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Asteroids
Get the Excalibur Ship, and Unlock classic Mode:
Enter the following codes on the "Password" screen.

Open Cheat Menu:
CHEATONX (In the middle of a game, press the SELECT button to bring up the menu. Press Up or Down to choose a level and Right or Left to choose a zone. Press A to toggle invulnerability.)

Get the Excalibur Ship:
PROJECTX

Unlock Classic Mode:
QRTREATR