*BSD News Article 46448


Return to BSD News archive

Path: sserve!newshost.anu.edu.au!harbinger.cc.monash.edu.au!simtel!news.kei.com!news.mathworks.com!solaris.cc.vt.edu!ceharris
From: ceharris@mal.com (Carl Harris)
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: Re: IRQ 9 unsafe?
Date: 6 Jul 1995 10:38:32 GMT
Organization: Maladjusted Communications
Lines: 35
Message-ID: <3tgef8$ffn@solaris.cc.vt.edu>
References: <gztQkHQ@quack.kfu.com> <3t940q$cf4@anshar.shadow.net> <3tc4dn$9k2@csugrad.cs.vt.edu>
Reply-To: ceharris@mal.com
NNTP-Posting-Host: mal9000.mal.com
NNTP-Posting-User: ceharris
X-Newsreader: TIN [version 1.2 PL2]

Jeff Aitken (jaitken@csugrad.cs.vt.edu) wrote:
: Don Whiteside (dwhite@anshar.shadow.net) wrote:
: :   IRQ 9 is indeed a cascaded IRQ 2.

: Pardon my ignorance, but what exactly does that mean? 


In the original IBM PC, there was only one peripheral interrupt
controller, or PIC -- an Intel 8259A.  For all practical purposes, every
one of the available IRQ lines on that PIC were reserved (clock
tick, keyboard, COM1, COM2, floppy disk, hard disk, LPTx) except IRQ 2,
which was "reserved by IBM."

Needless to say, this situation made it devilishly difficult to put 
anything else that needed an interrupt line into the machine.  So when
the AT was designed, IBM added a second PIC (also an 8259A chip).  (Then
IBM went on to reserve a good many of those IRQs.  ;-\ )

Since, in general, there is only one external (maskable) interrupt
input line on a CPU, only one PIC can be attached directly to it.  The
8259A was designed to solve this problem by allowing cascading.  In a
cascaded configuration, the PIC's INT line (which indicates that an
interrupt is pending on one of its IRQ lines) is attached to an IRQ pin
on another PIC.  In the case of the AT, the second PIC's INT pin is
attached to the first controller's IRQ 2 pin.

I don't have my Intel chip handbook handy, and I don't recall if there was
something magical about using IRQ 2 for the cascaded controller, or whether
IBM chose IRQ 2 because they hadn't already used it for something else.

--
Carl Harris
EXECUTIVE Scapegoat (and Systems Engineer)
ceharris@mal.com
http://www.mal.com/~ceharris