Emulating map circuitry!

Unexpectedly I got quite a bit of time to work on Xevious today and made the most of it.

Pretty good progress actually, as you can see from here...

Somewhere in Area 1

OK, maybe you can't see anything different, but there is some significant work behind it.

Yesterday's code rendered from a static dump of the background tile video RAM from MAME.

Today's code reads the map ROM data from an arbitrary point and copies it to video RAM. The interface to the MAP ROM data is (conceptually) the same as the arcade code - a pair of latches. The Neo Geo implementation emulates the glue logic between the 3 map ROMs on the PCB the same way MAME does.

I don't know if I'll keep it this way, but it reduces 64KB of map data down to 8KB.

Next step is to implement the scrolling, which won't be trivial with the Neo Geo sprites, but I've got a pretty good idea how to do it. I also vaguely recall reading an article on how to do it many years ago. Either way, I'm sure I'll work it out. But I wouldn't expect to have it finished for at least a few days.