I read this post by Krikzz in the Everdrive 64 Version 3.0 thread that got me thinking:
Sounds awsome Krikzz. Keep up the good work! Btw, what would be able to use the extra 64kb of rom memory? Potential homebrew?
It is not extra 64Kb, it means that actual max rom size for v2 is 64Mbyte minus 32Kb (if sram used), or 64Mbyte minus 128Kbyte (if flashram used). Whole 64Mbyte avaialble only for games with used eeprom or wihich have no save meory at all. V3 can use whole 64Mbyte of ROM memory with any type of saves
We have 3 Nintendo 64 games that are 64 MB: Conker's Bad Fur Day, Resident Evil 2, and Pokemon Stadium 2. Conker's Bad Fur Day uses EEP16K for saving though so it won't have any issues.
Resident Evil 2 saves with SRAM so the last 32 KB of the ROM get cut off. I took a look at the ROM in a hex editor and it seems the last 180 KB was all left blank by the developers so we have nothing to worry about with this game either.
Pokemon Stadium 2 on the other hand will get 128 KB cut off as it saves with FLASH. I took a look at the ROM and it seems only the last 72 KB are blank so we are losing a small piece of the ROM. This explains why the Pokemon images aren't working. It also explains why we were able to change the save-type to an incorrect one and then the Pokemon images would suddenly work again. Of course with an incorrect save-type selected you won't be able to save.
The solution is to convert the ROM to save some other way, EEP16K, or even SRAM is fine as it shouldn't cause any issues with only 32 KB missing. I wouldn't have any idea how to do that or even where to begin. It does look like Krikzz is going to have this game fixed for Everdrive Version 3.0, but I'd really like to see it working in our Version 2.0s.
Also regarding Battletanx not working, I dumped my copy of the game last night with the Z64. This new ROM worked just fine in emulator, but the Everdrive 64 won't play it. The trainer ROM definitely does work, but why? The Trainer simply added some cheat codes they didn't do any patching or fixing.