Date   : Tue, 26 Sep 2000 12:45:20 +0100
From   : Tom Seddon <T.W.Seddon@...>
Subject: Re: Font Implosion / Explosions?

>===== Original Message From Thomas Harte <t.harte@...> ====
>No, no, I'll explain why I'm asking properly. On my little Electron emulator
>I'm now getting good results from bit multiplexing (where the '6502' is
>tricked into handling extra bits with the effect that you can, for example,
>redraw the Repton graphics in 256 colours) with the exception of anything
>that is passed through the ROM implosion / explosion parts, because the data
>there is calculated and not pre-stored so that someone can just draw over
>them. I put no thought into this because I assumed only the OS would be
>doing anything, but now that I've found some games which do I've been
>wondering how it works to see if anything can be done. Because I really
>wanted someone to redraw Chuckie Egg!

How does this bit multiplexing work?

The chapter in the BBC Micro AUG which gives a memory map of the Beeb has a 
brief description of tables inthe OS rom which are used to convert 1bpp data 
(the BBC font, lives at &C000 on OS 1.20) into 2bpp (mode 1/mode 5) and 4bpp 
(mode 2), and other tables for plotting single pixels in the different modes. 
I seem to remember it's just a set of bit operations, with perhaps a bit of 
shifting. You can spot some games which use it because they only ahve 1 colour 
sprites.

I can't remember what the technique is exactly, but from memory you prepare a 
byte with all set pixels set to col. 3 or col. 15 (all bits set), then AND it 
with a mask for each pixel to set each pixel correctly. Last I used this 
technique was about 8 years ago now so don't quote me though; and I've 
recently moved, so I don't have my AUG handy, not that I take it to work 
anyway :-| But if you download it you should find the information you're 
looking for.

It sounds like this is what you mean, rather than the placing of UDGs in 
locations other than page &C (which is what the Beeb means by font implosion 
and explosion)... ???

--Tom

-- 
--Tom
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| T.W.Seddon@...        | My other life is 3rd person perspective |
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