Path: utzoo!utgpu!watmath!clyde!mcdchg!chinet!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!killer!chasm
From: chasm@killer.DALLAS.TX.US (Charles Marslett)
Newsgroups: comp.sys.ibm.pc
Subject: Re: IRQ2 used by VGA?
Summary: IRQ2 on EGA and VGA cards
Message-ID: <5948@killer.DALLAS.TX.US>
Date: 30 Oct 88 02:50:11 GMT
References: <5206@watdcsu.waterloo.edu>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 35

In article <5206@watdcsu.waterloo.edu>, dmurdoch@watdcsu.waterloo.edu (D.J. Murdoch - Statistics) writes:
> I just bought a Microsoft bus mouse for my XT clone.  (They're offering an
> excellent educational discount.)  When installing it, I found that the only
> interrupt line I had free was IRQ2, which is listed as conflicting only with
> an EGA and a network adapter.  I don't have any plans to get a network adapter,
> but would like to upgrade from monochrome to colour.  Does the VGA also use
> IRQ2?  If so, is it possible to double up devices on IRQ lines without
> killing everything?
...
> Duncan Murdoch

The IRQ2 line is not driven by the IBM VGA card or several of the VGA
clone cards (including older VGA Extra/EM cards from STB) -- other VGA
clones do drive it (including the older VGA Extra and current VGA Extra/EM
cards from STB) so as to be more compatible with the motherboard VGA on
PS/2 computers.

Normally, the difference is moot unless you run software that enables the
display interrupt since the line is an open collector output through a
resistor (should I say, usually -- nothing being for sure in this industry
:-).  If you get trash on the screen when moving the mouse or if you get
spurrous mouse movements when you change the screen features (often color
palettes or turning on or off the screen border) you will know the software
you are using is playing with the EGA interrupt.

Even in the worst case, the line can be cut in the EGA/VGA card (probably
voiding your warranty ;-) and there will be no conflict other than losing
the feature I just mentioned (you'll probably get more "snow" on the screen
when you run programs that play with the palette a lot and do not use the
BIOS or wait loops).

Charles Marslett
Chief BIOS fiddler
STB Systems, Inc.      <-- apply all standard disclaimers
chasm@killer.dallas.tx.us