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From: perkins@bnrmtv.UUCP (Henry Perkins)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Graphics driver
Message-ID: <2436@bnrmtv.UUCP>
Date: Tue, 25-Aug-87 19:09:14 EDT
Article-I.D.: bnrmtv.2436
Posted: Tue Aug 25 19:09:14 1987
Date-Received: Fri, 28-Aug-87 00:47:05 EDT
References: <1096@ark.cs.vu.nl>
Organization: BNR Inc., Mountain View, California
Lines: 32
Summary: Direct writes to CGA memory are fastest

In article <1096@ark.cs.vu.nl>, lesmem@cs.vu.nl (Marco Lesmeister) writes:
> Could anyone tell me what the fastest way is to control the screen in, for
> instance games and simulators? I want to move a C64-sprite like figure
> as fast as possible across the screen in CGA-mode.

If you want to modify a CGA screen, the fastest way to do it
is with direct writes to CGA memory, located in B800:0000 to
B800:1F40 (even scan lines) and B800:2000 to B800:2F40 (odd
scan lines).  A device driver would be s l o w .

You're trying to do something that IBM really didn't provide
for.  There is no hardware support for sprites or anything
resembling them; you'll have to save the original screen
contents so you can restore things when the "sprite" moves,
and you'll need to do your own collision calculations.  You
can't even do color table animation, because the color
palettes are fixed with two non-intersecting sets of 3 colors
each.  Since each byte of memory represents 4 pixels, you'll
need to OR and AND with pixel masks for horizontal movement.
Vertical movement is harder, because you need to add or
subtract your screen memory offset depending on whether you're
in an even or odd row.

Good luck; you'll need it, plus lots of patience.

(I did the animation for Championship Golf, published by
Activision's GameStar division.)
-- 
{hplabs,amdahl,ames}!bnrmtv!perkins         --Henry Perkins

It is better never to have been born.  But who among us has such luck?
One in a million, perhaps.