Path: utzoo!attcan!uunet!mcsun!ukc!strath-cs!glasgow!kcl-cs!howard
From: howard@kcl-cs.UUCP (JR Howard)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: INT2 on XT-AT-386
Message-ID: <2067@xenon.kcl-cs.UUCP>
Date: 26 Feb 91 11:09:36 GMT
References: <7504@uceng.UC.EDU>
Sender: memex@cs.glasgow.ac.uk (Memex information engines)
Organization: Department of Computing, King's College London
Lines: 26

In article <7504@uceng.UC.EDU> rogden@uceng.UC.EDU (rob ogden) writes:
>My docs show that an XT has one INT controller chip and
>that INT 2 is not used. Furthermore, the docs show that the
>AT&386 machines use INT2 as an input from a second controller
>chip. Two questions.
>
>1) On an AT class machine, can an interface card share the INT2
>   with the motherboard's #2 controller chip.
>
>2) On any ISA bus machine, can any two interface cards share the
>   same interupt number? For example, #5&7 seem to be used by cards
>   that are not parallel I/O.
>

To sum this up quickly - look at the XT & AT (ie. 8 & 16 bit) bus pinouts.  If
I remember correctly, INT2 on an XT is INT9 on an AT in the same position.
This solves the gain or loss problems of XT & AT compatability.  As for
sharing it depend on how or what controls the INT.  If the service enquiries
via the ports are based on 'if its' or 'what is' the device causing the
interrupt then sharing is possible.  However I suspect its actually a case of
looking closly at the particular devices and not the standard.

Finally - #5&7 may be working because the clone manufacturer has 'yet again'
implemented the assignments on HIS and not THE standard.

JR