From: Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: X-mode
Date: Fri, 24 Jan 1997 20:27:03 +0000
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Don writes:
>I'm writing my first game in Djgpp and I'm trying to use Mode-X because 
>of the many advantages.  The only thing I don't like is the non-linear
>addressing.  Whenever you plot a pixel you have to select the bit-plane
>right?!  Okay I wrote a function to do that, but obviously it is
>somewhat slower than say a 13h function.  My biggest problem is
>blitting.  How can I blit in mode X without plotting one pixel at a
>time (as apposed to a say memcpy command to blit whole lines).  

Copy the image plane-by-plane, rather than pixel-by-pixel. Select plane
0, and then copy every fourth pixel from your source image to adjacent
bytes in video memory. Ditto for planes 1-3. You should be able to draw
pretty much anything with only four writes to the plane select register,
if you order the operations correctly.

>Basically I want a fast and easy way to copy a buffer containing an
>image to the video buffer in mode-X.

If your engine is based around blitting a buffer from system ram to
video memory, it would be much faster in mode 13h. Quite apart from
having to alter the plane registers, the planar video organisation
wreaks havoc with your bus transfer rate (you can't read/write adjacent
strips of four pixels with 32 bit instructions if they are scattered all
over the place), and most modern cards can accept data a lot faster in
linear modes (on my Matrox, blitting to a VBE 2.0 linear framebuffer
320x200 mode is nearly 20% faster than the same resolution in mode 13h,
and over twice as fast as in mode-X).

IMHO, mode-X only shines when you are doing all your rendering directly
into video memory, and you are able to make use of the write planes to
copy groups of four pixels simultaneously (ie. you are doing lots of
solid-color fills, or you can fit all your sprites in an offscreen area
of video RAM and copy them from there). It was essential on a 386 and
well worth using on a 486, but on a pentium you'll almost certainly get
better results from mode 13h, or best of all a VBE2.0 linear framebuffer
mode...

Of course that's just my opinion. If you want some others, pop over to
rec.games.programmer, where several hundred people will be delighted to
discuss the issue at great length (before getting sidetracked onto the
respective merits of MS-DOS and Win95, and whether PC's or console
systems make better gaming platforms :-)

/*
 *  Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
 *  Ghoti: 'gh' as in 'enough', 'o' as in 'women', and 'ti' as in 'nation'.
 */