To use the codes, first you have to activate them.
At the main title screen, type "CHEAT". Afterwards,
hold down the following keys at once during the 
game to receive their effects.
You can now use these codes:

Code              Effect
------------------------------
TAB+O           - Fire options.
TAB+R           - Rapid fire.
TAB+S           - Shield menu.
[Backspace]+H+S - Toggle Cheat Mode On/Off.