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Control-Alt-Delete

From Academic Kids

This article is about Control-Alt-Delete, the keyboard shortcut. For the web comic, see Ctrl+Alt+Del (webcomic).
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A humorous image created as a parody of Control-Alt-Delete

Control-Alt-Delete (often abbreviated to Ctrl-Alt-Del) is a computer keyboard command on IBM PC compatible systems that can be used to reboot the computer. It is given by simultaneously pressing the Control, Alt, and Delete 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). 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. David 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 really famous."

Colloquially, the combination is also known as a three-finger salute, or more esoterically, a Vulcan nerve pinch.

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

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 and treated as the 80x86 non-maskable-interrupt signal, which, unless the system is horribly hung, invokes a soft reboot.

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. Theoretically, the system's other processes should continue normally--in practice, using this key combination to terminate a program/process will result in "secondary damage" that may destabilize the system. As such, it is strongly reccomended that, following a process kill, you save your work in any other applications and restart the computer.

Entering the combination twice in succession will theoretically 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.

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).

The keystroke can pose a security risk, in that a user can place the machine into single user mode after the machine reboots. Single user mode does not require a password and works in superuser mode, which would in effect give the user unrestricted access to the machine. Linux systems can be configured to ignore the keystroke combination.

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

it:Control-alt-canc

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