Donkey Kong Country 2 and Open Bus

Summary

Donkey Kong Country 2 has a pretty well-known bug in the old SNES emulator ZSNES where some stages have spinning barrels that don’t work properly. One of the earliest pictured here, in the first stage of Krem Quay (third world): Barrel Bayou After you jump into the barrel, you’re supposed to be able to completely control its rotation by pressing left and right on the d-pad, with the barrel only rotating while you’re holding left or right. In ZSNES, this is horribly bugged. Tapping left or right makes the barrel spin forever in that direction, until you press the opposite direction…which simply makes it spin forever in the opposite direction. This is more than just annoying - it makes these stages significantly more difficult than the developers intended, since later on the spinning barrels show up over spikes and other hazards: Klobber Karnage This used to be somewhat documented in threads on the ZSNES forums, but those unfortunately seem to have gone offline since last I looked at them, and I can’t find the relevant threads indexed in the Wayback Machine. This bug is caused by ZSNES not emulating open bus behavior. I believe this was originally discovered by Anomie roughly two decades ago, who subsequently fixed the same bug in Snes9x. This original fix hardcoded the specific addresses to return the values that the game depends on rather than properly emulating open bus, but it fixed DKC2 and probably didn’t break anything else. The bug was never fixed in ZSNES, which is now a long abandoned project (last release in 2007). Purely out of curiosity, I wanted to dig into this a little more to figure out what exactly in the game code causes these barrels to spin forever in an emulator that doesn’t emulate open bus behavior. On older platforms like the SNES, reading from an invalid memory address usually does not crash the program. There are cases where accessing specific invalid addresses can cause the hardware to lock up, but I don’t believe this can happen on SNES. I...