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1987-01-18
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SIMCGA - Simulate CGA with Hercules Monochrome Card
Written in October 1986 by
Chuck Guzis
153 North Murphy Ave.
Sunnyvale, CA 94086
First Edition, October 1986
First revision, January 1987
Suggestions and bug reports will be appreciated.
===========================================================================
NOTE TO USERS:
This document and SIMCGA, Copyright (C) 1987 by C. P. Guzis
All rights reserved.
SIMCGA may be freely copied and distributed for one-time personal
use. Under no circumstances may SIMCGA be sold or used for commer-
cial purposes without the express written consent of C. P. Guzis.
Further, C. P. Guzis provides this package on an "as-is" basis
and does not warrant the performance or incidental effects of
this program.
===========================================================================
This memory-resident utility allows you to "fool" most software requiring
a Color Graphics Adapter into using your Hercules (or compatible) mono-
chrome adapter in the graphics mode. Graphics images are reproduced
in normal aspect ratio, using as much of the available screen area as
is possible.
The trick used here is to program the HGC to display more lines of 3
lines per character time instead of 4 (The CGA displays 2). A service
routine hooked into the hardware timer interrupt (int 8) copies one line
to the third displayed line to give a filled-out image.
The method used here works and results in some useful color gray-scaling,
owing to incidental "dithering", but it does have a few minor drawbacks -
1. Since the copy operation occurs in real-time concurrent with the
display, some "ghosting" on moving images is inevitable. The
copy rate has been selected so that some CPU capacity will be
left for doing useful work. "Turbo" PC's and the PC/AT might
be able to copy larger pieces of the screen more quickly, but
this is a one-size-fits all program.
2. Since the image is expanded vertically by 50%, small line details
will be affected by the line doubling technique. This is mostly
an aesthetic annoyance.
3. Those utilities which "read" the CGA registers at 03d4 et seq.
are going to find no registers to read. This does not seem to
affect many programs.
4. 40x25 text mode is not supported - the hardware just isn't there.
This mode is mapped into 80x25 and again, doesn't seem to bother
many utilities.
5. When in text mode (modes 0, 1, 2 and 3), the normal monochrome
character attributes are not available - this is a problem with
the way the HGC handles the attribute memory map.
6. The multiiple text page feature as used by the CGA is not
supported with SIMCGA. A lack of hardware here is the problem,
but perhaps a software "swap" could be engineered to work. Thus
far, however, I have heard of no production code which uses this
feature.
7. Those utilities which bypass the BIOS and write the 6845 registers
directly to obtain certain effects will not work because of the
same reason as number 3 above - the registers do not exist!
8. Trying to use this program with a real CGA card also installed
gives very strange results owing to the contention for the same
memory space between the HGC and CGA.
9. A "native" HGC graphics program which programs the HGC registers
directly may leave you in text mode with no text displayed! This
is because the HGC was left displaying text from its second page,
while the BIOS is busily writing in the first page, or vice versa.
Use "SETMONO" before running "native" HGC programs, or, if you
forgot, run "SETMONO" afterwards and things will be put right again.
10. The second edition of this program allows a clean Code-Alt-Del
reboot.
11. In text modes, the copying operation is inactive, so your PC will
run full-speed while displaying text.
Quite a few programs have been tested, including Flight Simulator, PC-CAD,
BASICA and some games with no hitches. Yes, for the first time, you can
run *all* the games that came with your PC-DOS on the supplemental programs
diskette.
This package contains three programs - SIMCGA, SETCGA, SETMONO. SIMCGA
is the memory-resident part and must be run before the CGA simulation
feature is available. SETMONO will restore the monochrome-only mode;
SETCGA will reset CGA compatibility after a SETMONO. If this feature
is used routinely, you might want to put this in your AUTOEXEC.BAT file -
SIMCGA
SETMONO
and then run SETCGA when you need the CGA feature.
------------------
Some of the product names used in this document are probably registered
trademarks of IBM, Hercules or Microsoft or some other vendor.