SysRq

There is a key labeled SysRq (short for System Request) on keyboards for PCs that no longer has a standard use. On IBM AT keyboards, it was a key of its own, but on most more modern keyboards, the same function is reached through the key combination Alt+Print Screen. The BIOS keyboard routines simply ignore SysRq; therefore so do the DOS input routines as well as the keyboard routines in libraries supplied with many high-level languages.

The key is not totally ineffective, 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 function, but most programs have no need to do that. Sometimes software has the potential to completely lock up the system. When this happens, a BIOS interrupt is the only input that can be generated. The SysRq BIOS interrupt handler can be configured as a form of "panic button" to gracefully terminate the program or reboot the system.

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 turning off the computer, even when the computer is otherwise hung and no longer reacts to mouse actions or normal key presses.