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.