SysRq
Encyclopedia
Keyboard (computing)
In computing, a keyboard is a typewriter-style keyboard, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches...
s for PC
IBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
s that has no standard use. This key can be traced back to the operator interrupt key used on IBM 3270
IBM 3270
The IBM 3270 is a class of block oriented terminals made by IBM since 1972 normally used to communicate with IBM mainframes. As such, it was the successor to the IBM 2260 display terminal. Due to the text colour on the original models, these terminals are informally known as green screen terminals...
-type console keyboards of the IBM System/370
System/370
The IBM System/370 was a model range of IBM mainframes announced on June 30, 1970 as the successors to the System/360 family. The series maintained backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the...
mainframe computer
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
, which was used to cause the operating system such as VM/370 or MVS
MVS
Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers...
to allow the console to give input to the operating system.
History
Introduced by IBMIBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
with the PC/AT
IBM Personal Computer/AT
The IBM Personal Computer AT, more commonly known as the IBM AT and also sometimes called the PC AT or PC/AT, was IBM's second-generation PC, designed around the 6 MHz Intel 80286 microprocessor and released in 1984 as machine type 5170...
, it was intended to be available as a special key to directly invoke low-level operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
functions with no possibility of conflicting with any existing software. A special BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
routine — software interrupt hexadecimal 15, subfunction hexadecimal 85 — was added to signal the OS
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
when SysRq was pushed or released. Unlike most keys, when it is pressed nothing is stored in the keyboard buffer
Keyboard buffer
A keyboard buffer is a section of computer memory used to hold keystrokes before they are processed.Keyboard buffers have long been used in command-line processing...
.
The specific low level function that the SysRq key was meant for was to switch between operating systems. When the original IBM-PC was created in 1980, there were three leading competing operating systems: PC-DOS
PC-DOS
IBM PC DOS is a DOS system for the IBM Personal Computer and compatibles, manufactured and sold by IBM from the 1980s to the 2000s....
, CP/M-86
CP/M-86
CP/M-86 was a version of the CP/M operating system that Digital Research made for the Intel 8086 and Intel 8088. The commands are those of CP/M-80. Executable files used the relocatable .CMD file format...
, and UCSD p-System, while Xenix
Xenix
Xenix is a version of the Unix operating system, licensed to Microsoft from AT&T in the late 1970s. The Santa Cruz Operation later acquired exclusive rights to the software, and eventually superseded it with SCO UNIX ....
was added in 1983-1984. The SysRq key was added so that multiple operating systems could be run on the same computer, making use of the capabilities of the 286
Intel 80286
The Intel 80286 , introduced on 1 February 1982, was a 16-bit x86 microprocessor with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086 and 8088...
chip in the PC/AT.
A special key was needed because most software of the day operated at a low level, often bypassing the OS entirely, and typically made use of many hotkey combinations. The use of Terminate and Stay Resident (TSR) programs
Terminate and Stay Resident
Terminate and Stay Resident is a computer system call in DOS computer operating systems that returns control to the system as if the program has quit, but keeps the program in memory...
further complicated matters. To implement a task switching or multitasking
Computer multitasking
In computing, multitasking is a method where multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for...
environment, it was thought that a special, separate key was needed. This is similar to the way “Control-Alt-Delete
Control-Alt-Delete
Control-Alt-Delete is a computer keyboard command on IBM PC compatible systems that can be used to reboot the computer, and summon the task manager or Windows Security in more recent versions of the Microsoft Windows operating system...
” is used under Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
.
On 84-key keyboards (except the 84-key IBM Model M space saver keyboard), SysRq was a key of its own. On the later 101-key keyboard, it shares a physical key with the Print Screen
Print screen
Print screen is a key present on most PC keyboards. It is typically situated in the same section as the break key and scroll lock key...
key function. One must hold down the Alt key
Alt key
The Alt key on a computer keyboard is used to change the function of other pressed keys. Thus, the Alt key is a modifier key, used in a similar fashion to the Shift key. For example, simply pressing "A" will type the letter a, but if you hold down either Alt key while pressing A, the computer...
while pressing this “dual-function” key to invoke SysRq.
The default BIOS keyboard routines simply ignore SysRq and return without taking action. So did the MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
input routines. The keyboard routines in libraries supplied with many high-level languages followed suit. Although it is still included on most PC keyboards manufactured, and though it is used by some debugging software, the key is of no use for the vast majority of users.
In 2010, Lenovo removed the SysRq key from some of their new laptops.
Other uses
Under DOSDOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
, malfunctioning foreground software could make the entire computer unresponsive. Third-party TSRs
Terminate and Stay Resident
Terminate and Stay Resident is a computer system call in DOS computer operating systems that returns control to the system as if the program has quit, but keeps the program in memory...
existed that used SysRq to provide a form of “panic button” to terminate the program and return the user to the DOS command prompt.
In Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, the kernel can be configured to provide functions for system debugging
Debugging
Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...
and crash recovery. This use is known as the “Magic SysRq key”.
Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
has used SysRq for various OS
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
- and application
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
-level debugger
Debugger
A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...
s. In the CodeView debugger
CodeView
CodeView was a standalone debugger created by David Norris at Microsoft in 1985 as part of its development toolset. It originally shipped with Microsoft C 4.0 and later. It also shipped with Visual Basic for MS-DOS, Microsoft Basic PDS, and a number of other Microsoft language products...
, it was sometimes used to break into the debugging during program execution. For the Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
remote kernel debugger, it can be used to force the system into the debugger.
On the Hyundai/Hynix
Hynix
Hynix Semiconductor Inc. chips and flash memory chips. Founded in 1983, Hynix is the world's second-largest memory chipmaker, the largest being Samsung Electronics. Formerly known as Hyundai Electronics, the company has manufacturing sites in Korea, the U.S., China and Taiwan...
Super-16 computer, pressing will hard boot the system (it will reboot when is unresponsive, and it will invoke startup memory tests that are bypassed on soft-boot).
In embedded systems, SysRq key is usually used to assert low-level on RESET# signal.
See also
- Print ScreenPrint screenPrint screen is a key present on most PC keyboards. It is typically situated in the same section as the break key and scroll lock key...
- Serial console
- Magic SysRq key