This is the codes:
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IDKFA    - All guns, keys and full ammo and armour.

IDDQD    - God mode: Invincibility.

IDBEHOLD - Displays a menu to get rad suit, berserk light amp etc.
           Press the first letter to get that power-up.

IDCLEVxx - Skip to level xx where xx is a number from 01 to 32.

IDCLIP   - Walk through walls.

IDDT     - Type this a couple of times when viewing the map.
           Displays all walls, items and monsters.