Regarding the interface:
Port $80 is used to send a value to select a register. Port $81 is for associated data; both support reading and writing. Similar to the operation of integrated circuits such as the PSG or a VDP, it allows sending many values with a minimal interface.
This is how '''KachorroSoft''' created the incredible colour ports of '''AMC''' and '''MadMixGame''' with graphics from the CPC; '''Phantis''' from MS-DOS CGA and CPC; and '''Hundra''' from the ZX Spectrum.
* PCW Plus enables fantastic '''loading screens''' never seen before on 8-bit systems.