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The Ctrl + Alt + Del key combination to reset is known to me to work on older PC/MS-DOS PCs, some versions of Atari TOS, on runlevel 3 / multiuser.tager Linuxes (it can be disabled).

The older of these is apparently IBM PC. Did it originate on this platform or was it used on older systems also? Any stories about how these keys were chosen?

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was it used on older systems also

Just for curiosity: some kind of key combination to reset had other computers (non PC) too, e.g. the CPC6128 had Control + Shift + ESC.

Otherwise, Wikipedia has an interesting insight:

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 Ctrl+Alt+Esc, 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 Ctrl+Alt+Del 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 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."

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  • 1
    The Commodore 64 had RunStop\Restore which would do a soft reset - return you to the starting screen and 'READY' prompt but leave any program in memory intact.
    – Alan B
    Nov 22 '20 at 13:25
  • I wouldn't imagine that wikipedia has an article about ctrl-alt-del, thanx!
    – Krackout
    Nov 22 '20 at 15:36
  • 2
    @Krackout, Wikipedia has an article on everything.
    – Mark
    Nov 22 '20 at 18:13
  • Except, the CPC isn't an 'older' system then the IBM-PC, but came some 4 years after the IBM-PC, so their choice is rather copied from the PC.
    – Raffzahn
    Nov 22 '20 at 18:44
  • Especially in the early days, many PC programs were self-booting, and could only be exited by either using control-alt-del or by power-cycling the machine, with the former being 20-60 seconds faster depending upon how much memory was installed.
    – supercat
    Nov 22 '20 at 19:18
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[This question is kind of related, if not a duplicate]

The older of these is apparently IBM PC. Did it originate on this platform or

Yes, part of the BIOS.

was it used on older systems also?

No.

Any stories about how these keys were chosen?

Sorry, no fancy story story, just practical considerations.

Using three keys and having them far enough apart to need two hands to operate made it rather error-proof, while the combination had to be something that wasn't likely to be used otherwise CTRL and ALT are modifiers, thus free to be used, while DEL is a 'new', non-ASCII key, thus risk of collision with any existing software to be ported is virtual zero.

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    "DEL is a 'new', non-ASCII key" Isn't delete 0x7f, and backspace 0x08?
    – DrSheldon
    Nov 22 '20 at 5:16
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    ASCII says nothing about keyboards. But yes, ASCII has had DEL = 0x7f in all versions. Whether there was a key to transmit it is a separate question. ASR33 had a delete key; VT100 had backspace, not delete. Nov 22 '20 at 14:04
  • 1
    @DrSheldon Do you know any prior ASCII terminal able to transmit a code like CTRL-ALT-DEL? DEL on the IBM-PC keyboard is a new added function key, not a key producing the 7Fh character of ASCII - which is produced by pressing SHIFT-~ (left of 1). So, yes, new key, therefore a good choice for a new function.
    – Raffzahn
    Nov 22 '20 at 14:25
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    @another-dave: Interestingly, the purpose of DEL wasn't to send a code that would cause the receiving device to discard a byte, but was rather to be punched onto a paper-tape byte that already held a different value. Some visual display devices represented DEL with a black square; if a printer were to do so, that would tie in very nicely with its purpose: to delete a typo on a write-once medium, overprint it with something that will obscure whatever was there previously.
    – supercat
    Nov 22 '20 at 19:22
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    @Raffzahn "Do you know any prior ASCII terminal able to transmit a code like CTRL-ALT-DEL?" No ASCII terminal can transmit CTRL-ALT-DEL. It's not an ASCII character. An IBM PC keyboard doesn't send ASCII codes to its system unit. Instead it sends scan code information (up or down status whenever a key is pressed or released). The keyboard driver turns scancodes into ASCII characters and sends them to the OS/application. It can also recognise CTRL-ALT-DEL and reset the PC.
    – Graham Nye
    Nov 22 '20 at 21:46

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