Path: utzoo!mnetor!uunet!husc6!mit-eddie!uw-beaver!cornell!batcomputer!sun!naughton
From: naughton@sun.soe.clarkson.edu (Patrick Naughton)
Newsgroups: comp.sys.ibm.pc
Subject: Re: EGA's memory
Message-ID: <453@sun.soe.clarkson.edu>
Date: 22 Feb 88 06:41:34 GMT
References: <2171@bsu-cs.UUCP>
Organization: Clarkson University, Potsdam, NY
Lines: 24

From article <2171@bsu-cs.UUCP>, by pervect@bsu-cs.UUCP (Barrett Kreiner):
> My card has the full 256k, and you start counting at $A000, that means
> that it runs to $DFFF.  This doesn't run true, because I have rom routines
> for my hard disk at $C800.  So how does one access the high 128K of the card?

All four planes are mapped by the EGA into the same 64K block from
$A000:0000 to $A000:FFFF.  The sequencer and graphics controller take
care of which planes are getting written to/read from.  This is what all
of the references to "latching" are all about.  when you do a "read"
from $A000:0000 the EGA actually "reads" 32 bits of data (one byte from
each of four planes) into internal RAM "latches" then the sequencer and
graphics controller can change this data (say...  AND it with something
and rotate it) then the next "write" to anywhere in the EGA map will put
all 32 bits into the planes.  All of this action may be enabled/disabled
with various incantations that I won't go into here... 

I hope this helped...

-Patrick	 ___________________________________________
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