[stella] Trackball coprocessor

[Glenn Saunders <cybpunks2@xxxxxxxxxxxxx>]

How hard do you think it would be to make a microcontroller that could poll 
a high-resolution trackball or roller controller FOR the 2600 so that the 
2600 would only need to poll it once per frame?  The microcontroller would 
return back all the data that was missed by not reading during the 
screen.  I'm just thinking that if there was a way to implement some kind 
of handshaking with such a microcontroller so that it passed back a 
direction and a delta since the last read, that you'd get the best possible 
results.  I'm pretty sure that's how most classic hardware does it, i.e. 
dedicated hardware that constantly polls the controller rather than the 
main CPU.  POKEY does this to simplify paddle reads for the Atari 8-bit, 
for instance.

With only 4 bits to work with you'd only be able to yield 8 pixels of 
movement maximum per frame for a driving controller.  That means a total of 
64 positions per 360' rotation instead of 16.  That would probably be 
enough for Tempest.  Maybe not quite enough for this one.

You'd only yield 4 with a trackball.  If you used both controller ports you 
could double that, but sacrifice 2-player simultaneous games.  Or if it 
were possible to spool the data back in two or more nibbles at a time then 
you could have it buffer as large a number as you wanted.

It would be really cool to play 2600 games with the precision of a Logitech 
Mouseman without big kernel sacrifices.  We could also go back and hack 
Missile Command, Centipede, Millipede, Gyruss, and Reactor...

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