lfsr math in pitfall!

A few weeks ago I attended VCF West at the Computer History Museum in Mountain View. One of my favorite talks was Making Games in the 1980s by David Crane and Garry Kitchen about the unique constraints of programming for the Atari 2600. Crane mentioned one hack in particular from his game Pitfall! that inspired this blog post. Pitfall! is a platform game in which you control a character named Harry through a jungle filled with crocodiles, logs, ladders, and, of course, pits. One of its main innovations is allowing movement back and forth between different screens, creating the illusion of an enormous world (at least by the standards of the time). Each screen is unique, with different arrangements of obstacles and enemies. Even the patterns of the trees in the background aren’t static; you can see the difference if you look closely at the treelines of the two screenshots below: Each screen is represented by a single byte, with different bits controlling the objects to spawn, the layout…