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SIMCGA.DOC
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1987-04-10
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SIMCGA - Simulate CGA with Hercules Monochrome Card
Written in September 1986 by
Chuck Guzis
153 North Murphy Ave.
Sunnyvale, CA 94086
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,
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.
2. Since the image is expanded vertically by 1/2, 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.
Quite a few programs have been tested, including Flight Simulator, PC-CAD,
BASICA and some games with no hitches.
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 a SETCGA when you need the CGA feature.