From:	eplmst AT lu DOT erisoft DOT se (Martin Stromberg)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: strange interrupt chaining problem with keyboard interrupt
Date:	10 Oct 2000 09:22:37 GMT
Organization:	Ericsson Erisoft AB, Sweden
Lines:	31
Message-ID:	<8run4t$531$2@antares.lu.erisoft.se>
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To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp
Reply-To:	djgpp AT delorie DOT com

Peter Remmers (Peter DOT Remmers AT t-online DOT de) wrote:
: I disassembled keyb.com and got some very interesting insights.
: Num-lock, scroll-lock and caps-lock are special in that they
: cause keyb to change the LEDs on the keyboard. This means
: keyb has to send commands to the keyboard which make the keyboard
: cause an interrupt to announce that its ACK (0xFA) is ready to be
: picked up.
: Keyb.com's interrupt handler sets up it own stack (512 bytes) and
: points DS to the BIOS data segment on entry.
: It also has a reentrance counter. If the reentrace counter is
: greater than zero it does NOT set up the stack again!
: And that's the problem: it expects an ACK from the keyboard, and
: it expects to be reentered by the ACK interrupt, and that means
: it expects SS and DS to have certain values when the interrupt
: occurs.
: Now, if I hook the keyboard interrupt and set up DJGPP's execution
: environment I (or the wrapper) overwrite keyb's SS and DS
: registers, which is fatal in the case of the keyboard's ACK
: interrupt - or any other case of reentry, for that matter.

: In which way the shift keys are special, I don't know. But KEYB
: for whatever reason treats them special, too, because they also
: cause the keyboard to send an ACK, which ends like explained above.

I think I know. Some keyboards have another LED that lights up when
the shift keys is pressed (e. g. Microsnoft Natural keyboard).


Right,

						MartinS

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