Path: utzoo!attcan!lsuc!maccs!cs4g6ag
From: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn)
Newsgroups: comp.sys.ibm.pc.programmer
Subject: Re: reading directly from screen
Message-ID: <26255B40.21913@maccs.dcss.mcmaster.ca>
Date: 13 Apr 90 04:53:19 GMT
References: <399@techbook.UUCP> <2724@unocss.unomaha.edu>
Organization: McMaster University, Hamilton, Ontario
Lines: 22

In article <2724@unocss.unomaha.edu> ho@fergvax.unl.edu writes:
$Trouble with this might be that on EGA systems, if I'm not mistaken, it
$is possible to move the "beginning" of display memory, such that it is
$no longer at segment 0xb800.

   In fact, on CGA and Hercules systems as well, there is no guarantee
that the current display page starts at segment B800 (CGA) or B000 (Herc).
These cards have the ability to maintain multiple text pages, one of which
is displayed at any time.  I've never met a system that was running with
the text screen anywhere other than B800 or B000, but I'm sure there
are some programs out there that leave the card using a different page.

   The moral of this story is that you should not assume that the
display page is at a given address ... instead, have your program
initially test several likely locations (start with B000 and B800,
and if these fail, try others nearby) by noting their original contents,
printing something, and seeing if they change accordingly.
-- 
               More half-baked ideas from the oven of:
****************************************************************************
Stephen M. Dunn                               cs4g6ag@maccs.dcss.mcmaster.ca
     <std_disclaimer.h> = "\nI'm only an undergraduate ... for now!\n";