SMS wires each button to a seperate pin on the connector in a variant on the Atari style controller port. Megadrive has a Master System backwards compatability

SMS wires each button to a seperate pin on the connector in a variant on the Atari style controller port. Megadrive has a Master System backwards compatability mode (and the same 9 pin port), so its controller ports are going to be similar 8 bit IO ports. It adds extra buttons using a spare line as a select / multiplex output and reading the input lines multiple times. Nintendo used shift regiters in its controllers, and the SNES does use a 16 bit shift register, but some of the bits are unused (4 extra buttons deemed sufficient it seems).