Re: [stella] VDEL...

Subject: Re: [stella] VDEL...
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Sat, 23 Feb 2002 15:47:34 +0100
> Uhm... there is something done in Combat with VDEL you 
> can't do quicker & shorter without?!?

I don't know the Combat kernel too well, so I can't really
say. But if you want to do a two line kernel with single
line positioning freedeom, using VDEL for one of the objects
frees you from some of the timing constraints, which probably
was a good thing during the early days of VCS programming when
everyone was still learning. Otherwise the code for updating
the two players, missiles and one playfield register would
have to fit into the HBLANK. With VDEL you can update one
of the players at anytime you want during the two scanlines.

> Set aside all loads, what would prevent you from doing
> without VDEL?!?
> Actually, if done without VDEL, you save bytes & cycles 
> both in- and outside the kernel...

If you are using the stack, you would have to save and restore
the stack pointer, which would require one extra byte in the
RAM. And I don't think the timing with the TSX would work
properly. All attempts at this method that I have seen were
always off by one or two pixels for one of the writes. This
isn't much of a problem for a score display, since the digits
usually don't use all 8 pixels anyway, but a 48 pixels graphics
display doesn't seem possible with this trick.

> - Just imagine what cool stuff could've been done if 
> VDEL would delay any GRPX write exactly until the next 
> line! 

I can imagine a lot if interesting effects that would be
possible, if the VCS had different hardware. But then it
would be less of a challenge and less fun to program for,
don't you think? ;-)

Ciao, Eckhard Stolberg

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