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A 102-key PC US English keyboard layout with system request circled

System request (often abbreviated SysRq, SysReq, Sys Rq or Sys Req) is a key on keyboards for PCs that has no standard use. This key can be traced back to the operator interrupt key used on IBM 3270-type console keyboards of the IBM System/370 mainframe computer, which was used to cause the operating system such as VM/370 or MVS 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-level operating system functions with no possibility of conflicting with any existing software. A special BIOS routine — software interrupt hexadecimal 15, subfunction hexadecimal 85 — was added to signal the OS when SysReq was pushed or released. Unlike most keys, when it is pressed nothing is stored in the keyboard 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 many hotkey combinations. The use of TSR programs further complicated matters. To implement a task 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 under Windows NT.

On 84-key keyboards, SysReq 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 the Ctrl key and Alt key while pressing this “dual-function” key to invoke SysReq.

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 the Print 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.

In Linux, the kernel can be configured to provide functions for system debugging and crash recovery. This use is known as the "Magic SysRq key".

Microsoft has used SysRq for various OS- and application-level debuggers. In the CodeView debugger, it was sometimes used to break into the debugging during program execution.. For the Windows NT remote kernel debugger, it can be used to force the system into the debugger.

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).

References

1. ^ ^{a b} What is the SysRq key for?. comp.os.msdos.programmer FAQ. Retrieved on 2008-01-08.
2. ^ Ralf Brown's Interrupt List. Retrieved on 2008-01-08.
3. ^ Linux Magic System Request Key Hacks. Linux kernel. Retrieved on 2008-01-08.
4. ^ Returning Control to CodeView. Microsoft Knowledge Base. Retrieved on 2008-01-08.
5. ^ How to enable a remote kernel debugger connection on a computer that is running Windows Server 2003 with Service Pack 1. Microsoft Knowledge Base. Retrieved on 2008-01-08.