[Coco] How register $FF9B works

[Paul T. Barton]

Robert,

I assumed that if any video page was
swapped, the currently visible
screen would also move. I guessed wrong,
apparently.

In any of the NoCan(x) series, $FF9B works
the same. The differences are how they
access the memory above the 2MB boundary.

Paul

--- Robert Gault wrote:
--snipped--
> That is not what you want to see but it may be
> correct. A stock Coco does not use $FF9B
> so that loop will do nothing.
--snipped--
> With extended memory and $FF9B toggling the
> memory/video extra bits, the 
> video/memory will not point where it should
> (ie. where Basic thinks it 
> should be) and as a result CLS will not clear
> the memory to which the 
> video lines are pointing. Either video will
> point correctly but memory 
> is swapped or both memory and video will be
> wrong. It depends on which 
> Barton extended memory board is in use.
> 
> I don't remember exactly how the latest board
> works regards $FF9B but 
> you want a program that will exercise $FF9B
> while ensuring that CLS 
> clears memory to which the video lines point.
> So if $FF9B changes the 
> video lines, you must also make sure that the
> correct block of memory is 
> used by CLS. That is a function of Super
> Extended Basis and the MMU 
> block numbers are hard coded at $E0E1-$E0F0.
> 
> In short, you will probably have to coordinate
> POKEs to $FF9B with POKEs 
> to the default MMU block numbers.

OK, I'll try to (again) figure it out,
and have a look at $E0E1-E0F0.

I'm home sick with the flu and when that
usually happens, all rational thought
processes are suspect. :)

Thanks,
Paul T Barton




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