For Doom, a game in which players race through maze-like levels to acquire weapons and treasure while killing assassins and monsters, Carmack organized the code so that players were at less risk of damaging the core game when they introduced new sounds, graphics, and levels. He did this by creating a subsystem that separated the sound and picture "wads" from the core code that governed gameplay. Every time someone booted up the game, the main program would look for the wad of sounds and images to load in. The mod writer could simply point the main program to the new wads. Carmack also made available the source code for the Doom level editing and utilities program to give mod makers powerful tools.
This was a radical idea not only for games but, really, for any type of media. Although some level editing programs had been released in the past, few programmers-let alone owners-of a company had released the guts of what made their products work. It was an ideological gesture that empowered players and, in turn, loosened the grip of game makers. It was also, as it turned out, mighty good business.
Weeks after Doom's release, hackers began creating level editing programs. These tools let players change existing rooms in the game-relocate walls, move floors, or make other minor adjustments. Soon, two students collaborated to release the Doom Editor Utility, which explained how to build a level from the ground up. "You can do almost anything to any level," they pointed out, "move, add, or remove monsters and power-ups, change the wall colors and positions, create new lifts, doors, acid pools, crushing ceilings . . . or even create a new level from scratch!" For the hard-core Doom player, the ability to create new acid pools was pure joy.
The Doom Editor Utility was a watershed in the evolution of the participatory culture of mod making. Anyone with the interest could create a level of a complex game, the equivalent of writing a new chapter into a book, and then, via the Internet, publishing that creation. The tools let the player tweak what was there, or dress the game up with sounds, images, or ideas. The mod maker didn't need to be a hard-core programmer or an artist. Soon a University of Michigan student named Greg Lewis dived deeper into Doom code and came up with a program called DeHackEd, which allowed a user to modify not the wads but the game's core code, the executable file. Mod tools turned game players into game makers. Soon, the mod makers were swapping their Doom levels for free in forums on AOL, CompuServe, and across the Internet. Gamers who had been flunking out of school because of deathmatching now had an even more addictive pastime: all-night hacking parties.