Control-Alt-Delete (often abbreviated to Ctrl-Alt-Del, also known as the "three-finger salute") is a computer keyboard command on 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. It is invoked by pressing the Delete key while holding the Control and Alt keys. It forces a soft reboot, brings up the task manager (on Windows, BeOS, and KDE-based Linuxes) or a jump to ROM monitor. These keys are sometimes referred to in computer manuals as interrupt keys, since they are often used to interrupt the operation of a malfunctioning program.
This keyboard combination was implemented by David Bradley, a designer of the original IBM PC. Bradley originally designed Control-Alt-Escape to trigger a soft reboot, but he found it was too easy to bump the left side of the keyboard and reboot the computer accidentally. He switched the key combination to Control-Alt-Delete, a combination that was impossible to press with just one hand (this is not true of later keyboards, such as the 102-key PC/AT keyboard or the Maltron keyboard). More advanced operating systems use its status as a "reserved" combination for various purposes, but often retain the ability to trigger a soft reboot in certain configurations or circumstances. Bradley is also known for his good-natured jab at Bill Gates, at that time the CEO of Microsoft, and also the creator of many of Microsoft's programs: "I may have invented Control-Alt-Delete, but Bill Gates made it famous". He afterwards elaborated that it was made more famous due to Windows NT logon procedures ("Press Ctrl + Alt + Delete to log on"). However, while Bradley implemented the key sequence in the ROM BIOS, he did not invent it; the then chief programmer of the IBM PC Project, Mel Hallerman, did.
As computers became ubiquitous, so too, has the jargon. Control-Alt-Delete can also mean "dump," or "do away with.
Under Windows 3.0 and earlier (and Windows 3.1 running in Standard mode), Control-Alt-Delete simply rebooted the computer as in MS-DOS. In Windows 3.1 running in 386 Enhanced mode, Windows 95, Windows 98, and Windows Me, this keystroke combination is recognised by the Windows keyboard device driver. According to the value of the LocalReboot option in the [386Enh] section of system.ini, Windows performs one of several actions in response:
Killing tasks/processes is useful, for instance, if a program has entered an infinite loop. Theoretically, the system's other processes should continue normally—in practice, using this key combination to terminate a program/process in Windows 3.1 can result in resources and memory being leaked. As such, it is strongly recommended that, following a process kill in these versions of Windows, any work should be saved in any other applications and Windows should be restarted. Such damage is much less likely in newer versions of DOS-based Windows because of resource tracking.
In Windows 9x, pressing the combination a second time if the process listing has not appeared would display a blue screen from which the user can reboot the system by pressing the combination a third time; other times the system restarts on the second Ctrl-Alt-Delete combination. This allows the user to over-ride any "stuck" process, since no user-level program is able to define its own response to the Control-Alt-Delete key combination. However, this functionality does not always work.
In Windows NT, and thus on its successors, including Windows 2000, Windows XP, Windows Server 2003, Windows Vista and Windows Server 2008, this keystroke combination is recognized (as a special system-wide "keyboard hook") by the Winlogon process, which in response instructs GINA to perform one of the following tasks:
Windows NT is designed so that, unless security is already compromised in some other way, only the WinLogon process, a trusted system process, can receive notification of this keystroke combination. This is because the kernel remembers the Process ID of the WinLogon process, and allows only that process to receive the notification. This keystroke combination is thus called the Secure Attention Sequence. A user pressing Control-Alt-Delete can be sure that it is the operating system (specifically the WinLogon process), rather than a third party program, that is responding to the key combination, and that it is therefore safe to enter a password. It was chosen as the secure attention key in Windows (instead of, for example, the System Request key), because on the PC platform no program could reasonably expect to redefine this keystroke combination for its own purposes.
