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From: hd7x@vax5.cit.cornell.edu
Newsgroups: comp.sys.ibm.pc
Subject: Re: what display combinations can coexist in a PC?
Message-ID: <3871.26250e21@vax5.cit.cornell.edu>
Date: 13 Apr 90 03:24:17 GMT
References: <28045@ut-emx.UUCP>
Distribution: comp
Lines: 24

In article <28045@ut-emx.UUCP>, keshava@walt.cc.utexas.edu (Sanjay Keshava) 
writes:
> Does anyone have a summary on which combinations of display adapters
> can coexist in a PC?  In particular, I would like 2 VGA cards
> displaying different screens, eventually one will be upgraded to a
> TIGA board.  Simultaneous display would be great, but I can live with
> a soft-switch to toggle between displays.  Suggestions of other
> combinations (eg CGA-VGA, Mono-VGA, Herc-EGA, etc) are also welcome.

Basically, you can only have one monochrome adapter and one color adapter. 
This is because all IBM color adapters use the same memory location for text
display, and there would be address conflicts.  But monochrome adapters use
different addresses.  In short, you can have Herc simultaneously with
CGA,EGA,or VGA, but you cannot have CGA-VGA or EGA-VGA, for example.  In
addition, if you go with Herc+IBM standard, you must make sure to set HGC HALF
to avoid address conflicts.

But, there is one exception to this rule that will allow your dream of two VGA
displays.  On an IBM PS/2, you CAN have the internal VGA coexist with an IBM
8514/A adapter and monitor.  For this to work, one of the displays has to be
using one of the 8514/A's advanced modes (1024x768) while the other displays
standard VGA (640x480).

-Sanjay Aiyagari (HD7X@vax5.cit.cornell.edu)