World Library  
Flag as Inappropriate
Email this Article

Break key

Article Id: WHEBN0000004636
Reproduction Date:

Title: Break key  
Author: World Heritage Encyclopedia
Language: English
Subject: Keyboard layout, Power management keys, Secure attention key, Scroll lock, Computer keyboard
Collection: Computer Keys, Out-of-Band Management
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Break key

Break/Pause key on PC keyboard

The Break key is a key on a computer keyboard. The Break key originated with 19th century telegraph practice. While the Break key might no longer have a well-defined purpose, it can be used by software 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 games, such as Deus Ex and the Call Of Duty series.

Contents

  • History 1
    • Teleprinter 1.1
  • Sinclair 2
  • BBC Micro 3
  • Modern keyboards 4
  • Keyboards without Break key 5
  • Usage for breaking the program's execution 6
  • References 7
  • See also 8

History

A standard telegraph circuit connects all the keys, sounders and batteries in a single series loop so the sounders actuate only when both keys are down (closed or "marking", after the ink marks made on paper tape by early printing telegraphs). The receiving operator thus has to hold his key down or close a built-in shorting switch to let the other operator send. The receiving operator could interrupt the sending operator by opening his 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.)

Teleprinter

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-0's 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 Sinclair ZX80 and ZX81 computers, the Break is accessed by pressing Space. On the Sinclair 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 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. The latter two behaviours were inherited by the successor to Acorn MOS, RISC OS.

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 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 Pause key is different from all other keys in that it sends no scancodes at all on release; therefore it is not possible for any software to determine whether this key is being held down.[1]

On modern keyboards, the Break key is usually labeled Pause with Break below, sometimes separated by a line, or Pause on the top of the keycap and Break on the front. 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. These may use the following substitutes for Break:

  • Ctrl+Fn+F11 or Fn+B or Fn+Ctrl+B (on certain Lenovo laptops).[2][3]
  • Fn+Esc on Samsung.

Substitutes for Pause:

  • Fn+P or Fn+Ctrl+P or Fn+Alt+P (on certain Lenovo laptops).[3]

Apple keyboards do not have the Pause/Break key, as Mac OS X does not use it.[4]

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.[5][6] Although these two are often considered interchangeable,[7] compilers and execution environments usually assign different signals to these.[8] 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,[9][10] 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.

References

  1. ^
  2. ^ http://forum.thinkpads.com/viewtopic.php?f=60&t=98708
  3. ^ a b http://forum.thinkpads.com/viewtopic.php?t=105807
  4. ^ Computer Hope, Pause key http://www.computerhope.com/jargon/p/pausekey.htm
  5. ^
  6. ^
  7. ^
  8. ^ [1] Archived October 22, 2012 at the Wayback Machine
  9. ^
  10. ^

See also

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.