- System request
System request (often abbreviated SysRq or Sys Req) is a key on keyboards for PCs that has no standard usecite web|title=What is the SysRq key for?|work=comp.os.msdos.programmer FAQ|url=http://www.faqs.org/faqs/msdos-programmer-faq/part2/|accessdate=2008-01-08] . This key can be traced back to the operator interrupt key used on
IBM 3270 -type console keyboards of the IBMSystem/370 mainframe computer Fact|date=January 2008, which was used to cause the operating system such asVM/370 orMVS to allow the console to give input to the operating system.Original purpose
Introduced by
IBM with the PC/AT, it was intended to be available as a special key to directly invoke low-leveloperating system functions with no possibility of conflicting with any existing software. A specialBIOS routine — software interrupt hexadecimal 15, subfunction hexadecimal 85cite web|title=Ralf Brown's Interrupt List|url=http://www.ctyme.com/intr/rb-1522.htm|accessdate=2008-01-08] — was added to signal the OS when SysRq was pushed or released. Unlike most keys, when it is pressed nothing is stored in thekeyboard buffer .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 manyhotkey combinations. The use of TSR programs further complicated matters. To implement atask switching or multitasking environment, it was thought that a special, separate key was needed. This is similar to the way “Ctrl + Alt + Del” is used underWindows NT .On 84-key keyboards, SysRq was a key of its own. On the later 101-key keyboard, it shares a physical key with the
Print Screen key function. One must hold down theAlt key while pressing this “dual-function” key to invoke SysRq.The default BIOS keyboard routines simply ignore SysRq and return without taking action. So did the
MS-DOS input routines. The keyboard routines in libraries supplied with many high-level languages followed suit. Although still included in most PC keyboards manufactured, and some debugging software uses it, the key is of no use for the vast majority of users, except that it is usually combined with thePrint Screen key.Other uses
Under
DOS , malfunctioning foreground software could make the entire computer unresponsive. Third-party TSRs existed that used SysRq to provide a form of “panic button” to terminate the program and return the user to the DOS command prompt.Fact|date=January 2008In
Linux , the kernel can be configured to provide functions for systemdebugging and crash recovery.cite web|title=Linux Magic System Request Key Hacks|work=Linux kernel|url=http://lxr.linux.no/linux/Documentation/sysrq.txt|accessdate=2008-01-08] This use is known as the "Magic SysRq key ".Microsoft has used SysRq for various OS- and application-leveldebugger s. In the CodeView debugger, it was sometimes used to break into the debugging during program execution.cite web|title=Returning Control to CodeView|work=Microsoft Knowledge Base|url=http://support.microsoft.com/kb/43648|accessdate=2008-01-08] . For theWindows NT remote kernel debugger, it can be used to force the system into the debugger.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|accessdate=2008-01-08]On the Hyundai/Hynix Super-16 computer, pressing Ctrl+SysRq will hard boot the system (it will reboot when Ctrl+Alt+Del is unresponsive, and invoke startup memory tests that are bypassed on soft-boot).Fact|date=January 2008
References
See also
*
Print Screen
*Serial console
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