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From: evans@syd.dit.CSIRO.AU (Bruce.Evans)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: sharing hardware interrupts
Message-ID: <1991May30.125133.527@syd.dit.CSIRO.AU>
Date: 30 May 91 12:51:33 GMT
References: <1991May29.014824.16278@ux1.cso.uiuc.edu>
Organization: CSIRO Division of Info Tech, Sydney, Australia
Lines: 29

In article <1991May29.014824.16278@ux1.cso.uiuc.edu> phil@ux1.cso.uiuc.edu (Phil Howard KA9WGN) writes:
>Suppose you have two or more different serial ports in a PC.  Surely
>there is a register somewhere on there that indicates its status as to
>whether or not it has changed state, or has a state that needs to be
>dealt with.  When any one of the serial ports causes an interrupt (for
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ it won't
>the sake of discussion let's say it is #4 and nothing else uses #4)
>the software driver will get into control, scan all the known serial
>port addresses for states to deal with, and performs the appropriate
>action for all of them that need it.
>
>Given the above software, why could it not be possible to have two or
>more devices share the same interrupt.

Because if 2 devices are driving the same interrupt line, the one driving
it low wins. For some reason the PC interrupt h/w is designed so that
low == no interrupt. This is the reverse of what is needed for easy
interrupt sharing.

It is easy to have 2 ports sharing the same interrupt line if you don't
want them both to generate interrupts in the same application. The OUT2
port is wired as an interrupt-line enable. Turning it off allows another
serial device to drive the shared line. It is important for applications
that use it to restore it when they finish. I often have to clean up
after Windows has left it hot for COM1 in order to use an old version of
Mirror on COM3. Outputting 3 to port 3FC hex turns off the OUT2 bit (8)
for COM1.
-- 
Bruce Evans		evans@syd.dit.csiro.au