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Playstation - Space Invaders cheats

Stage select
Go to the main menu, choose either 1 or 2 players and push the 
Circle instead of X to start to play. Go to the selection screen 
to choose any stage.

Enable classic mode and unlock credits
Activate the "Stage select" code, select stage 00 to play.
Finish stage 00 to unlock an ending, classic mode and the credits. 
To access classic mode, press Right while choosing the players 
number. Go to the options screen to watch the credits. 

5 shots
Pause the game play and press Down, Left, Circle, Down, Right, Right, 
Right on your controller.

9 lives
Pause the game play and press Right, Right, Right, Down, Circle, Left, 
Down on your controller.