Memory mapped registers

Note: Most of the register names come from neogeo_spex.pdf

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. Only word access.

NeoGeo CD registers
NEO-MGA / LC8953 / LC98000 ?