Control-Alt-Delete is a computer keyboard command on IBM PC compatible systems. It is given by simultaneously pressing the Control, Alt, and Delete keys. This command is a hook to the BIOS of a PC running DOS; it causes the computer to hard reboot.
This keyboard combination was chosen so that a reboot would be hard to cause accidentally. The code for this type of bailing out was written by David Bradley (IBM programmer) who allegedly once said "I may have invented it, but Bill made it famous."
In Microsoft Windows, this keyboard command is intercepted and is used to perform the following tasks:
- to bring up the Task Manager to kill errant processes
- to bring up the Login dialog
- to bring up the "Windows Security" dialog, where the user can lock the computer, change password, shut down, and so on.
The Ctrl alt del key combination was chosen as the Secure attention key in Windows, because on the PC platform no program could expect to intercept this key combination, since the BIOS had previously used it exclusively. In the new situation, a user pressing Control-Alt-Delete can be sure that the Operating System (rather than a third party program) responds to the key combination, and that it is therefore safe to enter a password.
It is also the method of choice of bringing up the Task Manager, because a stuck program is not able to intercept the Control-Alt-Delete sequence. All other key combinations could potentially be exclusively tied up by a process that is stuck.
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 they could always use Control-Alt-Delete.
In Linux, the command is usually also intercepted, and can be used to initiate a soft reboot in situations where a normal shutdown procedure is unavailable - for example, if a user does not have the permission necessary to shut down the computer (usually reserved for root).
Colloquially, the combination is also known as the three finger salute, or more esoterically, a Vulcan nerve pinch.
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