On: 5 ways to reboot a PC. None of them reliable.

[smallblacksun]

An actual IBM-PC could reliably be rebooted using the keyboard controller (as mentioned in the article). The issues come with IBM-PC "compatible" machines. I'm sure commodore would have the same problem if anyone had bothered to clone them.

[yuhong → smallblacksun]

To be more precise, an IBM PC-AT. It is closely related to the A20 line enable.

[rbanffy → yuhong]

Why should anyone need to mess with the A20 line and the keyboard controller to simply reset a computer?

[yuhong → rbanffy]

Well, you do need to turn on A20 so it will jump to right address, but if you are in protected mode it should already be on.

[rbanffy → yuhong]

Isn't that kludgy? I mean, using a keyboard controller (in effect, a second computer) to reset the one you can't?

[Tuna-Fish → rbanffy]

The reason was that IBM bought the rest of the chips "as they were" from Intel, so all the logic that they made themselves ended in the in-house keyboard controller. x86 had an external interrupt line for reset, and an io pin in the keyboard controller was tied to this. That was a reasonable way to get true hard reset back then.

[yuhong → Tuna-Fish]

And what I meant by related was that another pin in the same keyboard controller was tied to the A20 gate, so it was toggled in a similar way.