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Control-Alt-Delete

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This article is about the key combination. For other meanings, see Control-Alt-Delete (disambiguation).
Control-Alt-Delete highlighted on a QWERTY keyboard.

Control-Alt-Delete (often abbreviated to Ctrl-Alt-Del) 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 given by pressing the Delete key while holding the Control and Alt keys.

This keyboard combination was designed 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 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", alluding to the three keystrokes required to logon to a Windows operating system; the jab has often been mispercieved as a jab at the supposed instability of Windows as Control-Alt-Delete also reboots the operating system (Windows ME or older) or opens a task manager to kill crashed programs (Windows 2000 or newer).

This Dutch protester wants to reset government policies using Control-Alt-Delete.

Colloquially, the combination is also known as a three-finger salute, Three Fingered Death Grip or, more esoterically, as a Vulcan nerve pinch [5].

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

File:CloseProgram.png
The Close Program window on Microsoft Windows 9x. The window is called "Task List" in Windows 3.1x and Windows NT 3.x.

In DOS-based Windows (including both Windows 3.x and its successors; 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.x presents a blue screen to the user inviting them to press Enter to end the current task or press Control-Alt-Delete again to perform a soft reboot.
    • Windows 95, Windows 98, and Windows Me, present a window which lists currently running processes (but only those with at least one visible window), 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.x 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.

Entering the combination twice in succession in DOS-based Windows will trigger a soft reboot, even if Windows has not yet been able to display the process listing (due to problems caused by other processes). 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.

Windows NT

File:Windows Task Manager.PNG
The Windows Task Manager in Windows Vista.
File:Drumline-taskman.JPG
The Windows Security Dialog in Vista. From here, the Task Manager can be launched.

In Windows NT, and thus on its successors, including Windows 2000, Windows XP, Windows Server 2003, Windows Vista and Windows Server "Longhorn" this keystroke combination is recognized (as special sort of a 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, 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 is Windows Vista and Windows Server Longhorn, regardless of whether or not the computer is part of a domain.
  • If the computer is not a part of a domain e.g. on Windows XP with the welcome screen and fast user switching turned on, Ctrl-Alt-Del directly invokes the Task Manager.

The design of Windows NT is such 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 (because the kernel remembers Process ID of WinLogon process and allows only that process registering it). This keystroke combination is thus a secure attention key. In Windows NT, it is called 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 that 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.

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 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.

Mac OS

On 1980s Apple microcomputers (such as the Apple II or the Apple III), if a user initiated the Control-Option-Reset key function, the computer would initiate a soft boot. In Mac OS X and older Mac Operating Systems, the Ctrl-Alt-Del (technically Control-Option-Delete) keystroke does nothing. A similar key command, Command-Option-Esc, opens the Force Quit window. On Mac laptops, as well as on older Macs that support a keyboard power button, pressing Command-Control-Power will instantly hard-reboot the system.

References

  1. "Windows 3.1 Resource Kit SYSTEM.INI 386ENH Section A-L". Microsoft's KnowledgeBase article 83435. Retrieved January 7. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  2. Linux manual pages for kill(2) and reboot(2).
  3. Gary S. Terhune (2004-01-11). "Lost Ctrl-Alt-Del function on W98, 2nd". Newsgroup: microsoft.public.win98.gen_discussion. uAIVMjC2DHA.2336@TK2MSFTNGP09.phx.gbl. {{cite newsgroup}}: Check date values in: |date= (help) — a report of the effect of LocalReboot in Windows 95
  4. Geoff Chappell (1998-05-06). "Is this possible?". Newsgroup: comp.os.ms-windows.programmer.vxd. 6iouc1$dgh$2@reader1.reader.news.ozemail.net. {{cite newsgroup}}: Check date values in: |date= (help) — a report of differences in LocalReboot between Windows 3.x and Windows 95
  5. ^ Soni Pitts. "Ctrl-Alt-Delete Your Life". HOME::Self-Improvement/Inspirational. Retrieved 2005-12-17.

See also

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