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From: ballou@martigny.ai.mit.edu (Kenneth Robert Ballou)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Soundblaster IRQ and Port settings
Date: 23 Apr 1993 18:29:22 GMT
Organization: Oberon Software, Inc.
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References: <C5MGx0.Dxy@news.Hawaii.Edu> <3130@shaman.wv.tek.com> <pdb059-210493145738@kilimanjaro.jpl.nasa.gov>
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In article <pdb059-210493145738@kilimanjaro.jpl.nasa.gov> pdb059@ipl.jpl.nasa.gov (Paul Bartholomew) writes:
>In article <3130@shaman.wv.tek.com>, andrew@frip.WV.TEK.COM (Andrew
>Klossner) wrote:
>> 
>> []
>> 
>> 	"These LPT1, COM1, disk controller are call devices.  There are
>> 	devices that requires exclusive interrupt ownership, eg. disk
>> 	controller (I6) and keyboard (I1).  There are also devices that
>> 	does not require exclusive ownership, ie. it will share an
>> 	interrupt with another device, eg. LPT1"
>> 
>> No.  In a standard ISA bus, the one that almost all non-laptop PCs use,
>> two separate interface cards cannot share an interrupt.  This is due to
>> a screwup in the bus design.  For example, if your Soundblaster wants
>> to drive interrupt number 7, then it must hold a certain bus wire to 0
>> or 1 at all times, depending on whether or not it wants an interrupt.
>> This precludes letting another card assert interrupt number 7.
>> 
>> When two or more devices in an ISA bus PC share an interrupt, it's
>> because they're implemented by a single card.
>
>Interesting.  Would you care to explain to me then, how my SoundBlaster
>Pro card and my printer card are sharing IRQ 7 successfully?  I assure
>you that they are both set to IRQ 7 and that I have no problem.  My
>computer is a DTK 286-12 IBM clone.

Simple.  First, Andrew is correct, although I can see where there might be
some confusion.  It is indeed possible to have two cards *configured* to use
the same interrupt.  They can not *share* the interrupt in the sense that it
is not possible to have both cards active at the same time.

Here is an example.  For some time, I was short of "free interrupts."  I had a
tape controller (not a "floppy tape") that needed one of IRQ0-IRQ7.  (It's an
*old* tape drive.)  My solution was to use IRQ3 (also used for COM2, where my
modem is).  I did this because I reasoned I would never be using the modem and
the tape simultaneously.  When kermit runs, it installs its own interrupt
handler for IRQ3 and uses the serial port.  If the tape drive were to generate
an interrupt, kermit would not have a clue what to do with/for the tape
controller.  (And since the tape controller would not be serviced, it would
most likely "hang.")  Likewise, when the tape backup software runs, it
installs an interrupt handler for IRQ3.  That handler won't do anything for
the serial port.

Under DOS, printing is NOT interrupt-driven.  Instead, the BIOS polls the
status of the parallel port to determine when another byte may be sent.
That's why you can have your sound card and LPT1 both configured to use IRQ7.
Try that on NT and see how far you'll get. :-)
-- 
Kenneth R. Ballou				VOICE:		 (617) 494-0990
Oberon Software, Inc.				FAX:		 (617) 494-0414
One Memorial Drive, Cambridge, MA  02142	Internet:     ballou@oberon.com

The views and opinions expressed above are my own and do not necessarily
reflect those of my employer.
