You have five main buttons in Descent if you manage to map all movement to directional inputs: Shoot gun, launch missile, shoot flare, change gun, change missile (you can complicate those by allowing cycling in multiple directions). Okay, there's also the map screen. Descent 2 added a lot of stuff like the after burner, headlight, guide bot, energy transfer, etc but IIRC none of that was in Descent 1.