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From: connery@bnrmtv.UUCP (Glenn Connery)
Newsgroups: comp.sys.ibm.pc
Subject: Re: screen base address
Message-ID: <2827@bnrmtv.UUCP>
Date: Fri, 16-Oct-87 12:41:20 EDT
Article-I.D.: bnrmtv.2827
Posted: Fri Oct 16 12:41:20 1987
Date-Received: Sun, 18-Oct-87 13:09:46 EDT
References: <965@cup.portal.com>
Organization: Bell Northern Research, Mtn. View, CA
Lines: 18

In article <965@cup.portal.com>, Steven_N_Jones@cup.portal.com writes:
>     Jim Frost suggests finding the display base address by asking BIOS
> for the display mode and then trying B000:0000 or B800:0000 depending
> on the result.  This won't work on a Compaq, though, because it uses
> a single offset for both the monochrome and color display modes.

Sorry, you are wrong.  If the Compaq did that then most software would not
run on it.  The Compaq dual mode monitors change the video memory address
when you switch modes, so that in 80 x 25 black and white the segment
address is B000 and in 640 x 200 graphics mode the segment address is
B800.  The method suggested (checking the mode to determine what screen
base address to use) is the one used by most software (in the absence of
a get_screen_address call which should have been added years ago) and
Compaq made their rep. on complete compatibility.
-- 

Glenn Connery, Bell Northern Research, Mountain View, CA
{hplabs,amdahl,3comvax}!bnrmtv!connery