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The acronym IWAD is generally interpreted as "Internal WAD" and refers to a WAD file which contains all of the game data for a complete game. The first four bytes of an IWAD file are the ASCII characters, "IWAD".
A complete IWAD file usually contains the following:
- Levels for each level in the game
- PLAYPAL, COLORMAP and ENDOOM lumps
- TEXTURE and PNAMES lumps for the game's texture information
- A complete set of sounds
- A complete set of music lumps
- Graphics for the menu, status bar, heads-up display, font, intermission and finale screens
- A complete set of sprites
- A complete set of patches (matching those found in PNAMES)
- A complete set of floor textures (flats)
When the game executable starts, one of the first things it does is attempt to locate an IWAD file. This is done based on name; the executable has the names of the Doom, Heretic or Hexen IWAD files stored in an internal list. Valid names are:
- DOOM1.WAD (Shareware Doom)
- DOOM.WAD (Registered Doom or The Ultimate Doom)
- DOOM2.WAD (Doom II)
- TNT.WAD (Final Doom - TNT: Evilution)
- PLUTONIA.WAD (Final Doom - The Plutonia Experiment)
- HERETIC1.WAD (Shareware Heretic)
- HERETIC.WAD (Registered Heretic)
- HEXEN.WAD (Hexen Demo or Full)
- HEXDD.WAD (Hexen: Deathkings of the Dark Citadel)
- STRIFE0.WAD (Strife Teaser)
- STRIFE1.WAD (Strife)
- VOICES.WAD (Strife Voices)
The executable sets an internal "mode" dependent on the IWAD it finds; because of this, what constitutes "a complete set" in the above lists depends on the name of the IWAD. For example, different music lumps will be used depending on whether the executable is configured to play Doom or Doom II. The executable also behaves differently in other ways: for example, loading of PWADs is disabled when loading a shareware IWAD.
Because of this, it is possible to use the executable from one Doom game with the IWAD file of another, although there are some exceptions to this due to version issues. For example, the Doom II executable cannot be used with a Final Doom IWAD, as that executable was created before Final Doom was released, though it is possible to treat Final Doom IWADs as PWADs to render the maps.