchangeable,[19] compilers and execution environments usually assign different signals to these.[20] Additionally, in some kernels (e.g. miscellaneous DOS variants) Script error: No such module "key". 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,[21][22] while Script error: No such module "key". is often translated instantly (e.g. by INT 1Bh under DOS). Because of this, Script error: No such module "key". 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

References

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  1. ^ a b Page Module:Citation/CS1/styles.css has no content.Québec clavier normalisé CAN/CSA Z243.200-92, Pictogrammes ISO 9995-7 [Quebec standardized keyboard CAN/CSA Z243.200-92, Pictograms ISO 9995-7] (PDF) (in français). Office québécois de la langue française. 2011-08-19.
  2. ^ Page Module:Citation/CS1/styles.css has no content."Line Break". 33 Keyboard: General Description and Principles of Operation (PDF). Skokie, Illinois: Teletype Corporation. 1971. p. 7.
  3. ^ a b Page Module:Citation/CS1/styles.css has no content."Line Break". AN/TGC-14(V) Teletypewriter I & O Maintenance. Study guide (PDF). Texas: Department of Communications and Missile Training. Sheppard Air Force Base. 1972. p. 3-5.
  4. ^ Page Module:Citation/CS1/styles.css has no content.Schur, Lee David (1973). "PL/1 (RUSH). Using the terminal". Time-shared computer languages: an introduction to conversational computing. Internet Archive. Reading, Mass., Addison-Wesley Pub. Co. p. 270. ISBN 978-0-201-06761-3.
  5. ^ Page Module:Citation/CS1/styles.css has no content."Section 2 Familiarization and Checkout" (PDF). Tektronix 4010 and 4010-1 Users Manual. Tektronix, Inc. 1972. p. 2-3.
  6. ^ Page Module:Citation/CS1/styles.css has no content."3.2.2 Special Function Keys" (PDF). Lear Siegler ADM-3A Operators Manual. Lear Siegler. p. 3-8. BREAK. This key activates the standard teletypewriter BREAK function, normally used to interrupt an incoming message.
  7. ^ Page Module:Citation/CS1/styles.css has no content."Chapter 3 Programmer Information". VT100 User Guide (3rd ed.). Digital Equipment Corporation. 1984. Typing the BREAK key causes the transmission line to be forced to its zero state
  8. ^ Page Module:Citation/CS1/styles.css has no content."What Does the Break Key Do in Windows & Virtual Machines?". Lenovo HK. Retrieved 2025-05-17.
  9. ^ Page Module:Citation/CS1/styles.css has no content."Keyboard 101- and 102-Key". PS/2 Hardware Interface Technical Reference. IBM. October 1990. p. 19. Archived (PDF) from the original on 2022-10-09. Except for the Pause key, all keys are make/break... Also, except for the Pause key, all keys are typematic
  10. ^ Page Module:Citation/CS1/styles.css has no content."Keyboard shortcut for Break, Pause, ScrLK, SysRq function on ThinkPad 6 rows Precision keyboard - ThinkPad - Lenovo Support US".
  11. ^ Page Module:Citation/CS1/styles.css has no content."Ctrl-Break key - Thinkpads Forum".
  12. ^ a b c Page Module:Citation/CS1/styles.css has no content."T530 Pause/Break key - Thinkpads Forum".
  13. ^ Page Module:Citation/CS1/styles.css has no content."Dell Inspiron 17 specification" (PDF). Archived (PDF) from the original on 2022-10-09.
  14. ^ a b c d Page Module:Citation/CS1/styles.css has no content."Break/Pause Key No Longer Available on Some Dell XPS, Latitude, and Precision Laptops | Dell US". www.dell.com. Retrieved 2023-05-05.
  15. ^ Page Module:Citation/CS1/styles.css has no content."Dell community". 7 January 2012.
  16. ^ Page Module:Citation/CS1/styles.css has no content.DELL. "Commonly-Used Keystroke Shortcuts". Dell.com. Retrieved 18 February 2018.
  17. ^ Page Module:Citation/CS1/styles.css has no content."CTRL+C (Break)". Msdn.microsoft.com. 2010-03-30. Retrieved 2010-10-22.
  18. ^ Page Module:Citation/CS1/styles.css has no content."Debug Break". Msdn.microsoft.com. 2010-03-30. Retrieved 2010-10-22.
  19. ^ Page Module:Citation/CS1/styles.css has no content."Ctrl-Break definition of Ctrl-Break in the Free Online Encyclopedia". Encyclopedia2.thefreedictionary.com. Retrieved 2010-10-22.
  20. ^ Page Module:Citation/CS1/styles.css has no content."Re: trouble passing ^C through to processes running in a *shell*". www.gnu.org. Archived from the original on October 22, 2012.
  21. ^ Page Module:Citation/CS1/styles.css has no content."Vim documentation: vim online". Vim.org. Retrieved 2010-10-22.
  22. ^ Page Module:Citation/CS1/styles.css has no content."ClipX — DOS 5.0 Ref. - int 23h ctrl-c handler". Ousob.com. Archived from the original on 2016-03-04. Retrieved 2010-10-22.
  • Page Template:Sister-inline/styles.css has no content.Script error: No such module "Sister project logo". Media related to Script error: No such module "Commons link". at Wikimedia Commons

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