Basics of the WAD format

A "WAD" (standing for Where is All the Data?) is a data package format that was originally developed for and used in Doom. It functions in the same way a modern .PAK file operates, acting as a single mass of game data and providing minimal lump compression where appropriate. Inside a .WAD file, data entries are referred to as lumps.

An important restriction inside the WAD package is the eight character entry name length for all lumps. When importing lumps into a .WAD, the filename extension is lost (this is because it is not needed - Doom is programmed to "know" the names of special lumps it needs to operate properly).

Other restrictions include a maximum height of 128 pixels and 32768 pixels in width on Patch entries and Texture subentries.

Being a 16-bit format, it is reasonable to assume that the maximum number of lumps that can exist in a WAD is 65536 (assuming an entry can exist on index number 0).