System request

related topics
{system, computer, user}
{math, number, function}

System request (often abbreviated SysRq or Sys Req) is a key on keyboards for PCs that has no standard use.[1] 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[citation needed], 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.

Contents

History

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[2] — was added to signal the OS when SysRq was pushed or released. Unlike most keys, when it is pressed nothing is stored in the keyboard buffer.

The specific low level function that the SysRq key was meant for 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[3], while Xenix was added in 1983-1984. The SysRq key was added so that multiple operating systems could be run on the same computer, making use of the capabilities of the 286 chip in the PC/AT.[4]

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 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 (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. One must hold down the Alt 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.[1] The keyboard routines in 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 debugging software, the key is of no use for the vast majority of users.

Full article ▸

related documents
Multiple-image Network Graphics
Java Platform, Enterprise Edition
Batch processing
StrongARM
Sendmail
Coda (file system)
Carrier sense multiple access
Myrinet
Revision Control System
Physical Layer
Wireless Markup Language
UAE (emulator)
Image and Scanner Interface Specification
Internet Relay Chat takeover
Gender changer
Multics
Pulse-amplitude modulation
Distributed switching
Simple DirectMedia Layer
Video editing software
TX-0
Star network
Codec
Wireless community network
QRP operation
Red Hat Linux
Real-time computing
Video coding
Samba (software)
Backward compatibility