All source code here is reverse engineered from Atari original ROM data, and remains their copyright. I've been fascinated by Atari's Star Wars arcade machine since I first saw it in an arcade in October 1983

By 1987 I owned my own upright machine, and was curious how it all worked. I understood the basics of the hardware, CPU, ROM, RAM, etc and had a fairly good clue how the AVG vector generator worked too, thanks to having an Asteroids machine and spending many hours fixing it! But the mathbox part was new to me, and I had no idea how it actually worked.

By the early 1990's I'd even written a small piece of self taught 6809 assembler and got some vector lines showing to try to understand more. Loss of interest and lack of information prevented any further progress.

More recently I've recovered the interest and also found much more information and disassembly tools thanks to the Internet. Many years of embedded system programming have helped my understanding of software much more, and thanks to Jed Margolin (pretty much Atari's  "Mr Star Wars ") having his own website explaining much of the 3D maths, it seemed like time to dig deeper into the theory of operation of this wonderful arcade game.

I'm now to a point where I've taken the original ROM code and disassembled it into useable ASM source files that can be asembled back into working game code. By altering parts of the code and running in MAME, I've managed to work out how parts of the game work. I've also been slowly commenting the parts of the code that I think I understand.

Maximum 6 fireball slots
Maximum 3 tie fighter slots

3D object handling. Tie fighters and bunker/tower explosion fragments are handled differently from bunkers/towers/trench. Functions to convert object points to AVG vectors are different depending on object type. There is a table of data to determine which points need visible vector lines generating. There is another table of data for how many XYZ points are in each object.

Starting wave completion bonus. You can select Easy, Medium or Hard starting waves, with bonus points of nothing, 400,000 and 800,000. But the game code table also has bonus points of 200,000 and 600,000 for non-existant starting waves 2 and 4.

The large blue 'Star Wars' 3D-like text on the attract screen is stored as vector instructions in the main ROM code, and copied to vector RAM.


Hidden text:
DURFEY GOT WIRED
HALLY WAS THE WHIPCRACKER

The text above is not exact, the first character of each phrase is not correct, so the 'D' and 'H' of Durfey and Hally are hex C4 and 28 for some reason. Erik Durfey was the technician, Mike Hally was project leader.

 

Text is stored as ASCII, but the last character of a string has bit 7 set. There is a data table of pointers to text string start sddresses. There are data tables for text position and text colour, stored as vector instructions ready to copy into vector RAM.