Date: Thu, 5 May 94 13:36:32 EDT
From: arra AT dsd DOT camb DOT inmet DOT com (Arra Avakian)
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Video memory: Native and Windows DOS


I am using both C (djgpp) and Ada (djgpp port of DOS), and want to have
Ada or C code directly reference the PC character mode video memory,
normally at something like address 0xb8000.

When running under GO32, the documentation says the first 1M memory is
mapped 1:1, so it would appear that it would be possible to reference
0xb8000 directly, but this causes a GP protection. Further
documentation mentions that this area is (re?)mapped to 0xe00b8000, and
in fact, this address (0xe00b8000) works fine.

However, neither 0xe00b8000 nor 0xb8000 work when running the same
program in a DOS box under windows 3.11. I notice that some library
routines use dosmemput, dosmemget, or movedata, which I suspect switch
modes before accessing the "dos" memory. However, I want to directly
access the video memory without switching modes. I believe that an real
mode program running in a Windows DOS box is free to directly access
video memory, with suitable address mapping "magic" to make it work
even when the DOS box is not "full screen".

Is there a way for a djgpp-compiled C or GNAT-compiled Ada program to
directly access video memory in both native DOS AND also in a Windows
DOS box? Even a solution that required a different video memory address
in the two cases is acceptable, as long as the program can dynamically
determine the correct video address based on its operating
environment.

Arra

P.S. Please forgive me if this question is already answered in the
documentation.  I have looked for the answer, but I may have missed
it!

=====================
 arra AT inmet DOT com
 Arra Avakian
 Intermetrics, Inc.
 733 Concord Ave
 Cambridge, MA 02138
 (617)661-1840 x4500
=====================