The Moon Landing Simulator for the HP-25 and HP-25c handheld programmable calculators was a simple RPN type-in program (macro) that simulated a lunar lander descent. The player controlled the timing and duration of landing thrusters with limited fuel. Each turn the thrust combined with gravity to determine the new descent speed of the module. The objective was to touch down on the lunar surface at or below a speed threshold. Running out of fuel or landing too fast caused the player to lose the game.
The game was published as type-in source code in the manual that was bundled with the calculator. The "user interface" was fairly cryptic, if simple.