> 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.