Message-ID:	<322F8464.5CF0@gbrmpa.gov.au>
Date:	Fri, 06 Sep 1996 09:54:44 +0800
From:	Leath Muller <leathm AT gbrmpa DOT gov DOT au>
Reply-To:	leathm AT gbrmpa DOT gov DOT au
MIME-Version:	1.0
To:	Philip Hodder <phodder AT pncl DOT co DOT uk>
CC:	djgpp AT delorie DOT com
Subject:	Re: Need help: Allegro & UniVBE
References:	<322D21F1 DOT 46A5 AT ohio-state DOT edu> <50nh66$k6t AT tube DOT news DOT pipex DOT net>

> I believe that's mentioned in the ALLEGRO.TXT file. However it does say -
> oh, hell, I'll cut and paste! Here 'tis:
> 
>    Mode-X scrolling is reliable and will work on any card. Unfortunately a
>    lot of VESA implementations (including UniVBE!) can only handle
>    horizontal scrolling in four pixel increments, so smooth horizontal
>    panning is impossible. This is a shame, but I can't see any way round it.

> Doesn't say WHY exactely - I'd guess it's how UniVBE deals with memory.
> Vertical scrolling should be OK (?)

I thought that it was because it only worked with 4 byte offsets (well,
that
might be obvious... :) because of the way SVGA graphics cards receive
info.
off the bus (generally PCI). I am very vague on this, but I am pretty
sure
this has something to do with it.

If you test the speed rates of a gfx card in VGA and SVGA on faster CPU
machines,
(assuming that you are using a linear framebuffer, and something like
Univbe)
you will notice that VGA peeks at about say 34 Mb/sec, whereas SVGA can
get
say 44 Mb / sec. If you speed test this on a slower machine (say my P166
verses
our other two P100's) you will notice that SVGA will slow down, but VGA
won't.

There is a workaround though: Use 640x480 in 32 bit colour... ;)

Leathal.

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