SysRq

The magical SysRq key is a key-combination that can be pressed when the kernel seems to be unresponsive.

It is enabled with the CONFIG_MAGIC_SYSRQ kernel configuration.

/proc/sys/kernel/sysrq, /proc/sysrq-trigger, drivers/tty/sysrq.c, Documentation/sysrq.txt

The SysRq key was added with on the PC/AT keyboard.