Path: utzoo!utgpu!watmath!att!ucbvax!hplabs!hp-pcd!hpcvlx!everett
From: everett@hpcvlx.HP.COM (Everett Kaser)
Newsgroups: comp.sys.ibm.pc
Subject: Re: HELP:  with EGA/VGA page-to-page memory copy
Message-ID: <101000062@hpcvlx.HP.COM>
Date: 28 Jul 89 15:31:59 GMT
References: <1495@dukeac.UUCP>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 12

When writing to EGA/VGA memory, you have to do a read first, even though you
don't use the data, then do the write.  The read latches the current video RAM
data into internal latches in the video controller, then when you do the write,
the video controller can combine the two according to the current replacement
rule.  If you don't do the read first, your write data is getting combined with
whatever garbage is in the internal latches.  Hence, I don't think you can use
memcpy.  I could be all wet, but this is what I've found in the little bit
of EGA programming I've done.

Everett Kaser                   "Your thoughts create your reality."
!hplabs!hp-pcd!everett
everett%hpcvlx@hplabs.hp.com