martinpiper / bombjack Goto Github PK

Hello! I have small, personal project where I try to document Bomb Jack hardware using game disassembly. While some things seem to be obvious I've stumbled upon $9A00 register/address. Do you happen to know what it does / how it works?

I've noticed that game zerofills $9A00-$9AC8 at $015F address. Then only writes to $9A00 during VBLANK after sprite table at $9820 is updated. Routine at $0958 sorts sprites by size. 32*32 are placed at the beginning of the table and 16*16 ones at the end. Upon exit $9A00 is written with number of 32*32 sprites + 3.

While sprites count makes sense the rest is mystery. I've quickly checked Senjyo and Star Force but there's no sign of $9A00 being used there that way. Couldn't google any results either, so I was hoping maybe you know the details? :)