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From: james@raid.dell.com (James Van Artsdalen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Question on ROM shadowing
Message-ID: <2538@dell.dell.com>
Date: 6 Aug 89 00:31:48 GMT
References: <1989Aug1.111017.1094@cs.dal.ca>
Sender: news@dell.dell.com
Reply-To: james@raid.dell.com (James Van Artsdalen)
Distribution: na
Organization: Institute for Applied Cosmology
Lines: 27

In <1989Aug1.111017.1094@cs.dal.ca>, lane@cs.dal.ca (John Wright/Dr. Pat Lane) wrote:

> Even when the ROM is shadowed, is the video buffer not still kept on
> RAM chips on the video adapter?

Yes.  The video buffer is not shadowed, only the option ROM.  The idea
is that a large number of fetches are done to execute the code from
the option ROM to print each character.

> Does this not limit the speed of video processing, both because of
> the speed of the on-board RAM and because of having to go through the
> bus (especially if it's an 8-bit board - or does this even make any
> difference)?

Yes it limits performance.  But if you use shadowing and never
actually write to the video RAM, you'll never see changes on the CRT.
One could cache the video buffer I suppose, but this is difficult as
one would need to flush the video buffer fairly often (each time a
different video page is chosen for example, or in [EV]GA graphics
modes).

8 bit boards are in principle slower, but 16 bit boards are harder to
make work.  I don't know how much slower the 8 bit boards are in real
life.
-- 
James R. Van Artsdalen          james@raid.dell.com       "Live Free or Die"
DCC Corporation       9505 Arboretum Blvd Austin TX 78759       512-338-8789