Hints for disassembly of 6809 code

   The last 2 bytes are the reset vector.  This is where the CPU loads the
program counter on a reset.  Start tracing from the address stored here.

   Remember that a PUL instruction that pulls the PC off of a stack is in
effect performing a RTS.

   On Williams' games, $9000-BFFF is RAM and $C000-CFFF is I/O.  All other
locations are ROM, except in Sinistar, where $D000-DFFF is also RAM.

   I've started using lower-case labels for data, and upper-case for code.

   It will be interesting to compare and contrast different game code.  It
looks like Stargate and Robotron are very similar, as are Joust, Bubble
and Sinistar.  Of course, there are multiple versions of some of these
games, so we need to indicate this in the data files.  I've chosen to name
them by the color of the labels that originally covered the EPROMs:
Joust - Red, Yellow and Green ROMs
Robotron - Yellow/Orange and Blue ROMs
Stargate, Bubbles, Sinistar - I only have one ROM version.  If you have
others, please let me know.

   Also, as Defender banked switched many of the ROMs into the same
addresses, it is difficult to disassemble.  Watch for my article about
running Defender on a Joust/Robotron/Bubbles machine.

   As an aside, my Joust ROMs (as purchased in the machine) were very 
slightly different from another set of Green ROM images that I got.
It appears that some of the graphics might have been touched up.

   Another way to tell ROM images apart is the default highscore that is
listed when you clear the scores.  The red Joust ROM prints 107212, while
the yellow and green ROMs print 109102.  The yellow/orange Robotron prints
131682, while the blue ROM prints 151782.

   Look for Robert J Mical's name in the Sinistar code.  He went on to
write Intuition, the graphical user interface for the Amiga.