Date   : Tue, 30 Jan 1996 11:43:29 GMT
From   : lamcw <lamcw@...>
Subject: Re: Vertical blank interrupt

Robin.Watts@... Mon Jan 29 14:10 GMT 1996 wrote:

;If I read this right, you actually emulate the changing of the palette
;at a point partway down the screen by doing it.

;We cheat and have "patches" for the specific modes.

;How do you know when to trigger the interrupt? I'd love to know so we
;can think about improving 6502Em.


That's right, I store PALETTE values in a table as execution progresses per
frame. I also store some video ULA numbers. The trick is to 'align' the
table with the screen. For that you need to know something about when
the VBI takes places.

I looked at my code last night but didn't understand it fully (it has been
over a year since I coded it). It does something with R7 of the CRTC.

For Elite you'll need the ULA numbers to know when to switch screen modes
(the upper part is mode 3 and the lower bit is 1... I think).

Cheers,

Chris Lam,
MacBeebEm Keeper.