Asteroids DMACNT signal and vgen instruction set

Hi,
  I'm currently reimplementing the Atari Digital Vector Generator in a CPLD,
reverse engineering it as I go. I've reverse-engineered the DVG down to
microcode/state machine level, and have a documented instruction set listing
that covers everything except the "SVEC" instruction. I've even created a
state transition diagram for the DVG state machine. No guarantees on
accuracy, but I'm pretty sure it's correct.

  What I'd like to know is how the DMACNT signal is used. It appears to have
the same purpose as DMAGO (start the vector state machine running), and is
generated as the result of reading from or writing to address $3800 (IIRC).
Thing is, as near as I can tell, the Asteroids code never writes to 3800.
What the heck is DMACNT there for, if it's never used?

  Also, does anyone happen to have any info on the programming aspect of the
DVG? I'm trying to figure out how the SVEC instruction is used, and how its
parameters affect the size of the vector. What I would really like to find is
a few example SVECs and the matching VCTR instructions.

  I'm also trying to find out if instruction $00 does cause the DVG to halt
(as has been suggested), or if there are certain instruction parameter values
that will cause similar effects. As usual, this sort of thing would be easy
to find out if I had an Asteroids PCB to play with, but I haven't seen one
for sale in ages (well, not on this side of the 'Pond anyway).

  If anyone wants to take a look at my notes, drop me an email and I'll send
you the PDF. I don't want to release it too widely yet, because it's an
incomplete work-in-progress and I don't want to see it all over the 'Net
until I've at least had a chance to debug it.

Thanks,

-- 
Phil.                              | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk              | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/          | Panasonic CF-25 Mk.2 Toughbook
... Artificial Intelligence: The other guy's opinion.
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