From http://www.mjmwired.net/kernel/Documentation/sysrq.txt

Linux Magic System Request Key Hacks
Documentation for sysrq.c version 1.15
Last update: $Date: 2001/01/28 10:15:59 $

*  What is the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It is a 'magical' key combo you can hit which the kernel will respond to
regardless of whatever else it is doing, unless it is completely locked up.

*  How do I enable the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
configuring the kernel. When running a kernel with SysRq compiled in,
/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
the SysRq key. By default the file contains 1 which means that every
possible SysRq request is allowed (in older versions SysRq was disabled
by default, and you were required to specifically enable it at run-time
but this is not the case any more). Here is the list of possible values
in /proc/sys/kernel/sysrq:
   0 - disable sysrq completely
   1 - enable all functions of sysrq
  >1 - bitmask of allowed sysrq functions (see below for detailed function
       description):
          2 - enable control of console logging level
          4 - enable control of keyboard (SAK, unraw)
          8 - enable debugging dumps of processes etc.
         16 - enable sync command
         32 - enable remount read-only
         64 - enable signalling of processes (term, kill, oom-kill)
        128 - allow reboot/poweroff
        256 - allow nicing of all RT tasks

You can set the value in the file by the following command:
    echo "number" >/proc/sys/kernel/sysrq

Note that the value of /proc/sys/kernel/sysrq influences only the invocation
via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
allowed.

*  How do I use the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
On x86   - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
           keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
           also known as the 'Print Screen' key. Also some keyboards cannot
	   handle so many keys being pressed at the same time, so you might
	   have better luck with "press Alt", "press SysRq", "release Alt",
	   "press <command key>", release everything.

On SPARC - You press 'ALT-STOP-<command key>', I believe.

On the serial console (PC style standard serial ports only) -
           You send a BREAK, then within 5 seconds a command key. Sending
           BREAK twice is interpreted as a normal BREAK.

On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,  
             Print Screen (or F13) - <command key> may suffice.

On other - If you know of the key combos for other architectures, please
           let me know so I can add them to this section.

On all -  write a character to /proc/sysrq-trigger.  eg:

		echo t > /proc/sysrq-trigger

*  What are the 'command' keys?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
'r'     - Turns off keyboard raw mode and sets it to XLATE.

'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
          console. NOTE: See important comments below in SAK section.

'b'     - Will immediately reboot the system without syncing or unmounting
          your disks.

'c'	- Will perform a kexec reboot in order to take a crashdump.

'o'     - Will shut your system off (if configured and supported).

's'     - Will attempt to sync all mounted filesystems.

'u'     - Will attempt to remount all mounted filesystems read-only.

'p'     - Will dump the current registers and flags to your console.

't'     - Will dump a list of current tasks and their information to your
          console.

'm'     - Will dump current memory info to your console.

'v'	- Dumps Voyager SMP processor info to your console.

'0'-'9' - Sets the console log level, controlling which kernel messages
          will be printed to your console. ('0', for example would make
          it so that only emergency messages like PANICs or OOPSes would
          make it to your console.)

'f'	- Will call oom_kill to kill a memory hog process

'e'     - Send a SIGTERM to all processes, except for init.

'i'     - Send a SIGKILL to all processes, except for init.

'l'     - Send a SIGKILL to all processes, INCLUDING init. (Your system
          will be non-functional after this.)

'h'     - Will display help ( actually any other key than those listed
          above will display help. but 'h' is easy to remember :-)

*  Okay, so what can I use them for?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Well, un'R'aw is very handy when your X server or a svgalib program crashes.

sa'K' (Secure Access Key) is useful when you want to be sure there are no
trojan program is running at console and which could grab your password
when you would try to login. It will kill all programs on given console
and thus letting you make sure that the login prompt you see is actually
the one from init, not some trojan program.
IMPORTANT:In its true form it is not a true SAK like the one in   :IMPORTANT
IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT
       It seems other find it useful as (System Attention Key) which is
useful when you want to exit a program that will not let you switch consoles.
(For example, X or a svgalib program.)

re'B'oot is good when you're unable to shut down. But you should also 'S'ync
and 'U'mount first.

'C'rashdump can be used to manually trigger a crashdump when the system is hung.
The kernel needs to have been built with CONFIG_KEXEC enabled.

'S'ync is great when your system is locked up, it allows you to sync your
disks and will certainly lessen the chance of data loss and fscking. Note
that the sync hasn't taken place until you see the "OK" and "Done" appear
on the screen. (If the kernel is really in strife, you may not ever get the
OK or Done message...)

'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
Again, the unmount (remount read-only) hasn't taken place until you see the
"OK" and "Done" message appear on the screen.

The loglevel'0'-'9' is useful when your console is being flooded with
kernel messages you do not want to see. Setting '0' will prevent all but
the most urgent kernel messages from reaching your console. (They will
still be logged if syslogd/klogd are alive, though.)

t'E'rm and k'I'll are useful if you have some sort of runaway process you
are unable to kill any other way, especially if it's spawning other
processes.

*  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
That happens to me, also. I've found that tapping shift, alt, and control
on both sides of the keyboard, and hitting an invalid sysrq sequence again
will fix the problem. (ie, something like alt-sysrq-z). Switching to another
virtual console (ALT+Fn) and then back again should also help.

*  I hit SysRq, but nothing seems to happen, what's wrong?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are some keyboards that send different scancodes for SysRq than the
pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
code (84 is decimal for 0x54). It's probably best to put this command in a
boot script. Oh, and by the way, you exit 'showkey' by not typing anything
for ten seconds.

*  I want to add SysRQ key events to a module, how does it work?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to register a basic function with the table, you must first include
the header 'include/linux/sysrq.h', this will define everything else you need.
Next, you must create a sysrq_key_op struct, and populate it with A) the key
handler function you will use, B) a help_msg string, that will print when SysRQ
prints help, and C) an action_msg string, that will print right before your
handler is called. Your handler must conform to the prototype in 'sysrq.h'.

After the sysrq_key_op is created, you can call the macro 
register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in
sysrq.h, this will register the operation pointed to by 'op_p' at table
key 'key', if that slot in the table is blank. At module unload time, you must
call the macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
will remove the key op pointed to by 'op_p' from the key 'key', if and only if
it is currently registered in that slot. This is in case the slot has been
overwritten since you registered it.

The Magic SysRQ system works by registering key operations against a key op
lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
a number of operations registered into it at compile time, but is mutable,
and 4 functions are exported for interface to it: __sysrq_lock_table,
__sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The
functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined
in the header itself, and the REGISTER and UNREGISTER macros are built from
these. More complex (and dangerous!) manipulations of the table are possible
using these functions, but you must be careful to always lock the table before
you read or write from it, and to unlock it again when you are done. (And of
course, to never ever leave an invalid pointer in the table). Null pointers in
the table are always safe :)

If for some reason you feel the need to call the handle_sysrq function from
within a function called by handle_sysrq, you must be aware that you are in
a lock (you are also in an interrupt handler, which means don't sleep!), so
you must call __handle_sysrq_nolock instead.

*  I have more questions, who can I ask?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
And I'll answer any questions about the registration system you got, also
responding as soon as possible.
 -Crutcher

*  Credits
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Written by Mydraal <vulpyne[AT]vulpyne[DOT]net>
Updated by Adam Sulmicki <adam[AT]cfar.umd[DOT]edu>
Updated by Jeremy M. Dolan <jmd[AT]turbogeek[DOT]org> 2001/01/28 10:15:59
Added to by Crutcher Dunnavant <crutcher+kernel[AT]datastacks[DOT]com>