Control-Alt-Delete

Control-Alt-Delete (often abbreviated to Ctrl+Alt+Del, also known as the "three-finger salute" or "Security Keys") 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 (before an operating system starts) 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 (former Microsoft CEO) 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 ("Press Ctrl + Alt + Delete to log on").
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 (or in a pre-boot environment, when no operating system is started yet), this keystroke combination is intercepted by the BIOS. The BIOS reacts by performing a soft reboot (also known as a warm 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 option in the section of system.ini, Windows performs one of several actions in response. If (default):
If , 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:
The key combination always invokes Windows Security in all versions and editions of Windows NT family except Windows XP. (See below.) 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.
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 (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).
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 (as the Option key is the equivalent of Alt key on a Mac keyboard) 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.
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."