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From: P88036@BARILVM.BITNET (Ephraim Vider)
Newsgroups: comp.windows.ms
Subject: Re: Dumn Question: what is ega.sys?
Message-ID: <91157.163904P88036@BARILVM.BITNET>
Date: 6 Jun 91 16:34:48 GMT
References: <1991Jun4.210335.20502@nntp-server.caltech.edu>
Organization: Bar-Ilan University Computing Center, Israel
Lines: 53

In article <1991Jun4.210335.20502@nntp-server.caltech.edu>,
nyet@nntp-server.caltech.edu (n liu) says:
>
>I've got a Trident 8900 card and the new hirez 256 color beta drivers.
>During installation, setup insists that i need to load ega.sys.
>
>I've experimented a little between loading it and not loading it, but
>haven't seen much difference.
>
>The startup screen looks like this:
>
>EGA Device Driver Version 2.10.18
>[copyright stuff]
>
>Using default value of BCH for interrupt 2FH function calls.
>Warning!! Overwriting an old copy of the EGA Device Driver.
>A copy of the EGA Device Driver is being installed.
(interrupt info deleted... )
>
>Incidentally, the "Warning!!" message goes away if i take out my Logitech
>mouse.sys ver 5.0.
>
>nye

here is my understanding of EGA.SYS :

The ega card control registers are write only. A system like windows has
to save and restore the values in those registers for each dos application
it runs.

The EGA.SYS device driver intercepts writes to the registers
and saves the last values written. This information is then available
through some interrupt 10h service (around ah=F0 I think).

The availability of this service is indicated using an INT 2Fh code.
When EGA.SYS loads it checks if such a service is already installed.

Microsoft's mouse driver, and probably logitech's also, gives the same
service. EGA.SYS sees this as an older version and overwrites it.

BTW: If you load MS mouse driver after EGA.SYS, it detects the
EGA service and does not load this part of itself.
Try loading the logitech driver after EGA.SYS and it might save a few
bytes of memory.

I'm not sure if EGA.SYS is really needed on VGA systems, but the
mouse driver will load this service if there is no EGA.SYS, and I can't
get any extra bytes of memory, so I left it there...

-eff

Ephraim Vider                           BITNET:   p88036@barilvm
Bar Ilan University                     INTERNET: p88036@vm.biu.ac.il