Continue game play
Press Up(2), Down(2), Left, Right, Left, Right, Down, Up, Down, Up at the game over screen to continue up to nine times.
Extra options
Enable the "Sound test" code. Play sounds 7, 4, 3, 7, 4, 8, and 1, pressing 1 after each sound. Options to play the game as a jet, switch the vertical controls, and adjust the difficulty level will be available.
Sound test
Press Right, Left, Down, Up at the title screen.
