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{{Short description|Computer key}}
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[[Image:KeyboardWithPrintScreenRinged.svg|thumb|A 104-key [[Personal computer|PC]] [[United States|US]] [[English language|English]] [[keyboard layout]] with System request circled|400x400px]]
'''System Request''' ('''SysRq''' or '''Sys Req''') is a key on [[IBM PC compatible|personal computer]] [[Computer keyboard|keyboards]] that has no standard use. Introduced by [[IBM]] with the [[IBM Personal Computer/AT|PC/AT]], it was intended to be available as a special key to directly invoke low-level [[operating system]] functions with no possibility of conflicting with any existing software. A special [[BIOS]] routine – software interrupt 0x15, subfunction 0x85<ref name="intlist">{{cite web|title=Ralf Brown's Interrupt List|url=https://www.delorie.com/djgpp/doc/rbinter/id/30/15.html|access-date=2018-06-21}}</ref> – was added to signal the [[operating system|OS]] when SysRq was pushed or released. Unlike most keys, when it is pressed nothing is stored in the [[keyboard buffer]].

== History ==

The specific low level function intended for the SysRq key was to switch between operating systems. When the original IBM-PC was created in 1980, there were three leading competing operating systems: [[PC DOS]], [[CP/M-86]], and [[UCSD p-System]],<ref>{{cite web|url=http://www-03.ibm.com/ibm/history/exhibits/pc25/pc25_fact.html|archive-url=https://web.archive.org/web/20060814072427/http://www-03.ibm.com/ibm/history/exhibits/pc25/pc25_fact.html|url-status=dead|archive-date=August 14, 2006|title=Original IBM PC Product fact sheet|date=August 12, 1981|publisher=IBM Information Systems Division|access-date=January 13, 2010}}</ref> while [[Xenix]] was added in 1983–1984. The SysRq key was added so that multiple operating systems could be run on the same computer, using the capabilities of the [[Intel 80286|286]] chip in the PC/AT.<ref>Personal interview with Sandy Meade, creator of the SysRq key</ref>{{vc|date=April 2013}}

A special key was needed because most [[software]] of the day operated at a low level, often bypassing the OS entirely, and typically made use of many [[hotkey]] combinations. The use of [[terminate-and-stay-resident]] (TSR) programs further complicated matters. To implement a [[Context switch|task switching]] or [[Computer multitasking|multitasking]] environment, it was thought that a special, separate key was needed. This is similar to the way "[[Control-Alt-Delete]]" is used under [[Windows NT]].

On 84-key keyboards (except the 84-key IBM Model M space saver keyboard), SysRq was a key of its own. On the later 101-key keyboard, it shares a physical key with the [[Print screen]] key function. The [[Alt key]] must be held down while pressing this dual-function key to invoke SysRq. This continues through modern PC keyboards.<ref name="modern">{{cite web|title=What Are the Sys Rq, Scroll Lock, and Pause Break Keys on My Keyboard?|date=26 September 2016 |access-date=March 2, 2020|url=https://www.howtogeek.com/125315/htg-explains-what-are-the-sys-rq-scroll-lock-and-pausebreak-keys-on-my-keyboard/}}</ref>

The default BIOS keyboard routines simply ignore SysRq and return without taking action. So did the [[MS-DOS]] input routines. The keyboard routines in [[Library (software)|libraries]] supplied with many high-level languages followed suit. Although it is still included on most PC keyboards manufactured, and though it is used by some{{which|date=July 2019}} debugging software, the key is of no use for the vast majority of users.

On the [[Hynix|Hyundai/Hynix]] Super-16 computer, pressing {{key press|Ctrl|SysRq}} will hard boot the system (it will reboot when {{key press|Ctrl|Alt|Del}} is unresponsive, and it will invoke startup memory tests that are bypassed on soft-boot).{{Fact|date=January 2008}}

== Modern uses ==
In [[Linux]], the [[Kernel (operating system)|kernel]] can be configured to provide functions for system [[debugging]] and crash recovery.<ref name="linux">{{cite web|title=Linux Magic System Request Key Hacks|work=Linux kernel|url=https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html|access-date=2018-06-21}}</ref>  This use is known as the "[[magic SysRq key]]".

[[Microsoft]] has also used SysRq for various [[Operating system|OS]]- and [[Application software|application]]-level [[debugger]]s. In the [[CodeView|CodeView debugger]], it was sometimes used to break into the debugging during program execution.<ref name="mskb43648">{{cite web|title=Returning Control to CodeView|work=Microsoft Knowledge Base|url=http://support.microsoft.com/kb/43648|access-date=January 8, 2008|archiveurl=https://web.archive.org/web/20081108021540/http://support.microsoft.com/kb/43648|archivedate=November 8, 2008|url-status=dead}}</ref> For the [[Windows NT]] remote kernel debugger, it can be used to force the system into the debugger.<ref name="mskb900093">{{cite web|title=How to enable a remote kernel debugger connection on a computer that is running Windows Server 2003 with Service Pack 1|work=Microsoft Knowledge Base|url=http://support.microsoft.com/kb/900093|access-date=January 8, 2008|archiveurl=https://web.archive.org/web/20121021034022/http://support.microsoft.com/kb/900093|archivedate=October 21, 2012|url-status=dead}}</ref>

On modern Windows and some Linux desktop environments "[[Alt key|Alt]] + [[Print screen]]" (which is the shortcut for SysRq on most modern keyboards<ref name="modern" />) copies an image of the current active window, instead of the entire screen as Print Screen alone would.<ref>{{cite web|title=Copy the window or screen contents|access-date=March 2, 2020|url=https://support.microsoft.com/en-us/office/copy-the-window-or-screen-contents-98c41969-51e5-45e1-be36-fb9381b32bb7}}</ref>

== Similar keys ==
[[IBM 3270]]-type console keyboards of the IBM [[System/370]] [[mainframe computer]], created in 1970, had an operator interrupt key that was used to cause the operating system such as [[VM/370]] or [[MVS]] to allow the console to give input to the operating system.

== See also ==
* [[Serial console]]
* [[Break key]]
* [[Scroll Lock]]

==References==
{{Reflist}}

==External links==
*{{Commonscatinline}}

{{Keyboard}}
{{keyboard keys}}

{{DEFAULTSORT:System Request}}
[[Category:Computer keys]]
[[Category:System administration|Request]]
[[Category:Out-of-band management]]
[[Category:IBM personal computers]]