Cheat mode:

Submit one of the codes below at the name screen. REMEMBER: The
procedure to do this can be found on page 7 of the manual.

Letters A, B, C, D collected
Enter HANG_ON[Period].

Invincible to bullets
Enter SEGA[Period].

Letters A, B, C, D, collected and invincibility
Enter SPECIAL[Comma].

Three extra lives after time reaches zero
Enter GP_WORLD.

All cheat features
Enter DOKI_PEN.