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. Applications that have 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."

/Talk