Date   : Thu, 29 Sep 1994 13:47:36 WET DST
From   : Bonfield James <jkb@...>
Subject: Re: CRTC and ULA emulation

Chris Lam writes:

>regarding the emulation of the CRTC & ULA, i've only come across 2 programs
>which use the split screen mode, namely ELITE and REVS. does anyone know of
>any more?

Many games use split palette modes, but I can only think of the above two for
changing screen modes. Maybe Sphere of Destiny does. Speaking of S.O.D, do
people realise that the speed (it's a "trailblazer" clone) of game is simply
because the animation of the 3D trail is done purely using colour palette
switching? Amazing! Exile also uses colour palette hacking (for the water), as
do many other games I imagine.

>well, these two programs change screen modes in between whole character lines
>which makes life simpler. it seems to me that the only things these video
>interrupts do are

But the SOD and Exile both change colour palette on line boundaries, not
character boundaries. This makes things complex again.

> a) change the value of &FE20 and therefore the 'mode' itself
>
> b) change &FE21 a few times to redefine the colours.
>
>i see no reason why they should modify the CRTC registers.

It depends on precisely what they're changing. The mode in FE20 is simply the
colour control (ie bits per colour) isn't it? If the resolution changes (eg
mode 4 vs mode 2) CRTC registers will surely need modifying. I'd be suprised
if Elite didn't do this somewhere, although I can't remember the modes it uses.

>also at which point on the screen does the vertical sync interrupt occur?

The bottom I hope ;-) The sync interrupt ideally occurs just as the raster has
stopped drawing this frame, so that we've got enough time to splat a few
things on the screen (starting at the top to be careful) before the beam
starts scanning at the top again. This is simply my 'gut' intuition on this
matter though.

>comments?

Your scheme would have problems with SOD and Exile I expect. Still, it's
further than I've got :-)

       James