Kisuishou Densetsu Astal

Untested:

Secret mode:
Press Left, Right, Left, Right, Up, Down, L, R, Start on second controller in the options screen.
Astal will shout and the phrase "Secret Mode" will appear at the top of the screen to confirm correct code entry.

99 lives:
Enter the secret mode code. Highlight the lives option and change it to 99.

Life refill:
Press Start to pause the game.
Quickly press Down, R, Up, L, X, A, Y, B, Z, C, Right, Left.

Invincibility:
Press Start to pause the game.
Quickly press Down, Y, Left, A, Down, B, Right, C.
Alternatively, pause and quickly press Up, Y, Left, A, Down, B, Right, C.

Suicide:
Press Start to pause the game.
Press L, A, R, C, B.
Press Start to return to the beginning of the current stage.

Stage select:
Enter the secret mode code.
Return to the title screen and press Up, Down, Left, Right, L, R, A, Y, C, Z, B, X.
A new stage select option will appear on the main menu.