The Doom engine powers Doom and Doom II, and is separate from the assets and resources (data) used by those games. In a more technical sense, the engine is the executable elements of the games, based on the corresponding source code, as opposed to WAD and lump files. Various versions of the engine were released by id Software for the PC games, one per release but often shared by both games, as the same executables are used for both, only renamed accordingly.