Date   : Fri, 06 May 2005 16:21:45 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: Chuckie Egg - better patch (and some mysteries

Thomas Harte wrote (with his own unique brand of line wrap! :-)

> Yes, I meant the Electron version, having been unaware that it was a
> rewrite. I'd just assumed that the reason for the 1 colour sprites was
> easy BBC/Electron compatibility but I guess that isn't the case.

Electron version is in Mode 5 versus the Beeb's Mode 2.  Presumably for
video bus hogging reasons.  So certainly the sprite plotting routines will
be different, if nothing else.  You can almost see how the game evolved
just by dissecting it.  The sprite routine is the very first thing in the
code (at &1902) and uses all Basic-friendly zero page locations, so you can
imagine the author first put together the sprite code, tested it in Basic,
and built the game slowly around this.  I can imagine that, with a new Mode
5-friendly sprite routine in place, the rest of the game would just bolt
transparently into this.

Perhaps the sprites are monochrome just because they were ripped straight
from the Spectrum original, as well as them being easier to plot at any
pixel position this way?

> [...] it seemed very much
> as if the game uses the ROM text output functions to throw its
> sprites onto the screen.
>
> Two possibile conclusions occur. The first is that I was mistaken.
> This is very likely as I'm rubbish with other people's code.
>
> The second is that I was right and the developers just found that
> to be an expedient way of doing things. A variation may be that
> they found it to be a more speedy way of doing things Electronwise,
> what with ROM accesses being always clocked at 2Mhz but RAM
> in modes 4-6 only 1Mhz.

The Beeb version definitely has a home-grown sprite plotter which pretty
much mimics the action of VDU 5 printing, but allows sprites to be
arbitrary dimensions.  I don't think it's particularly optimal - it plots
pixel-by-pixel (instead of trying to be more 'clever' and writing a whole
byte at once).  It's quite possible that the Electron code uses OS routines
instead of plotting the sprites pixel-at-a-time in Mode 5... I guess the
only way to find out is to take a look...


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