This game has a lot of passwords that will take you to various parts of the game, enter one of the following for a level select and press START afterwards to enter the level.

8547 (Level 1)
5431 (Level 2)
9892 (Level 3)
6315 (Level 4)
7452 (Level 5)
1697 (Level 6)
6425 (Level 7)

Enter password 0916 and press START to go to the Level Select Mode. Press UP or DOWN and to change level, though you won't really know which one you'll pick cause there's no indicator other than a beep.

SOUND TEST

But that is not all, there are also password codes for more unusual things such as a Sound Test, enter password 1965 and press START. Press B to toggle between sounds and then press A to play. If START is pressed again, you'll return to the title screen.

ENEMY VIEWER

Enter password 7777 and press START to go to the Enemy Viewer Mode. Press UP or DOWN and you'll see the enemy on the right side of the titlescreen change. If START is pressed again, you'll return to the title screen.

WEAPON SELECT

Enter password 8708 and press START to enable the Weapon Select. Another code can be entered after this one. In Man-Mode, PAUSE the game and press UP once for Gun 1, press UP twice for Gun 2. In Dino-Mode, PAUSE the game and press DOWN to select a weapon.

VIEW ENDING

Enter password 9425 and press START to go to the Ending of the game.