Re: [stella] I'm back! :) ...

Subject: Re: [stella] I'm back! :) ...
From: Manuel Polik <cybergoth@xxxxxxxx>
Date: Mon, 07 Oct 2002 22:10:14 +0200
Hi Ben!

>OK, actually I sat down and thought it out, and I
>suppose it wouldn't be as hard as I originally
>thought...basically just treat the whole 40 column
>playfield as a giant register and just do ROLs and
>RORs to get the masking bit where I want it.  The fact
>that the PF0 bits are packed into one byte will be a
>minor inconvenience, but shouldn't be anything too
>I suppose I will try to implement this strategy first
>for the playfield drawing routine, since that's a big
>ROM hog right now...

I used a simple ANDing table to do it:

    .byte %01111111
    .byte %10111111
    .byte %11011111
    .byte %11101111
    .byte %11110111
    .byte %11111011
    .byte %11111101
    .byte %11111110

If you are on a reflected byte, you can EOR #$07 the 
index, so the same table always does the job.

Of course, RORing and ROLing the complete thing back and 
forth, plus a CLC inbetween can result in a very clever 
solution, too. Cycle consuming, but ROM saving.


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