System request
| Industry | Computer hardware, computer software |
|---|---|
| Founded | June 16, 1911 in Endicott, New York, United States |
| Headquarters | Armonk, New York , United States |
System Request (commonly abbreviated as SysRq or Sys Req) refers to a key found on PC keyboards that lacks a standardized function. Originating with IBM’s PC/AT model, its purpose was to trigger low-level operating system operations without interfering with existing applications. The BIOS implements a routine—software interrupt 0x15, subfunction 0x85—that notifies the OS when the SysRq key is pressed or released. Unlike typical keys, pressing SysRq does not place any input into the keyboard buffer.
History
The initial concept behind the SysRq key was to facilitate switching between different operating systems. At the time of IBM’s 1980 original PC release, three main OS options existed: PC DOS, CP/M-86, and UCSD p-System, with Xenix joining the lineup around 1983–1984. The SysRq key enabled running multiple operating systems on a single computer by utilizing the Intel 80286 processor found in the PC/AT.
A dedicated key was necessary because most software during that era operated at a low level, frequently bypassing the OS and relying on numerous shortcut keys. The widespread use of terminate-and-stay-resident (TSR) programs further complicated multitasking efforts. Consequently, a unique key such as SysRq was devised to support task switching, analogous to the "Control-Alt-Delete" combination on Windows NT systems.
On 84-key keyboards (with the exception of the IBM Model M space saver), SysRq appeared as an individual key. Later, 101-key layouts merged it with the Print screen key; activating SysRq now requires holding down the Alt key while pressing this combined key—a method still employed on current PC keyboards.
Standard BIOS and MS-DOS keyboard handlers disregard SysRq input, as do many high-level programming language libraries. Although it remains present on most PC keyboards and is occasionally utilized by debugging utilities, SysRq is rarely used by everyday users.
On the Hyundai/Hynix Super-16 computer, pressing this key initiates a hard reboot followed by startup memory diagnostics, providing a recovery option when other inputs are unresponsive.
Modern uses
Within Linux, the kernel can be configured to leverage the "magic SysRq key" for system debugging and crash recovery purposes.
Microsoft has incorporated SysRq functionality into various operating system and application debuggers. For instance, in the CodeView debugger, it allowed interruption of program execution for debugging. In the Windows NT remote kernel debugger, SysRq can be used to force entry into debugging mode.
In contemporary Windows and some Linux desktop environments, pressing "Alt + Print screen"—the typical SysRq shortcut on many keyboards—captures an image of the currently active window instead of taking a screenshot of the entire display.
Similar keys
The operator interrupt key found on IBM 3270-style console keyboards for IBM’s System/370 mainframe computers (circa 1970) served a comparable purpose by enabling operating systems such as VM/370 or MVS to receive console input commands.