<s>
System	B-Device
Request	I-Device
(	O
SysRq	B-Device
or	O
Sys	B-Device
Req	I-Device
)	O
is	O
a	O
key	O
on	O
personal	B-Device
computer	I-Device
keyboards	B-Device
that	O
has	O
no	O
standard	O
use	O
.	O
</s>
<s>
Introduced	O
by	O
IBM	O
with	O
the	O
PC/AT	B-Operating_System
,	O
it	O
was	O
intended	O
to	O
be	O
available	O
as	O
a	O
special	O
key	O
to	O
directly	O
invoke	O
low-level	O
operating	B-General_Concept
system	I-General_Concept
functions	O
with	O
no	O
possibility	O
of	O
conflicting	O
with	O
any	O
existing	O
software	O
.	O
</s>
<s>
A	O
special	O
BIOS	B-Operating_System
routine	O
–	O
software	O
interrupt	O
0x15	O
,	O
subfunction	O
0x85	O
–	O
was	O
added	O
to	O
signal	O
the	O
OS	B-General_Concept
when	O
SysRq	B-Device
was	O
pushed	O
or	O
released	O
.	O
</s>
<s>
Unlike	O
most	O
keys	O
,	O
when	O
it	O
is	O
pressed	O
nothing	O
is	O
stored	O
in	O
the	O
keyboard	B-Device
buffer	I-Device
.	O
</s>
<s>
The	O
specific	O
low	O
level	O
function	O
intended	O
for	O
the	O
SysRq	B-Device
key	I-Device
was	O
to	O
switch	O
between	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
When	O
the	O
original	O
IBM-PC	O
was	O
created	O
in	O
1980	O
,	O
there	O
were	O
three	O
leading	O
competing	O
operating	B-General_Concept
systems	I-General_Concept
:	O
PC	B-Operating_System
DOS	I-Operating_System
,	O
CP/M	B-Operating_System
-86	I-Operating_System
,	O
and	O
UCSD	B-Language
p-System	I-Language
,	O
while	O
Xenix	B-Application
was	O
added	O
in	O
1983	O
–	O
1984	O
.	O
</s>
<s>
The	O
SysRq	B-Device
key	I-Device
was	O
added	O
so	O
that	O
multiple	O
operating	B-General_Concept
systems	I-General_Concept
could	O
be	O
run	O
on	O
the	O
same	O
computer	O
,	O
using	O
the	O
capabilities	O
of	O
the	O
286	B-General_Concept
chip	O
in	O
the	O
PC/AT	B-Operating_System
.	O
</s>
<s>
A	O
special	O
key	O
was	O
needed	O
because	O
most	O
software	O
of	O
the	O
day	O
operated	O
at	O
a	O
low	O
level	O
,	O
often	O
bypassing	O
the	O
OS	B-General_Concept
entirely	O
,	O
and	O
typically	O
made	O
use	O
of	O
many	O
hotkey	O
combinations	O
.	O
</s>
<s>
To	O
implement	O
a	O
task	B-Operating_System
switching	I-Operating_System
or	O
multitasking	B-Operating_System
environment	O
,	O
it	O
was	O
thought	O
that	O
a	O
special	O
,	O
separate	O
key	O
was	O
needed	O
.	O
</s>
<s>
This	O
is	O
similar	O
to	O
the	O
way	O
"	O
Control-Alt-Delete	B-Device
"	O
is	O
used	O
under	O
Windows	B-Device
NT	I-Device
.	O
</s>
<s>
On	O
84-key	O
keyboards	B-Device
(	O
except	O
the	O
84-key	O
IBM	O
Model	O
M	O
space	O
saver	O
keyboard	B-Device
)	O
,	O
SysRq	B-Device
was	O
a	O
key	O
of	O
its	O
own	O
.	O
</s>
<s>
On	O
the	O
later	O
101-key	O
keyboard	B-Device
,	O
it	O
shares	O
a	O
physical	O
key	O
with	O
the	O
Print	B-Device
screen	I-Device
key	I-Device
function	O
.	O
</s>
<s>
The	O
Alt	B-Device
key	I-Device
must	O
be	O
held	O
down	O
while	O
pressing	O
this	O
dual-function	O
key	O
to	O
invoke	O
SysRq	B-Device
.	O
</s>
<s>
This	O
continues	O
through	O
modern	O
PC	B-Device
keyboards	I-Device
.	O
</s>
<s>
The	O
default	O
BIOS	B-Operating_System
keyboard	B-Device
routines	O
simply	O
ignore	O
SysRq	B-Device
and	O
return	O
without	O
taking	O
action	O
.	O
</s>
<s>
So	O
did	O
the	O
MS-DOS	B-Application
input	O
routines	O
.	O
</s>
<s>
The	O
keyboard	B-Device
routines	O
in	O
libraries	B-Library
supplied	O
with	O
many	O
high-level	O
languages	O
followed	O
suit	O
.	O
</s>
<s>
Although	O
it	O
is	O
still	O
included	O
on	O
most	O
PC	B-Device
keyboards	I-Device
manufactured	O
,	O
and	O
though	O
it	O
is	O
used	O
by	O
some	O
debugging	O
software	O
,	O
the	O
key	O
is	O
of	O
no	O
use	O
for	O
the	O
vast	O
majority	O
of	O
users	O
.	O
</s>
<s>
In	O
Linux	B-Application
,	O
the	O
kernel	B-Operating_System
can	O
be	O
configured	O
to	O
provide	O
functions	O
for	O
system	O
debugging	O
and	O
crash	O
recovery	O
.	O
</s>
<s>
This	O
use	O
is	O
known	O
as	O
the	O
"	O
magic	B-Application
SysRq	I-Application
key	I-Application
"	O
.	O
</s>
<s>
Microsoft	O
has	O
also	O
used	O
SysRq	B-Device
for	O
various	O
OS	B-General_Concept
-	O
and	O
application-level	O
debuggers	B-Application
.	O
</s>
<s>
In	O
the	O
CodeView	B-Application
debugger	I-Application
,	O
it	O
was	O
sometimes	O
used	O
to	O
break	O
into	O
the	O
debugging	O
during	O
program	O
execution	O
.	O
</s>
<s>
For	O
the	O
Windows	B-Device
NT	I-Device
remote	O
kernel	B-Operating_System
debugger	B-Application
,	O
it	O
can	O
be	O
used	O
to	O
force	O
the	O
system	O
into	O
the	O
debugger	B-Application
.	O
</s>
<s>
On	O
modern	O
Windows	O
"	O
Alt	B-Device
+	O
Print	B-Device
screen	I-Device
"	O
(	O
which	O
is	O
the	O
shortcut	O
for	O
SysRq	B-Device
on	O
most	O
modern	O
keyboards	B-Device
)	O
copies	O
an	O
image	O
of	O
the	O
current	O
active	O
window	O
,	O
instead	O
of	O
the	O
entire	O
screen	O
as	O
Print	B-Device
Screen	I-Device
alone	O
would	O
.	O
</s>
<s>
IBM	O
3270-type	O
console	O
keyboards	B-Device
of	O
the	O
IBM	B-Device
System/370	I-Device
mainframe	B-Architecture
computer	I-Architecture
,	O
created	O
in	O
1970	O
,	O
had	O
an	O
operator	O
interrupt	O
key	O
that	O
was	O
used	O
to	O
cause	O
the	O
operating	B-General_Concept
system	I-General_Concept
such	O
as	O
VM/370	B-Application
or	O
MVS	B-Application
to	O
allow	O
the	O
console	O
to	O
give	O
input	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
