- This article is about Control-Alt-Delete, the keyboard shortcut. For the web comic, see Ctrl+Alt+Del (webcomic).
Control-Alt-Delete (often abbreviated to Ctrl-Alt-Del) is a computer keyboard command on IBM PC compatible systems. It is given by simultaneously pressing the Control, Alt, and Delete keys.
This keyboard combination was originally designed to trigger a soft reboot, and was therefore chosen so that it would be hard to issue accidentally. On the original PC/XT 83-key keyboard, it was impossible to press all three keys together with just one hand, although this is not true of later keyboards, such as the 102-key PC/AT 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.
Colloquially, the combination is also known as a three-finger salute, or more esoterically, a Vulcan nerve pinch.
DOS and all real mode systems
On a PC running DOS or a system that runs in real mode, this keystroke combination is recognised by the keyboard handling code in the BIOS, which in response invokes a soft reboot. The code for this type of bailing out was written by David Bradley, an IBM programmer, who allegedly once said, "I may have invented it, but Bill [Gates] made it famous."
DOS-based Windows
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 restart the computer.
- Windows 95, and its successors, present a window which lists currently running processes, and can be used to manually "kill" them.
Killing tasks/processes is useful, for instance, if a program has entered an infinite loop.
Entering the combination twice in succession triggers 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.
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.
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 only the superuser has permission to initiate programmatically).
Windows NT
In Windows NT (and thus on its derivatives Windows 2000 and Windows XP), this keystroke combination is recognised (as a system-wide "keyboard hook") by the WinLogon process (more specifically: by the default GINA loaded by that logon process) which in response performs one of the following tasks:
- If nobody is logged in, bringing up the login dialog to allow the user to log in
- If a user is logged in, bringing up the Task Manager to allow the user to terminate errant processes
- If a user is logged in, bringing 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.
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 it is the first to register the keyboard hook). This keystroke combination is thus a secure attention key. 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. All other keystroke combinations could potentially be exclusively tied up by a process that is stuck, but a stuck user program is not able to intercept the Control-Alt-Delete sequence.
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 privilege of being able to shut down or reboot the computer, where previously (on MS-DOS and other variants of Windows) they could always use Control-Alt-Delete.
References
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- Linux manual pages for kill(2) and reboot(2).