Many games separate game logic and engine to allow user modified game play, so called 'mods'. To make such mods portable across CPUs and operating systems Quake 3 allows to compile regular C code to a Quake3 specific byte code. The byte code is then translated to native instructions by the game when loading the mod to achieve near native performance. This talk explains how the virtual machine works and how it's possible that Quake 3 supports both native shared objects as well as byte code VM's.