ACE Engine

The ACE Engine is an unclassifiable project somewhere between enhancement mod and source port. The core principle is to exploit known vulnerabilities in the original DOOM2.EXE (specifically the version 1.9 from Doom II, not the similarly-numbered versions from Ultimate Doom or Final Doom) to execute arbitrary code when loading a specially crafted (and technically invalid) map. In this case, the arbitrary code being loaded is the ACE Engine, which allows to graft some modern features to the game, such as bug fixes, raising some of the static limits, or a limited subset of DECORATE. In essence it can be thought of as a source port that has to be launched from vanilla Doom in DOS.