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From: hpa@casbah.acns.nwu.edu (H. Peter Anvin)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: IRQ 2 on AT-class machines
Message-ID: <1991May4.060625.15307@casbah.acns.nwu.edu>
Date: 4 May 91 06:06:25 GMT
References: <22603@yunexus.YorkU.CA> <160@thor.sdrc.com> <3WC+R0F@jwt.UUCP>
Organization: Northwestern University
Lines: 23

In article <3WC+R0F@jwt.UUCP> john@jwt.UUCP (John Temples) writes:
[that sometimes you have to specify "IRQ 9" and sometimes "IRQ 2" when
using this particular interrupt line on the AT.  The line is strictly
speaking IRQ 9 but it is often used as a substitute for IRQ 2 which is used
for chaining the secondary interrupt controller carrying IRQ 8-15 to the
primary carrying IRQ 0-7 and therefore is lost.]

The AT BIOS is part of the reason: the default interrupt handler for IRQ 9
(INT 49h I think) is just a jump to INT 09h, which is where the IRQ 2
handler should have been if IRQ 2 hadn't been used for chaining and thus is
unavailable.

Only 16-bit cards can use IRQ 8-15, with the exception of IRQ 9 which is
carried in the 8-bit part of the ISA bus in the position for IRQ 2.

DISCLAIMER: I think this is right.  The information I have gotten on this
is contradicting, to say the least.  (And much of it has been clearly
wrong).
-- 
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