Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!mintaka!spdcc!iecc!johnl
From: johnl@iecc.cambridge.ma.us (John R. Levine)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: sharing hardware interrupts
Message-ID: <1991May30.004511.791@iecc.cambridge.ma.us>
Date: 30 May 91 00:45:11 GMT
References: <1991May29.014824.16278@ux1.cso.uiuc.edu> <08VP31w164w@bluemoon.uucp>
Organization: I.E.C.C.
Lines: 37

In article <08VP31w164w@bluemoon.uucp> jamaass@bluemoon.uucp (Jeffrey A. Maass) writes:
>phil@ux1.cso.uiuc.edu (Phil Howard KA9WGN) writes:
>> I have read that two devices cannot share and concurrent use the same
>> interrupt (IRQ).  ...
>
>Phil: You _CANNOT_ share interrupts on a PC/XT/AT, and the reason
>is in hardware, not dumb software. ...

The real answer, like the answer to most questions about crufty old PC
harware, is "it depends."  Most hardware cards are built on the assumption
that the interrupt line is unshared.  When the card doesn't want to
interrupt, it firmly turns the IRQ line off, when it does want to interrupt,
it turns the line on.  The problem with putting two cards on the same
interrupt line is that in the typical case that one wants to interrupt and
the other doesn't, the IRQ line is pulled both ways and the result is usually
no interrupt at all.  I once had all sorts of bizzaro printer problems until
I removed an unused Ethernet card that happened to be jumpered to the same
IRQ line.

The major exception to this is serial ports.  Most serial port cards I have
come across, even the $12.99 Taiwanese specials, connect to the IRQ line in a
way that allows any of several cards to turn on the IRQ line and have it
work.  For example, my internal Telebit modem and the $12.99 serial port for
my fax modem share the same interrupt.  Works fine.

Dumb software remains a problem, since any time you get an interrupt on a
shared line, the software has to poll all of the devices sharing the line to
find out which ones have something to say.  Too many device drivers,
particularly in DOS land (I run Unix) weren't written to support shared
interrupts even when the hardware allows it.

Regards,
John Levine, johnl@iecc.cambridge.ma.us, {spdcc|ima|world}!iecc!johnl
-- 
John R. Levine, IECC, POB 349, Cambridge MA 02238, +1 617 492 3869
johnl@iecc.cambridge.ma.us, {ima|spdcc|world}!iecc!johnl
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