When Ray Kassar was fired as CEO of Atari in 1983, James Morgan was hired to take his place. One of the changes Jim implemented was to put all projects on hold for 30 days while he reviewed everything that had been going on while Ray was in charge. The success of the 7800 was deemed high priority at the time and so most 2600 and 5200 projects were put on hold or outsourced.
Tod Frye (the games programmer) was pulled off SwordQuest: Airworld to work on Xevious instead and while the gameplay in the 2600 version is amazingly accurate, some corners still had to be cut. One issue Frye faced with the 2600 was that it only had one fire button. Tod got around this problem by allowing the fire button have a double function by acting as both the fire and bomb button. Pressing fire will shoot a shot and drop a bomb that slowly arcs to where the targeting sight is. The enemy AI also got a bit of a rework so they all act similiar instead of having distinct attack patterns.
The most impressive feat that Tod achieved was to implement a scrolling background as was seen in the arcade version. His background wasn't as detailed as it's arcade cousin but it was still a major accomplishment for the 2600. Not only that but Frye was able to have multiple enemies appear on the screen at once with little or no flicker. The only thing that flickers in the entire game is your ship which he cleverly made up of two missile graphics to keep more sprites free . This unique design also makes your ship almost twice as large as it was in the arcade (which also has the side offect of making it harder to dodge enemy shots).
Proto - Front
Proto - Top
Proto - Back
In game pictures and video will be added as soon as time allows!