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From: scjones@thor.sdrc.com (Larry Jones)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: IRQ 2 on AT-class machines
Message-ID: <160@thor.sdrc.com>
Date: 1 May 91 18:16:40 GMT
References: <22603@yunexus.YorkU.CA>
Organization: SDRC, Cincinnati
Lines: 39

In article <22603@yunexus.YorkU.CA>, rreiner@yunexus.YorkU.CA (Richard Reiner) writes:
> I've seen the documentation for several cards claim that IRQ 2 must
> not be used for expansion cards on AT-class machines, as it is
> reserved for something or other (they never say what).
> 
> So what's the truth here?  

The truth is that many people are very confused and so you souldn't
believe everything you read (except for this definitive article ;-).

When IBM designed the AT, they doubled the number of interrupts by
adding a second interrupt controller (PIC) which is cascaded to
IRQ-2 on the original PIC.  What that means is that when one of
the new interrupts (IRQ-8 - 15) occurs, the second PIC raises the
IRQ-2 signal on the first PIC which then raises the CPU's
interrupt signal.  Since IRQ-2 was thus used internally, they
connected the bus line that was connected to IRQ-2 on the PC to
IRQ-9.  Many people miss this particular subterfuge and assume
that since IRQ-2 is used as a cascade you can't connect anything
to the IRQ-2 bus line.  This is incorrect.

So, the bottom line is that the IRQ-2 bus line is definitely
available for whatever you want to use it for.  The most commonly
defined use is video retrace, but you can't count on it so noone
actually uses it as far as I know.  As long as your video card
can be persuaded to not use it (which is frequently the case --
most don't use it by default), you can use it for anything you
want.  There is one small gotcha, though.  You need to tell the
driver to use IRQ-9 rather than IRQ-2 since that's what you're
really doing.  Processing an interrupt from the second PIC is a
little different than processing an interrupt from the first PIC,
though, and not all drivers support the second PIC.  Hopefully,
this shouldn't be much of a problem any more since the AT has
been around for quite a while now and everyone has had more than
enough time to update their driver code.
----
Larry Jones, SDRC, 2000 Eastman Dr., Milford, OH  45150-2789  513-576-2070
Domain: scjones@sdrc.com             Path: uunet!sdrc!scjones
Nobody knows how to pamper like a Mom. -- Calvin