You Can Now Play Doom In Doom With A Simple Mod

gamespot.com:

In yet another of the cavalcade of absurd ways to play Doom, you can now play the original Doom inside the DOS version of Doom II. With a DOS emulator and the right files, you can live out your metagame dreams.

YouTuber and programmer kgsws, as spotted by PCGamesN, found an exploit within the code of Doom. First, they find a command called “SpawnMapThing,” used to place enemies, items, and other games objects in Doom II’s world. With some fine tuned trickery--it’s worth watching the whole video if you are curious about how this works--placing a negative number can override the game’s states. Now, “states” indicate what version of a in-game object should be displayed. A state governs whether an item is flashing or an enemy is firing their weapon or a foe is starting a death animation. With some more bug fixes, kgsws associates the trigger of a state code with an executable, namely the .exe file for Chocolate Doom.

After that, kgsws spruces up the exploit. They add a movie theater room for playing Doom inside Doom II, complete with adjustable lights. They also added ways to play the game full screen or displayed on multiple walls. They even get the exploit working with Chocolate Heratic, though that needed further fixes.

The video’s description notes that this will only work with the DOS version of Doom II, not with GZDoom or other source ports. However, the DOS version of Doom is available on Steam and can be run with a DOX emulator like DOSbox. If you want to try this out yourself, you can download the required files on Github.