Pause the game and enter the following codes for the desired effect.

Effect	Code
4 more enemies killed	Right, Left, Right, Left, A, A, B, B.
Free bag of dollars	Right, Left, A, Right, Left, B, Right, Left.
Invulnerable	A, B, C, A, B, C, A, A.
Return to the first level	A, A, B, B, C, C, A, B.
Skip level	A, A, A, A, B, B, B, B.
Unknown	Right, Left, Right, Left, A, A, C, C.