Memory mapped registers

I/O registers
Always read/written as bytes (?)

System registers
Handled by a 74HC259 adressable latch on cart systems. Byte writes only.

Video registers
Handled by the GPUs. Byte writes only work on odd addresses and store the same data in the MSB and LSB.

NeoGeo CD registers
NEO-MGA / LC8953 / LC98000 ?