Date   : Sat, 09 Sep 2000 16:48:54 +0100
From   : "Tom Seddon" <T.W.Seddon@...>
Subject: Re: FRAK!

Richard Talbot-Watkins wrote:
> the right things, and as for Firetrack... HOW DID HE DO THAT SCROLLING!!!!

I had a bit of a go at reproducing the effect myself the other day. As you
found, big changes in R5 seem to make the screen jump a bit. On my emulator,
R5 moves the screen up and down (0 is normal position, 10 (the maximum) is
ten scanliens down; the timing for the frame is unaffected), and Firetrack
works fine. It would appear that it sets R5 to 0, 1, 2... 7, then back to 0
at which point the screen is scrolled using R12 and R13. Because the
blanking of the top and bottom few lines doesn't work under the emulator,
it's also possible to see that only one character line is drawn in advance;
there's nothing funny like drawing a few in advance and scrolling through
all of them.

On my real Beeb, however (again, as you too found) R5 appears to move the
screen in half scanline increments -- it appears this is the case whether in
interlace mode or not, as well.

My Beeb is not set up right now, but I'm going to have more of a play over
the next couple of days. Does anybody have any more suggestions or tips? And
as a question of my own, does anybody know whether changing R12 and R13
mid-frame should work? I can't tell whether the problem is my code or the
6845; I presume it's a 6845 limitation, because palette and mode changes
work as expected, but I could be wrong...

--
--Tom
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