Sega Master System / Mark III / Game Gear
The Game Gear has three face buttons: buttons 1 and 2 as in the Master System, and additionally a Start button.
When running in Game Gear mode, the Start button state may be read by inputting from port $00; bit 7 reflects its state, with active-low logic (
1 = not pressed,
0 = pressed).
When in Master System mode, the Start button acts identically to the Pause button.