"Doom" was programmed largely in the ANSI C language, with a few elements in assembly language, on NeXT computers running the NeXTSTEP operating system. The data, including level designs and graphics files, is stored in WAD files, short for "Where's All the Data". This allows for any part of the design to be changed without needing to adjust the engine code. Carmack had been impressed by the modifications made by fans of "Wolfenstein 3D", and wanted to support that with an easily swappable file structure, and released the map editor online.