Halley Wars (Game Gear)
Halley Wars is a sequel to the 1986 arcade shoot 'em up Halley's Comet. It was also previously released for the Famicom Disk System.
At the title screen, press Left, Left, Left, Left, Right, Right, Right, Right and then start the game. The menu will now have a level select and sound test added, plus settings for speed, weapon level, and lives.