Date   : Mon, 19 Aug 2002 11:13:25 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: Firetrack emulation again

Michael Foot <mjfoot@...> wrote:

> Thanks for the summary. I thought there may be something
> hardwired like that. However, it does not help with my initial
> point 2 with regards to FireTrack because it will still display
> blank lines when R9 goes from &7 to &F. Maybe my timing is
> wrong or something?

I was going to reply to your original post but then realised I
had no conclusive answers :(

What Firetrack appears to be doing is rather different to what
I'd imagined (and a very different method to my demo vertical
scroller).  It seems to have only 1 CRTC cycle per frame, and
it appears to ensure that each frame contains exactly 312
scanlines by:

a) Setting R5 to an amount 'n' which causes the vertical screen
   displacement to move by pixel amounts, thereby creating the
   vertical scrolling;

b) Setting R4 to be one character row less than normal, i.e.
   38 rows (304 scanlines) instead of 39 rows (312 scanlines);

c) Setting R9 to '15-n' for *one character row only* in order to
   make up the missing scanlines.

What it is also doing is using R8 to turn on/off the display
at line '8-n' of the displayed visible screen (which of course,
due to the offset caused by R5, is always at the same raster
line) so that we have a constant "top of visible screen" position.
(This is of course a much better method than the palette blanking
shenanigans that I was doing in my demo... doh).

I don't understand why R8 is only remaining 'screen enabled' for
90 scanlines, nor do I understand why you should be seeing the
change in R9 from 7 to 15 happening during visible screen refresh -
I think this should *always* be happening "offscreen" because
setting R9 to 15 will definitely result in 8 blank scanlines....

Very curious.... sounds like timing issues, but it's difficult
to interpret your results without knowing which moment in the CRTC
cycle you consider to be "VS" and the model you're using to
emulate your screen refresh... having said that, if it seems to get
my demo and Uridium working fine, there can't be too much wrong...

If I think of anything, I'll let you know...

Rich




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