delete

Control-Alt-Delete

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.

DOS and all real mode systems

On a PC running DOS or a system that runs in real mode, this keystroke combination is recognized by the keyboard handling code in the BIOS and treated as the CPU's NMI signal, which, except for rare exceptions, invokes a soft reboot.

Windows

DOS-based Windows

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:

  • If LocalReboot=Off it performs a soft reboot.
  • If LocalReboot=On:
    • Windows 3.1 presents a blue screen to the user inviting them to press Enter to end a task that has stopped responding to the system (if such a task exists) or press Control-Alt-Delete again to perform a soft reboot.
    • Windows 95, Windows 98, and Windows Me, temporarily halts the entire system, and presents a window which lists currently running processes, and can be used to notify them that they should end, or, when they don't respond, kill them. The user can press Control-Alt-Delete again to perform a soft reboot.

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.

Windows NT (and later versions)

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:

  • If nobody is logged in, bringing up the login dialog to allow the user to log in. Also used when the computer is locked to bring up the unlock dialog.
  • If the computer is configured as a part of a domain or it runs Windows 2000, the combination brings up the "Windows Security" dialog, where the user can lock the computer, change their password, log out, shut the computer down, or invoke the Task Manager. This is the default behavior in Windows Vista and Windows Server 2008, regardless of whether or not the computer is part of a domain. The options presented can be controlled through the use of Group Policy.
  • If Windows XP is not connected to a domain...

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.

OS/2

In OS/2, this keystroke combination is recognised by the OS/2 keyboard device driver, which notifies the session manager process. The normal session manager process in OS/2 versions 2.0 and later is the parent Workplace Shell process, which displays the "The system is rebooting" window and triggers a soft reboot. If it is pressed twice in succession OS/2 triggers an immediate soft reboot, without waiting for the session manager process.

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).

Linux

In Linux, this keystroke combination is recognised by the keyboard device driver in the kernel. In the absence of more specific instructions, which will usually only be during system initialisation, the kernel directly initiates a soft reboot in response. More commonly, the kernel will send a signal to the init process, which will perform an administrator-configured task, such as running a script, or displaying an "end current session" box in KDE.

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".

Macintosh

A Macintosh computer can be forced to shut down or restart, skipping the usual shutdown process. This should only be done when absolutely necessary:

  • On all systems, holding down the Power button forces an immediate shutdown. This skips the normal shutdown process
  • Some desktop systems have a Reset button (◁) which forces an immediate restart.
  • On MacBooks and PowerBooks, Control-Command-Power forces an immediate restart.

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.

Apple II and Apple III

On the Apple IIe, Apple IIc, and the Apple III, Control—Open-Apple—Reset would cause an immediate restart.

Effect on various computers

Platform Key combination Function
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.

See also

Parts of this article were originally based on Three-finger salute at FOLDOC, used with Foldoc license.

External links

References

General references

  1. Windows 3.1 Resource Kit SYSTEM.INI 386ENH Section A-L. Microsoft's KnowledgeBase article 83435. .
  2. Linux manual pages for kill(2) and reboot(2).
  3. |url=http://groups.google.com/group/microsoft.public.win98.gen_discussion/msg/54f1c50b9eab17fb}} — a report of the effect of LocalReboot in Windows 95
  4. |url=http://groups.google.com/group/comp.os.ms-windows.programmer.vxd/msg/a7332964a4c61ad6}} — a report of differences in LocalReboot between Windows 3.x and Windows 95

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