Control-Alt-Delete


Control-Alt-Delete is a computer keyboard command on IBM PC compatible computers, invoked by pressing the Delete key while holding the Control and Alt keys:. The function of the key combination differs depending on the context but it generally interrupts or facilitates interrupting a function. For instance, in pre-boot environment or in DOS, Windows 3.0 and earlier versions of Windows or OS/2, the key combination reboots the computer. Starting with Windows 95, the key combination invokes a task manager or security related component that facilitates ending a Windows session or killing a frozen application.

History

The soft reboot function via keyboard was originally designed by David Bradley. Bradley, as the chief engineer of the IBM PC project and developer of the machine's ROM-BIOS, had originally used
, but found it was too easy to bump the left side of the keyboard and reboot the computer accidentally. According to his own account, Mel Hallerman, who was the chief programmer of the project, therefore suggested switching the key combination to as a safety measure, a combination impossible to press with just one hand on the original IBM PC keyboard.
The feature was originally conceived only as a development feature for internal use and not intended to be used by end users, as it triggered the reboot without warning or further confirmation—it was meant to be used by people writing programs or documentation so that they could reboot their computers without powering them down. Bill Gates remembered it as "just something we were using in development and it wouldn't be available elsewhere". The feature, however, was detailed in IBM's technical reference documentation to the original PC and thereby revealed to the general public.
Bradley viewed this work as just one small task out of many: "It was five minutes, 10 minutes of activity, and then I moved on to the next of the 100 things that needed to get done." In a March 2018 email, one of Bradley's co-workers confirmed the command was invented in 1981 in Boca Raton, Florida.
Bradley is also known for his good-natured jab at Gates at the celebration of the 20th anniversary of the IBM PC on August 8, 2001 at The Tech Museum:
"I have to share the credit. I may have invented it, but I think Bill made it famous."; he quickly added it was a reference to Windows NT logon procedures.
During a question and answer presentation on 21 September 2013, Gates said "it was a mistake", referring to the decision to use Ctrl+Alt+Del as the keyboard combination to log into Windows. Gates stated he would have preferred a single button to trigger the same actions, but could not get IBM to add the extra button into the keyboard layout.

BIOS

By default, when the operating system is running in real mode, this keystroke combination is intercepted by the BIOS. The BIOS reacts by performing a soft reboot.
Examples of such operating systems include DOS, Windows 3.0 in Standard Mode as well as earlier versions of Windows.

Windows

DOS-based Windows

In Windows 9x and Windows 3.0 running in 386 Enhanced mode, the keystroke combination is recognized by the Windows keyboard device driver. According to the value of the LocalReboot option in the section of system.ini, Windows performs one of several actions in response. If LocalReboot=On :
If LocalReboot=Off, Windows performs a soft reboot.

Windows NT family

The Windows NT family of operating system, whose members do not have "NT" in their names since Windows 2000, reserve Ctrl+Alt+Delete for the operating system itself. Winlogon, a core component of the operating system, responds to the key combination in the following scenarios:
;Invoking Windows Security: When a user is logged onto a Windows computer, pressing Ctrl+Alt+Delete invokes Windows Security. It is a graphical user interface that allows user to lock the system, switch user, log off, change the password, invoke Windows Task Manager, or end the Windows session by shutting down, rebooting or putting the computer into sleep or hibernation; clicking "Cancel" or pressing the Escape key returns the user to where they were.
The key combination always invokes Windows Security in all versions and editions of Windows NT family except Windows XP. Prior to Windows Vista, Windows Security was a dialog box, did not allow user switching and showed the logon date and time, name of user account into which the user has logged on and the computer name. Starting with Windows Vista, Windows Security became full-screen.
;Secure attention: Login spoofing is a social engineering trick in which a malicious computer program, masquerading as Windows login dialog box, prompts for user's account name and password to steal them. To thwart this attack, Windows NT implements an optional security measure in which Ctrl+Alt+Delete acts as a secure attention key combination. Once the protection is activated, Windows requires the user to press Ctrl+Alt+Delete each time before logging on or unlocking the computer. Since the key combination is intercepted by Windows itself and malicious software cannot mimic this behavior, the trick is thwarted. Unless the Windows computer is part of a Windows domain network, the secure attention protection is disabled by default and must be enabled by the user.
;Windows XP behavior: Windows XP introduces Welcome Screen, a redesigned logon interface. The Welcome Screen of Windows XP, however, does not support the secure attention scenario. It may be disabled in favor of the classic plain logon screen, either explicitly by the user or as a consequence of the Windows XP computer becoming part of a Windows domain network. With that in mind, Windows XP uses the Ctrl+Alt+Delete in the following unique scenarios:
  1. At a logon prompt, the key combination dismisses Welcome Screen and invokes classic logon user interface.
  2. When a user is logged on to a Windows XP computer and Welcome Screen is enabled, pressing the key combination invokes Windows Task Manager instead of Windows Security.

    OS/2

In OS/2, this keystroke combination is recognized 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 "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.

Mac

Ctrl+Alt+Delete is not a keyboard shortcut on macOS. Instead, brings up the Force Quit panel. restarts the computer.
The original Mac OS X Server had an Easter egg in which pressing would show an alert saying "This is not DOS."

Linux

On some Linux-based operating systems including Ubuntu and Debian, is a shortcut for logging out.
On Ubuntu Server, it is used to reboot a computer without logging in.

Equivalents on various platforms

PlatformKey combinationFunction
TI-30XIISRestarts the calculator and clears RAM
TI-80, TI-81, TI-82, TI-83, TI-84,, Shows ROM version number. enters self test mode
TI-85, TI-86,,, Shows ROM version number. enters self test mode
TI-89Restarts the calculator and clears RAM
TI-89Force break without restarting RAM
TI-89,, Enter self test mode
Natural display Casio calculatorsRestarts the calculator and clears RAM and EEPROM. Continue pressing Shift to advance through self-test mode.
TI-99/4AResets machine back to startup screen.
Voyage 200Restarts the calculator and clears RAM
HP-48Restarts RPL, clearing the Stack and PICT, closing IO, and returning to the HOME directory
HP-48As above, but also purges the memory
Scientific Atlanta Explorer DHCT
Samsung cable boxes		 Reboots box
Foxtel Set-top-boxes Power cycles the machine.
Foxtel Set-top-boxes Power cycles the machine.
Foxtel Set-top-boxes Power cycles the machine.
C64Warm starts the machine, does not work if CPU has crashed or NMI is blocked in hardware or rerouted.
Amazon Fire TV Reboots the device

Cultural adoption

As computers became ubiquitous, so too, has the jargon. Control-Alt-Delete can also mean "dump," or "do away with".
The keystrokes are well known and infamous for escaping from problems in pop culture. For example, in the Billy Talent song "Perfect World", part of the lyrics include the sequence and associate it with resetting their memory and escaping from a situation: "Control-Alt-Deleted. Reset my memory."