[Coco] Update on new Coco 3 game engine

[John Kowalski]

On Wed Aug 14 00:26:53 EDT 2013, Richard Goedeken
<Richard at fascinationsoftware.com>wrote:

>Dividing the horizontal clock by 2 is a really good idea. Making this work on
>all Coco 3s could be tricky, because of the 2 different GIME chips. Is there
>any easy way to tell them apart? If not, I guess you have to run a timing
>loop with some heuristic to figure out which one you're on.

There are two things you can do:
A) In the 60hz IRQ, set up and enable the timer to trigger FIRQs.  In
the FIRQ, advance a counter.  In the next 60hz IRQ, disable the
timer/FIRQ again and check how many times the counter advanced.  The
count will be different from both '86 and '87 GIME ('87 will give a
higher count value than '86)

B) Use the 279.365 nsec source clock for the timer instead of the
63.695 usec source clock.  The timer frequency will still be off by
one count depending on GIME, but since you have to set the countdown
to a high value to set your sample rate, the audible difference
between the two will be imperceptible.

-- 
John Kowalski / Sock Master
http://users.aei.ca/twilight/sock