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From: news@cit-vax.Caltech.Edu (Usenet netnews)
Newsgroups: comp.sys.ibm.pc
Subject: Re: problems with Hercules compatible card
Message-ID: <1549@cit-vax.Caltech.Edu>
Date: Wed, 21-Jan-87 14:52:53 EST
Article-I.D.: cit-vax.1549
Posted: Wed Jan 21 14:52:53 1987
Date-Received: Thu, 22-Jan-87 00:34:24 EST
References: <8701200750.AA26234@ucbvax.Berkeley.EDU> <1442@nicmad.UUCP>
Reply-To: tim@tomcat.UUCP (Tim Kay)
Distribution: na
Organization: California Institute of Technology
Lines: 28
Summary: hercules half mode graphics

Organization : California Institute of Technology
Keywords: 
From: tim@tomcat.Caltech.Edu (Tim Kay)
Path: tomcat!tim

In article <1442@nicmad.UUCP> brown@nicmad.UUCP (Mr. Video) writes:
>The Hercules card and the CGA card can exist together only if the Hercules
>card is NEVER used in graphics mode.  The graphics address of the Hercules
>card end up in the CGA addresssing area.  The program that is run to put
>the Hercules card into mono mode only is HGC HALF.
>
>You MUST remove the CGA card before Hercules graphics mode is used.

I believe this isn't right.  A screen full of Hercules graphics requires
720x348/8 = about 32K bytes of memory.  The card comes with 64K bytes or
TWO pages of graphics memory.  A program can double buffer; there is a
control bit that determines which page is displayed.  The first page
starts at 0xb0000, while the second starts at 0xb8000.  The latter address
also happens to be the address of the CGA.  If you have a CGA, you use
the HGC HALF command to disable the second page of memory.  You can still
do graphics, just no double buffering.  I don't know how many programs use
that second page of memory.  If yours doesn't, and you don't have a CGA,
you could use the memory as a small ram disk.

Timothy L. Kay				tim@csvax.caltech.edu
Department of Computer Science
Caltech, 256-80
Pasadena, CA  91125