In real mode, which was all there was when these keys were assigned, Ctrl+Alt+Del didn't actually fire any special low level hardware interrupt. I didn't watch Bill's video yet, so I don't know what he may have been referring to, but it definitely doesn't do that. (Maybe he just meant that it reboots the computer?)
In real mode, Ctrl+Alt+Del is handled by the same BIOS code as any other keystrokes. Each of those three keys generates the usual keyboard interrupt, and BIOS code checks for Ctrl and Alt being down when Del is pressed. When that happens, the BIOS jumps to the POST (Power On Self Test) address.
Or alternatively, Ctrl+Alt+Del is handled (or not handled!) by any other code that has taken over the keyboard interrupt.
There are no low level hardware interrupts involved at all other than the normal keyboard interrupt that handles any other keystroke.
I wrote up the details over on Reddit a couple of days ago, including some code listings from the XT BIOS:
"We could have had a single button, but the guy who did the IBM keyboard design didn’t want to give us our single button.."
"You want to have something you do with the keyboard that is signalling to a very low level of the software - actually hard-coded in the hardware - that it really is bringing in the operating system you expect, instead of just a funny piece of software that puts up a screen that looks like a log-in screen, and then it listens to your password and then it’s able to do that," Mr Gates said.
Now what that means in terms of hardware/software I don't know, but it sounds a lot like some sore of interrupt to me.
Does it matter? I think not.
I don't think the combination of keys was ever picked for their ease of use, but instead became widely know only because the Windows of that time was not the most stable of systems.
But once again I don't thin any operating system running in real mode would've had the power protect the user from badly written programs.
Thanks for posting that transcript. Bill was seriously mixed up here! It's understandable since he was speaking off the cuff about events of 30 years ago.
The guy who did IBM's keyboard design had nothing to do with Ctrl+Alt+Del. The keyboard doesn't know anything special about Ctrl+Alt+Del or any other key combination. It just sends scan codes over the wire for each individual key press or release.
The guy who wrote (or specified) the PC BIOS is who gave Ctrl+Alt+Del its special meaning. As I explained in that Reddit post, the BIOS simply checks for those keys being down and jumps to the reset/POST address.
It's all done in software, and any real mode program running on the machine can disable or take over Ctrl+Alt+Del just like it can any other keystroke or combination. So any program in DOS, in real mode Windows, or in any other real mode operating system can be that "funny piece of software that puts up a screen that looks like a log-in screen".
It wasn't until we got chips that supported protected mode, so ordinary applications couldn't smash the interrupt vectors, that there was any possibility of a guarantee that Ctrl+Alt+Del be always handled by the OS itself and not intercepted by some other program.
Also, back in the real mode days, no one would ever press Ctrl+Alt+Del with the expectation that they would get a login screen or any other kind of operating system response. After all, when Ctrl+Alt+Del was working correctly it would reboot the computer completely!
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In real mode, which was all there was when these keys were assigned, Ctrl+Alt+Del didn't actually fire any special low level hardware interrupt. I didn't watch Bill's video yet, so I don't know what he may have been referring to, but it definitely doesn't do that. (Maybe he just meant that it reboots the computer?)
In real mode, Ctrl+Alt+Del is handled by the same BIOS code as any other keystrokes. Each of those three keys generates the usual keyboard interrupt, and BIOS code checks for Ctrl and Alt being down when Del is pressed. When that happens, the BIOS jumps to the POST (Power On Self Test) address.
Or alternatively, Ctrl+Alt+Del is handled (or not handled!) by any other code that has taken over the keyboard interrupt.
There are no low level hardware interrupts involved at all other than the normal keyboard interrupt that handles any other keystroke.
I wrote up the details over on Reddit a couple of days ago, including some code listings from the XT BIOS:
http://www.reddit.com/r/technology/comments/1n68nz/bill_gate...
Here a transcript of the video:
"We could have had a single button, but the guy who did the IBM keyboard design didn’t want to give us our single button.."
"You want to have something you do with the keyboard that is signalling to a very low level of the software - actually hard-coded in the hardware - that it really is bringing in the operating system you expect, instead of just a funny piece of software that puts up a screen that looks like a log-in screen, and then it listens to your password and then it’s able to do that," Mr Gates said.
http://www.heraldsun.com.au/technology/news/bill-gates-confe...
Now what that means in terms of hardware/software I don't know, but it sounds a lot like some sore of interrupt to me.
Does it matter? I think not.
I don't think the combination of keys was ever picked for their ease of use, but instead became widely know only because the Windows of that time was not the most stable of systems.
But once again I don't thin any operating system running in real mode would've had the power protect the user from badly written programs.
Thanks for posting that transcript. Bill was seriously mixed up here! It's understandable since he was speaking off the cuff about events of 30 years ago.
The guy who did IBM's keyboard design had nothing to do with Ctrl+Alt+Del. The keyboard doesn't know anything special about Ctrl+Alt+Del or any other key combination. It just sends scan codes over the wire for each individual key press or release.
The guy who wrote (or specified) the PC BIOS is who gave Ctrl+Alt+Del its special meaning. As I explained in that Reddit post, the BIOS simply checks for those keys being down and jumps to the reset/POST address.
It's all done in software, and any real mode program running on the machine can disable or take over Ctrl+Alt+Del just like it can any other keystroke or combination. So any program in DOS, in real mode Windows, or in any other real mode operating system can be that "funny piece of software that puts up a screen that looks like a log-in screen".
It wasn't until we got chips that supported protected mode, so ordinary applications couldn't smash the interrupt vectors, that there was any possibility of a guarantee that Ctrl+Alt+Del be always handled by the OS itself and not intercepted by some other program.
Also, back in the real mode days, no one would ever press Ctrl+Alt+Del with the expectation that they would get a login screen or any other kind of operating system response. After all, when Ctrl+Alt+Del was working correctly it would reboot the computer completely!