You can now play 'Doom' inside 'Doom II'

engadget.com:

Doom is playable on just about anything, from a DJ controller and Canon printer to an ATM and Minecraft. YouTuber kgsws took the Inception route, though. They put a playable version of Doom inside Doom II.

They used an exploit in the DOS version of Doom II, as Gizmodo notes. The modder, who explains the process in a 15-minute YouTube video, was able to get a modern port of Doom (Chocolate Doom, which uses the original source code) running inside Doom II as an animated texture. The original game is projected on a virtual display in a custom Doom II map that kgsws created.

They built more maps that showcase the pioneering first-person shooter, including having the same instance of Doom running on four walls surrounding the player and a cinema screen that's projecting the game. Additionally, kgsws showed off Heretic running inside of Doom.

You can try the hack for yourself, as kgsws shared the code on GitHub. You can snap up a DOS version of Doom II from Steam. You might need a reasonably beefy gaming PC to run Doom inside Doom II, though. "Both games are running independently of each other," kgsws explains. "That means you need double the memory. I would recommend you to get at least 16MB of RAM."