It is also a reliable method for bringing up the Task Manager (in Windows Server 2003 and older). All other keystroke combinations could potentially be exclusively tied up by a process that is stuck, but a user process is not able to intercept the Control-Alt-Delete sequence. It can be however disabled by Windows Group Policies. Ctrl+Shift+Esc also brings up the task manager in all Windows NT versions starting with NT 4.0, even if pressing Ctrl+Alt+Del is set to bring up the Windows Security dialog.
As a side effect, users who do not have physical access to the computer's power supply and power/reset switches can be denied the ability to shut down or restart the computer, where previously (on MS-DOS and other variants of Windows) they could always use Control-Alt-Delete. However, as both the Task Manager and Windows Security have options for shutting down the computer, this operation can still be executed unless the entire system (including the WinLogon process) is unresponsive. Also, it is sometimes impossible to access and/or see the Task Manager after a full-screen application has frozen, although the Windows Security dialog, which is on a separate "secure desktop" almost always appears.
In both cases, the system flushes the page cache, cleanly unmounts all disc volumes, but does not cleanly shut down any running programs (and thus does not save any unsaved documents, or the current arrangements of the objects on the Workplace Shell desktop or in any of its open folders).
In many Linux distributions, init is configured to switch run levels and to perform a soft reboot in response to the signal. Thus it provides a mechanism for a person with physical access to the keyboard to perform system shut down (a task that requires superuser rights to initiate programmatically). However, Linux systems can be configured to ignore the keystroke combination. The setting is usually in the inittab(5) configuration file under the keyword "ca".
In Mac OS X, a windowed process (application program) can be killed with the “Force Quit…” command in the Apple menu (Option-Command-Escape, ⌥⌘⎋). The Activity Monitor application (in “/Applications/Utilities”) can be used to quit or force-quit any other process, including invisible processes and applications belonging to other users.
|Acorn Machines (pre-1987)||Break||Processor reset, although confusingly always referred to as soft reset. Hold down Ctrl as well for so-called hard reset (reinitializes various settings); hold down Shift to boot from disk (or not to, if disk is the default).|
|Acorn and post-Acorn RISC OS machines.||Reset button||Processor reset, although confusingly always referred to as soft reset. Hold down Ctrl as well for so-called hard reset (reinitializes various settings); hold down Shift to boot from disk (or not to, if disk is the default). Hold down various other keys to restore CMOS settings to safe configurations.|
|Ctrl + Break||Perform a soft reboot.|
|Amiga||Ctrl + Left Amiga (or Commodore) + Right Amiga||Reboot the machine|
|Amstrad CPC 464 and CPC6128||Ctrl + Shift + Esc||Reset (cold)|
|Amstrad PCW||Shift + Extra + Exit||Reset (cold)|
|IBM PC under DOS||Ctrl + Alt + Del||Perform a soft reboot|
|IBM PC under Windows 3.x shell||Ctrl + Alt + Del||Close unresponsive applications or (if pressed twice) perform a soft reboot|
|IBM PC under Microsoft Windows (95, 98, and Me)||Ctrl + Alt + Del||Bring up simplistic task manager (actually "Close Program" dialog) or (if pressed twice) perform a soft reboot|
|IBM PC under Windows NT-based OS (NT, 2000, XP, 2003 and Vista)||Ctrl + Shift + Esc||Bring up the Windows Task Manager|
|Ctrl + Alt + Del||Also known as the Secure Attention Sequence; bring up the logins screen (when pressed in login screen), or the "Windows security" dialog or (configurable on Windows 2000 and later) the Windows Task Manager (when logged in)|
