Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!think!snorkelwacker!mit-eddie!bbn!granite!horvath
From: horvath@granite.cr.bull.com (John Horvath)
Newsgroups: comp.sys.ibm.pc
Subject: Re: CGA emulation on EGA???
Keywords: CGA emulation, EGA
Message-ID: <1989Dec11.211621.27725@granite.cr.bull.com>
Date: 11 Dec 89 21:16:21 GMT
References: <11747@phoenix.Princeton.EDU>
Reply-To: horvath@granite.UUCP (John Horvath)
Organization: Bull HN Information Systems Inc.
Lines: 34

> I'm looking for a CGA emulator for an EGA card/display on behalf of a
> friend, so his 4 year old daughter can play some games he has on his PC.

If you have EGA and BIOS, then you already have a CGA emulator
in the BIOS. Which works fine UNLESS the CGA program tries to set the
graphic mode by attempting to directly set the CGA's CRTC registers.
Its OK on CGA displays but when the program is run with EGA,
the CRTC registers aren't equivalent and get set to irrelevant values 
that screw up the EGA display mode. Fortunately, most CGA programs 
set the graphics mode using BIOS and are compatible with EGA and VGA.

You probably wouldn't have asked about this unless you tried running
the program and it bombed out. By chance is the CGA program your friend 
is trying to run, one of the Sesame Street games published by 
Hi Tech Expressions? I got one of these for my daughter and was unable
to run it on a VGA display. It would start up and then go into blizzard mode.
Using the Turbo debugger I found it was setting the CRTC registers directly 
instead of using BIOS. So I wrote a patch for it to use BIOS instead.
(while I was in there I also patched out that stupid song. No one should
be forced to listen to the theme to sesame street more than once a day. :{} ) 

I wrote twice to the distributer without any response,
so I guess they don't support this anymore. I can email the patches
for the "big bird's special delivery" game or specifics about how 
to find this code in other programs.


John Horvath
Internet: horvath@granite.cr.bull.com
mail:	Bull HN Information Systems Inc
        MA30-824A
        300 Concord Road
	Billerica, MA 01821
	USA