System request

Introduced by IBM with the PC/AT, it was intended to be available as a special key to directly invoke low-level operating system functions with no possibility of conflicting with any existing software.

A special BIOS routine – software interrupt 0x15, subfunction 0x85[1] – was added to signal the OS when SysRq was pushed or released.

When the original IBM-PC was created in 1980, there were three leading competing operating systems: PC DOS, CP/M-86, and UCSD p-System,[2] while Xenix was added in 1983–1984.

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.

On the Hyundai/Hynix Super-16 computer, pressing Ctrl+SysRq will hard boot the system (it will reboot when Ctrl+Alt+Del is unresponsive, and it will invoke startup memory tests that are bypassed on soft-boot).

A 104-key PC US English keyboard layout with System request circled
Tilde Exclamation mark At sign Number sign Dollar sign Percent sign Caret Ampersand Asterisk Parenthesis Parenthesis Underscore Plus sign Backspace Backtick 1 (number) 2 (number) 3 (number) 4 (number) 5 (number) 6 (number) 7 (number) 8 (number) 9 (number) 0 Hyphen-minus Equals sign Backspace Tab key Q W E R T Y U I O P Curly bracket Curly bracket Vertical bar Tab key Q W E R T Y U I O P Square bracket Square bracket Backslash Caps lock A S D F G H J K L Colon (punctuation) Quotation mark Enter key Caps lock A S D F G H J K L Semicolon Apostrophe Enter key Shift key Z X C V B N M Bracket Bracket Question mark Shift key Shift key Z X C V B N M Comma (punctuation) Full stop Slash (punctuation) Shift key Control key Windows key Alt key Space bar Alt key thumb Menu key Control key