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SubjectRe: i8042 problem
On Mon, 28 Jul 2003, Andries Brouwer wrote:

> > > > Well, are timeouts needed at all?
> > >
> > > Yes. We send a command to the keyboard. It may react, or it may not.
> >
> > But we need not wait for that actively. If we are unsure about a result
> > of a command, then we may send a command in question followed with an echo
> > request. This assures an IRQ will finally arrive and if no command
> > response arrives before an echo response, then the keyboard ignored the
> > command. I used this approach many years ago to differ between PS/2
> > keyboards (which respond with 0xfa,0xab,0x83 to a request for ID) and
> > genuine PC/AT ones (which respond with lone 0xfa). It worked.
> And what did you do for XT? :-)

Nice joke, but I'll answer seriously. No support was provided. Hooking
a PC/XT keyboard to the 8042, if supported, requires a different setup of
the command byte and is possibly done by the system firmware. You can
read the command byte to see which configuration is used.

Wrt polling vs IRQ-driven probing and setup: using IRQ is a natural
choice as you have to do keyboard detection in the IRQ handler anyway to
properly support hot plugging of a PC/AT or a PS/2 keyboard.

+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+ e-mail:, PGP key available +

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