Open your PDA, go to objectives.
Hold Left trigger then press B,A,X,Y

Collectors edition (Classic DOOM games) 
Enter during play 

Cheat                                      Effect 
-------------------------------------------------
Hold LEFT TRIGGER then Press (A) x4        GOD Mode 
Hold LEFT TRIGGER then Press (B) x4        All Weapons and Ammo 
Hold LEFT TRIGGER then press A,B,A,B       All keys, weapons, ammo 
Hold LEFT TRIGGER then Press (X) x4        Temp Invincibility 
Hold LEFT TRIGGER then Press (X) x3 (Y) x1 Berserk Mode 
Hold LEFT TRIGGER then Press (A) x2 (B) x2 End Level 
Hold LEFT TRIGGER then Press (B) x2 (A) x2 Give Chainsaw 
Hold LEFT TRIGGER then Press (X) x2 (Y) x2 Invisibility 
Hold LEFT TRIGGER then Press (Y) x3 (X) x1 Light Amplification Goggles 
Hold LEFT TRIGGER then Press (X) x1 (Y) x3 Radiation Suit 
Hold LEFT TRIGGER then Press (Y) x4        Show All Map