Doing mapper 69 (nice) instead, because Batman Return of the Joker is meant to be an awesome game.
Mapper 69 progress: Only the bottom half of the B in Batman is being displayed, and it's in the wrong position, and it's using the wrong palette, but it's better than the great big nothing that I had before.
Apparently there are some behavioural differences between certain FME-7 cartridges, depending on whether or not they used battery-backed RAM.
Success! (kind of)
A lot of the game is working properly, but there are IRQ timing issues, which makes sense because I just kinda hacked the IRQ counting in.
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