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From: perkins@bnrmtv.UUCP (Henry Perkins)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Screen Base Address
Message-ID: <2789@bnrmtv.UUCP>
Date: Thu, 8-Oct-87 17:20:07 EDT
Article-I.D.: bnrmtv.2789
Posted: Thu Oct  8 17:20:07 1987
Date-Received: Sun, 11-Oct-87 12:09:49 EDT
References: <5326@jhunix.UUCP>
Organization: BNR Inc., Mountain View, California
Lines: 27
Keywords: DOS Video address
Summary: Screen base address is a function of the screen BIOS, not of DOS.

In article <5326@jhunix.UUCP>, ins_agwa@jhunix.UUCP (Gunther Wil Anderson) writes:
> Does anyone know how to determine the screen base address from
> within DOS, independent of screen type (i.e. not by determining
> the screen type and then assuming that the base address is what
> it should be for that type)?

The screen base address isn't dependent on DOS; it's dependent on
the display board you're using, and the BIOS which supports it.
It's the BIOS which usually generates the screen addresses that
are used.  There is no better way than (1) determining the
hardware which is installed; (2) finding out the display mode; and
(3) making a computation/lookup based on knowing how these two
pieces of information go together.

As an example, you might determine that (1) there's a CGA card
installed, and (2) it's in 80x25 color mode, on page 1.  Then
you'd "know" that the screen base address is B800:1000.  Now it
MIGHT actually be an AutoSwitch EGA card switched to Hercules mode
with base address (corresponding to the upper left corner of the
screen) B000:0000, but you can't determine that from software if
the configuration information available to your software is
incorrect.
-- 
{hplabs,amdahl,ames}!bnrmtv!perkins         --Henry Perkins

It is better never to have been born.  But who among us has such luck?
One in a million, perhaps.