Re: [stella] playfield on-the-fly updates

Subject: Re: [stella] playfield on-the-fly updates
From: emooney@xxxxxxxxxxxxxxxx (Erik Mooney)
Date: Tue, 05 May 1998 02:33:08 GMT
>Before I reinvent the wheel, can anybody comment on the feasibility of
>producing a non-repeated playfield by writing PF0, PF1 and PF2 chasing the
>scanning beam before data required for the repeated section?  Of course,
>there would be two lots of writes required per scan line, but I'm interested
>to know if its been done.  What I'm asking, I guess, is when it comes to
>drawing the right half of the PF, does the hardware re-read the PF
>registers, or is it buffered internally somehow?

It's perfectly feasible, and used in dozens of games... check out Centipede
and Millipede for a couple very obvious examples.  The hardware re-reads
the PF registers when it comes time to reuse them.  There aren't any tricks
or standard routines to help you out, but I can tell you that the second
write to PF0 must finish between cycles 28 and 49 (that is, the first copy
of PF0 ends during cycle 28, and the second begins during cycle 49); the
second write to PF1 must finish between cycles 38 and 54, and the magic
numbers for PF2 are 49 and 65 cycles.

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