Activision was the first third-party Atari 2600 developer, but they didn't have to reverse-engineer the system from scratch -- they were all defectors from Atari. They did invent plenty of nifty tricks, though.<p>One podcast talks about Imagic, another VCS third-party developer: <i>One guy, Bruce Pederson, was amazing. Without any documentation whatsoever, he attached an in-circuit emulator to a VCS and reversed the entire video architecture - including the special registers that doubled or tripled the sprites along with the requisite code timing issues. Took him two days to do it. </i><p>Parker Brothers used existing carts to inform their efforts: <i>The 2600’s zany graphics chip was the key thing there. My first contribution was to write a disassembler for already-published cartridges. When analysis of the circuitry presented ideas for how the sprites and background worked, I wrote small test programs on the 2600 to verify our assumptions. With these combined efforts, we figured out how everything worked in a few months.</i><p>[1] <a href="http://2600gamebygamepodcast.blogspot.com/2014/02/q-with-imagics-michael-greene.html" rel="nofollow">http://2600gamebygamepodcast.blogspot.com/2014/02/q-with-ima...</a>
[2] <a href="http://www.retrogamer.net/retro_games80/the-making-of-star-wars-the-empire-strikes-back/" rel="nofollow">http://www.retrogamer.net/retro_games80/the-making-of-star-w...</a>