|IBM PC under OS/2||Ctrl + Esc||Bring up the Window List (unblocking the synchronous input queue)|
|Ctrl + Alt + Del||Perform a soft reboot|
|Ctrl + Alt + NumLock (twice)||Halt the system and begin a system dump to floppy disk|
|IBM PC under Linux||Ctrl + Alt + Del||Signal the init process (usually configured to soft reboot)|
|Alt + SysRq + function key||Magic SysRq key: Depending on the function key, performs a certain low-level function. Examples: sync (flush caches), reboot (forced soft reboot), unmount (remount filesystems readonly), etc...|
|IBM PC under other OS||Ctrl + Alt + Del||Often (but not always) configured to reboot|
|Sinclair ZX Spectrum||Break||Halted peripheral (cassette tape or printer) operations with the report D BREAK - CONT repeats, or halted BASIC programs with the report L BREAK into program.|
|Sun workstation||L1/Stop + A||Enter ROM monitor|
|Sun workstation (serial console)||Break||Enter ROM monitor|
|Alphas running OpenVMS||Ctrl + P||Enter ROM Serial Console or reboot, depending on setting in SRM|
|Apple II family machines||Ctrl + Reset||Enter the monitor or ROM BASIC|
|Ctrl + Open Apple + Reset||Reboot the machine|
|Ctrl + Option (Closed Apple) + Reset||Enter BIOS setup, then reboot|
|Ctrl + Option (Closed Apple) + Open Apple + Reset||Self-test, then reboot|
|Ctrl + Open Apple + Escape||Kill application|
|Apple Macintosh computers with power button on keyboard||Command + Power||Enter debugger|
|Control + Command + Power (sometimes known as a "Control Flower Power")||Reboot the machine|
|Mac OS (7 and later)||Option + Command + Esc||Force quit applications|
|S60 Platform (used on some mobile phones such as Nokia smartphones)||Green + * + 3 (while restarting the phone)||Wipes internal memory and resets the device|
|SGI workstation||Left Shift + Left Ctrl + Left Alt + Keypad Divide + F12||Restart X server (same as Ctrl + Alt + Backspace below)|
|Commodore 64||Run/Stop + Restore||Halt (soft reconfiguration) and return to READY prompt|
|Commodore 128||Reset||Reset to power on state in current mode Commodore + Reset Reset to C-64 mode. Run/Stop + Reset Reset to ML monitor preserving contents of BASIC memory|
|X Window System||Ctrl + Alt + Backspace||Restarts windowing system, logging the user out if using an X display manager, kills X otherwise|
|TI-30XIIS||On + Clear||Restarts the calculator and clears RAM|
|TI-80, TI-81, TI-82, TI-83, TI-84||Mode, Alpha, S||Shows ROM version number. [Enter] enters self test mode|
|TI-85, TI-86||2nd, Mode, Alpha, S||Shows ROM version number. [Enter] enters self test mode|
|TI-89||2nd + Left Arrow + Right Arrow + On||Restarts the calculator and clears RAM Esc + On Force Break without restarting RAM|
|TI-99/4A||FCTN-+||Resets machine back to startup screen.|
|Voyage 200||2nd + Hand + On||Restarts the calculator and clears RAM|
|HP-48||On + C||Restarts RPL, clearing the Stack and PICT, closing IO, and returning to the HOME directory (but not purging the memory)|
|On + A + F||As above, but also purges the memory|
|BeOS||Ctrl-Alt-Shift and click an applications entry in the Deskbar||Kills application|
|Zenith IBM-PC clones||Ctrl-Alt-Ins||Brings up hardware configuration menu|
|Scientific Atlanta Explorer DHCT||Volume Down + Volume Up + Info (on settop box; not remote)||Reboots box (starts up to blue EXPLORER screen)|
|Olivetti M20||Ctrl + Reset||Soft resets the machine|
|TI Explorer Lisp Machine||Left-Ctrl Left-Meta Right-Ctrl Right-Meta Abort||Restart the system|
|Xfce||Ctrl + Alt + Esc + click on window||Kill application|
|Foxtel Set-top-boxes||Back + Select (on box; not remote) (except UEC 720). Standby + Foxtel (on box; not remote) (UEC 720). Back + Select + Reset (on box; not remote) (iQ2)||Power cycles the machine. Pressing [Power], Up, Down, [Power] when lights illuminate on box forces firmware update.|
LocalRebootin Windows 95
LocalRebootbetween Windows 3.x and Windows 95
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