From:	"John M. Aldrich" <fighteer AT cs DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Urgent: memory protection and graphics
Date:	Thu, 20 Nov 1997 18:17:15 +0000
Organization:	Two pounds of chaos and a pinch of salt
Lines:	42
Message-ID:	<34747EAB.47A5@cs.com>
References:	<b127cd$17135 DOT 33c AT news DOT alberni DOT net>
Reply-To:	fighteer AT cs DOT com
NNTP-Posting-Host:	ppp220.cs.com
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

wolfman AT cedar DOT alberni DOT net wrote:
> 
> I am writing because I am working on a graphics library and I want to shut of memory
> protection so that I can write to the video buffer like: video_buffer[offset]=color; as
> well as for speed and for compatability with Watcom.  The problem is that I was working
> with line drawing and I was switching in and out of memory protection within the line
> drawing function.  I was disatisfied with the speed, so I descided to switch in and out
> of memory protection outside the function.  My line drawing speed went like a basillion
> times faster!  I am very tempted to shut off memory protection when the graphics mode is
> set and to turn it back on when the program exits.  Is there any proplem with this?  The
> faq and documentation is very vague about the subject.  I desperately need the speed
> increase.  What should I do?

Memory protection is there because, without it, you can easily hose your
computer's internals with a rogue pointer, leading to anything from
crashes to hard lockups to damaged devices (such as a SCSI drive mapped
into conventional memory).

The way many people have suggested to solve the problem is to develop
your code with near pointers enabled in as small a portion as possible,
and only enable them globally once you've gone through the major testing
phases and you are sure that your code is relatively bug-free.  You can
also use code optimizations like double-buffering, where you "draw" to a
buffer in protected mode memory, and only disable memory protection when
you want to blit each frame to the screen.  This should significantly
reduce the overhead from disabling/enabling, without as much risk to
your system.  Double-buffering is substantially faster and "cleaner"
than single-buffering in any case.

I'm sure other people can come up with even more possibilities.  :)

hth

-- 
John M. Aldrich, aka Fighteer I <fighteer AT cs DOT com>

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