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On computer keyboards, system request (SysRq) is a key which has no standard use. It was initially provided as a special key for directly invoking low-level operating system functionalities, especially for switching between different operating systems. It was mostly used for that purpose in the early 1980s, but later fell into disuse.
Nothing is stored in the keyboard buffer when the system request key is pressed, a property which is not found in any other keys on the keyboard. The system request key was first introduced by IBM, and it triggered a special BIOS routine to signal the operating system whenever the system request key was pressed. It helped in encouraging a multitasking environment for users.
When introduced, the system request key was a separate key on most keyboards, but in modern keyboards it shares the physical key with other functions (often "print screen"). The default BIOS keyboard routines and the keyboard routines in various high-level languages usually ignore system request functionality and return without any steps being taken. Currently the system request key is not of much use to a large majority of users, although it is still used in certain applications for troubleshooting or debugging, such as in Microsoft for OS-level or application-level debuggers.