> Megadrive has 2 scroll planes A and B, with 4-bit 8x8 tiles from 4 palettes.
> Colour 0 is transparent. A tile can be selected high or low priority by a bit
> and can be flips x and y. All stuff I'm sure you know, but just to remind you
> ;-)

Is the character (pattern) data packed like graphics in the NES (i.e. one byte holds the lsbits of a tileline and other bytes hold other bits)? If so, you might want to consider unpacking the tile data into something more accessible (I got a 10% speed boost in my NES emulator by doing so). I managed to do this while still using the same amount of memory.