During play press 'Space' to view the options screen. Move down to the "VIEW GAME SCREEN" option and push the joystick 'Left' and hit 'Fire'. The game screen should appear. Center the joystick and press 'Fire' again. The option screen should now reappear. Move down to the "SOUND ON/OFF" option and push the joystick 'Right' and hit 'Fire'. Now return to the "VIEW GAME SCREEN" option and push the joystick 'Right'. While holding the joystick in this direction, hold down the 'Right' mouse button and press the 'Left' mouse button. The option screen should change to cheat mode. Now go to "Resume Game" and press 'Fire'. You now have unlimited shields and can select any level on the options menu. If it doesn't work, repeat the whole procedure again by resuming the game and pressing 'Space' to go back to the options screen.