Enable cheat codes, then set the code to MA and press Select. Change the code to SV and press Start. The following commands must be entered using Controller Two:

Up -- Boosts weapon power to maximum for 10 seconds
Down -- Toggles voice mode
Left -- Change gunlock type
Right -- Skip area and disable invincibility
Triangle -- Toggles invincibility
Circle -- Increases bombs
Square -- Change weapon type
X -- Increases weapon power-up time by 30 seconds