Re: [stella] VDELP problem

Subject: Re: [stella] VDELP problem
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Sat, 26 Jul 2003 14:55:08 +0200
> I have a problem with the damn VDELP.
> When use only VDELP0 all is just perfect!
> But after adding VDELP1 I have following troubles:
> P0 and P1 sprites are showed interlaced (when they are on the same
scanline). I have the feeling that VDELP0 and VDELP1 use the same additional
buffer register.

No, there are individual buffer registers for GRP0 and GRP1, but
how they are written to isn't explaind that well in the Stella
programmers guide.

They are actually four graphics registers for the players, GRP0-A,
GRP0-B, GRP1-A and GRP1-B. With the VDEL registers you only select
the A or the B register for display for the corresponding player.

When you do STA GRP0, the contents of the accumulator will be written
to the GRP0-A register. At the same time the contents of the GRP1-A
register is copied to the GRP1-B register. The same thing happens
when you do STA GRP1. The contents of the accumulator will be written
to GRP1-A, and the contents of GRP0-A will be copied to GRP0-B.

With this you can create a two line kernel where you update GRP0
in the first scanline and GRP1 in the second scanline. That way
you get a double line display resolution but a single line positioning
resolution. Depending on which of the VDEL registers you turn on
you can have either player start displaying in the first scanline
or in the second scanline.

Ciao, Eckhard Stolberg

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