I used to develop for the classic Gameboy which used a serial cable to connect two units for multiplayer. The serial port could exchange exactly one byte per fr

I used to develop for the classic Gameboy which used a serial cable to connect two units for multiplayer. The serial port could exchange exactly one byte per frame, simultaneously in both directions. That was just enough to send the input state for the frame (U,D,L,R,A,B,Select,Start = 8 bits) from each device to the other. So, this sort of blind deterministic simulation was the only option. Identical hardware and predictable latency made this somewhat easier, but it was still devilishly tricky to get right. The simulation had to not be affected by which player is local and which is remote. That's a tall order for 8-bit assembly language on a 1MHz CPU.