Doom (1993 video game)
Because id planned to self-publish the game, as the game neared completion they had to set up the systems to sell the game. Jay Wilbur, who had been brought on as CEO and sole member of the business team, planned the marketing and distribution of "Doom". He felt that the mainstream press was uninterested in the game, and as id would make the most money off of copies they sold directly to customers—up to 85 percent of the planned US$40 price—he decided to leverage the shareware market as much as possible, buying only a single ad in any gaming magazine. Instead, he reached out directly to software retailers, offering them copies of the first "Doom" episode for free, allowing them to charge any price for it, in order to spur customer interest in buying the full game directly from id.
Doom (1993 video game)
"Doom" was programmed largely in the ANSI C programming language, with a few elements done in assembly language. Development was done on NeXT computers running the NeXTSTEP operating system. The data used by the game engine, including both level designs and graphics files, are all stored in WAD files, short for "Where's All the Data". This allows for any part of the game's design to be easily changed without needing to adjust the engine code. Carmack designed this system specifically to enable fans to be able to 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.