Invincibility
Hold Down/ Left + I + II and press Run. Alternatively, hold Down + Select before the stage text appears. Release the buttons when your ship stops flashing to be invincible while your weapons are at level one.
Level select
Press Select at the title screen to enter the options menu. Highlight the "Exit" option, then press Down, II, II, Left, I, Right, Select. A sound and red background letters will confirm correct code entry.
