Extra credits 
Add 1 credit per each hour on game play 

Free play 
After 18 hours game play 

Game play hour is cumulative. It counts when you actually play the game. It does 
not count when you just wait on the title and demo screens.

You can check your game play hours on the Load/Save option.

It saves your game play hour when:
1.) You gameover.
2.) You reset the game by software reset.
3.) You goto the Pause Menu and exit to the title.

Full power up
Press Start to the Pause Menu and then enter the code.
Full power up cheat is cumulative. After you use one, it deducts from the total.

For example:
At the beginning (stage 1), you have only 1 full power up cheat. 
At stage 3, if you never use it, you have 3 full power up cheats.
At stage 5, you have 5 full power up cheats, etc.

Instant double shot power-up:
Up, up, down, down, left, right, left, right, L2, R2 
Gives you 1 Speed Up, Missile, Double, 4 Options & ? (Force Field) 

Instant laser shot power-up:
Up, up, down, down, left, right, left, right, L1, R1 
Gives you 1 Speed Up, Missile, Laser, 4 Options & ? (Force Field) 

Weapon edit mode:
Successfully complete the game. Press R1 in the weapon selection screen to turn 
on weapon edit mode. Press L1 to return to the normal weapon selection.