[Coco] real-time mouse cursor

Arthur Flexser flexser at fiu.edu
Sat Jan 26 22:54:55 EST 2008

On Sat, 26 Jan 2008, Robert Gault wrote:

> Roger Taylor wrote:
> > Hopefully others are paying attention to your tutes so they might can 
> > dive into something graphics intensive.  I've known for ages most of 
> > what you're saying but the part about letting IRQ decide when to switch 
> > the screen and having the main code not accidentally get mixed up with 
> > the current state is something that carries a little bit of worry since 
> > IRQ can happen at any time during the main code execution, even if 
> > predictable at 60hz.
> > 
> > I mostly use the following VSYNC subroutine in all of my screen updating 
> > stuff.
> > 
> > VSYNC   TST     $FF02
> > a@      TST     $FF03
> >         BPL     a@
> >         RTS
> > 
> That's the way to use the PIA interrupt but as time is crucial, I'd 
> suggest in-line code rather than a subroutine. That will remove the 
> cycles needed to BSR or JSR and RTS.

Or better yet, replace the TST $FF03, BPL a@ with a SYNC instruction.
(Of course, time isn't really crucial if you're spending the bulk of it doing
nothing while waiting for the next interrupt.)


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