SysRq

There is a key labeled SysRq on keyboards for PCs that no longer has a standard use. The BIOS keyboard routines simply ignore it; therefore so do the DOS input routines as well as the keyboard routines in libraries supplied with high-level languages.

The key is not totally inactive, however. When it is pressed, nothing is stored in the keyboard buffer[?], but a BIOS function is called. The default handler of that function does nothing and simply returns. Programs can use SysRq simply by creating an interrupt handler[?] to replace the default stub, but most programs have no need for that functionality. Software that has the potential to completely lock up the system, so that a BIOS interrupt is the only input that could be generated, use SysRq as a form of "panic button."

In Linux systems, provided the kernel has been compiled with the correct option, the key can be used to perform a variety of functions in an emergency, such as syncing disks, killing processes and powering off the computer.

See also scroll lock, break key..