Re: [AL] VESA2

[William F B L. <zh...@nt...>]

----- Original Message -----
From: "J.P. Morris" <jp...@it...>
To: <all...@li...>
Sent: Sunday, July 27, 2003 2:11 PM
Subject: Re: [AL] VESA2


> On Sun, 27 Jul 2003 11:26:37 +0100
> "William F B Labbett" <zh...@nt...> wrote:
>
> > Hi All Allegro People!
> >
> > GOOD NEWS!
>
> Out of interest, why was this formatted like a poem, with 40 column lines?
> Was it written on a BBC Micro?

aha! the cheek! i think reasons can be complicated, plz forgive the
seriousness :)
I *think* the reason is psychological. I could be mistaken but i think it's
do with
what i, in my more reflective moments would have say is an irrational fear;
i think
i'm not in control of things if i leave it up to the computer or the program
to move
the cursor to the next line, so i hit return, usually unnecessarily early.
 I said reasons can be complicated because it must also be because i rarely
full screen
the window before writing mail so i write them in the little window you get
which makes things worse of course. I often find myself quite unrelaxed at
the computer
so i'm often unaware that i'm doing things in a silly fashion.

 I'm not sure it gets any deeper than that :)


> >  It's about VESA2...
> >
> > Does anyone know much about it?
> >
> > I'm mostly wondering what it
> > has that VESA doesn't and
> > also about things like
> > the ratio between people out
> > there who play games who
> > have cards that support it
> > and people out there who
> > don't.
>
> Before VESA, all videocards had to be programmed differently to make them
do
> anything above VGA spec (640x480x16).  Obviously, this sucked because you
could
> only develop for the cards that you happened to posess.
>
> The VESA standard defines a set of commands that are implemented on all
video
> cards (except a few really modern ones that are Windows-only), so if you
use
> those commands to do everything, the program should work on any decent
video
> card.  There is a little contention, however, about which modes are
available
> (for instance, 320x200x15, 320x200x16, 320x200x24 etc).  If you stick with
> 640x480 or 800x600, there shouldn't be any problem.

someone told me 1024 by 768 was a standard 8-bit VESA res
is that right?

> The trouble is, that when VESA was defined, everything was done in real
mode,

what is real mode plz?


> so there was a 64k limit on the amount of data that could be sent to the
video
> card.  This is enough to get you 320x200, which is covered quite
adequately
> without needing VESA.
>
> The solution is bank-switching.  You set bank 0, send the first 64k, then
> select back 1, send the next 64k until the screen is completely done.
> That's VESA 1.
>
>
> Time passed, and people started working in flat protected mode instead,
> with a limit of around 4194304k instead.

That's a massive difference!


  Using VESA 1 is pretty nasty,
> since you usually have to switch back to real mode or V86 mode to run the
> VESA command, which is (A) slow, and (B) dangerous.
>
> VESA 2 was designed, and it introduced the linear framebuffer, which is
now
> a staple requirement of all video systems.  With VESA 2, you only need to
> call the VESA commands at the start, to choose the video mode.  When you
do this,
> it redirects some of the higher memory addresses (often around the 2GB
area,
> where there isn't likely to be any physical memory in a DOS system) so
they
> point to the video display instead.
>
> That gives you the ability to write directly into the display area without
> having to copy the screen there in 64k chunks.  This is a lot faster and
> incredibly more convenient to do.
>
>
> Of course, Allegro (and any other graphics library) hides all this stuff
from
> you anyway, so it doesn't really matter unless you're writing the graphics
> library yourself (as I did before switching to Allegro for better
cross-platform
> support).
>
> >
> >  I don't want to write a
> > game and find out that only
> > a handful of people can use it.
>
> Why should this happen?  GFX_AUTODETECT should choose a driver that works.
> If you do have detection problems, add a switch on the command line to let
the
> user select a driver themselves.  Or some kind of INI file.
>
> >
> > Plz say as much as you can.
> >
>
> Was that enough? :)


 That was lots but as usual when people explain things to me it often
arouses lots more questions in me :)

thx





> >
> > will
> > _________________________________
> > 3^9*7^3*11^3*17^3*41^3*47^3*443^3*
> > 499^3*3583^3
> > The smallest cube whose sum of divisors
> > is also a perfect cube.
> > :)
> >
>
> --
> JP Morris - aka DOUG the Eagle (Dragon) -=UDIC=-  jp...@it...
> Fun things to do with the Ultima games            http://www.it-he.org
> Reign of the Just - An Ultima clone   http://rotj.it-he.org
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