"Doom" was programmed largely in the ANSI C programming language, with a few elements in assembly language, targeting the IBM PC and MS-DOS platform by compiling with Watcom C/C++ and using the included royalty-free 80386 DOS-extender. id developed on NeXT computers running the NeXTSTEP operating system. The data used by the game engine, including level designs and graphics files, are 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 designed this system so fans could easily modify the game; he had been impressed by the modifications made by fans of "Wolfenstein 3D", and wanted to support that with an easily swappable file structure along with releasing the map editor online.