TEXTURE1 and TEXTURE2

The TEXTURE1 and TEXTURE1 lumps define how wall patches from the WAD file should combine to form wall textures.

In an era where space is expensive, a map texture inside a WAD is to a texture seen inside the game, as a MOD file is to an MP3 file. In other words, it is decomposed into smaller textures in such a way that repeating areas are only stored once. At startup time (to be precise, during "R_Init: Init DOOM refresh daemon"), the patches inside the WAD are read and combined to, what they call, 'map textures'.

Binary data
The binary contents of the TEXTURE1 lump starts with a header of flexible size, followed by all the the the map textures.

All integers are 4 bytes long in x86-style little-endian order. Their values can never exceed 231-1, since Doom reads them as signed ints. Short integers are 2 bytes long and are otherwise equal to integers.