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From: bmarsh@cod.NOSC.MIL (William C. Marsh)
Newsgroups: comp.sys.ibm.pc
Subject: Re: What's IRQ 2 used for on an AT?
Message-ID: <1925@cod.NOSC.MIL>
Date: 31 May 90 21:01:32 GMT
References: <749@ucsvc.ucs.unimelb.edu.au>
Reply-To: bmarsh@cod.nosc.mil.UUCP (William C. Marsh)
Organization: Naval Ocean Systems Center, San Diego
Lines: 26

In article <749@ucsvc.ucs.unimelb.edu.au> U5533129@ucsvc.ucs.unimelb.edu.au (CARDIOLOGY, R.M.H.) writes:
>Can anyone out there tell me what the IRQ 2 line is used for on an AT?
>
>I've just set up an ethernet card to use it, but when I told the supplier he
>said he though it would clash with the second interupt controller.  I've been
>using it for several weeks without any problem.  Have I just been lucky, or
>is there a conflict there?

Actually, on the AT bus, the pin that was IRQ 2 in a PC/XT is actually called
IRQ 9.  The supplier in question is partially correct, IRQ 2 (on the original
interrupt controller) is now connected to the interrupt request pin from the
second interrupt controller (controllers are cascaded).  Some software does
not correctly handle this 'IRQ 2' on an AT, since it's really 'IRQ 9' (Though
the BIOS does help, and by default will revector IRQ 9 to where IRQ 2 would
have called, however, the software still has to unmask the proper interrupts
and send the EOI command to the correct controller).

More and more software (mouse drivers, etc) which use IRQ 2 know about this,
now that the AT has been around for five years.

Bill
-- 
Bill Marsh, Naval Ocean Systems Center, San Diego, CA
{arpa,mil}net: bmarsh@cod.nosc.mil
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"If everything seems to be coming your way, you're probably in the wrong lane."