Date   : Thu, 25 Aug 2011 15:54:19 +0100
From   : dl.harper@... (David Harper)
Subject: Tube ROM

On 25/8/11 15:05, Rick Murray wrote:
> As an aside, wasn't there a game (Elite?) that switched MODE partway 
> down the screen, like the exterior view was drawn in MODE 0 and the 
> cockpit at the bottom was drawn in MODE 2? On the same screen, at the 
> same time? I bet that was an interesting design! Best wishes, Rick. 
I did this for a couple of programs, and it was actually quite easy. You 
just had to swap between a couple of values of the ULA control register. 
You did the first swap on the V-sync interrupt, and allowed this also to 
set a VIA-timer interrupt which clicked in about half-way down the 
screen and which you used to swap back.

The actual border between the two halves wobbled a bit, so you had to 
have a blank band a few pixels high in which you did the swap, otherwise 
you got weird flickering at the changeover point.

It made complications in the screen memory map, so you couldn't do all 
the screen writes /via/ the MOS. Some had to be done by direct writes to 
screen memory.

Sorry, but I don't have the relevant programs any more.

David Harper