Nine continues:
Press X, A, L, R, Left, then hold L and press X, C, Z, A, Right(2) at the game start screen. A sound will confirm correct code entry.

Additional difficulty levels:
Hold X and press Z, C, L, B, Left, R, L at the game start screen. A sound will confirm correct code entry. Very easy and abnormal difficulty levels will be selectable options.

Rapid autofire:
Hold B and press Y, Right, Left, X, Z, L, R at the game start screen. The sound of an explosion will confirm correct code entry.