This is the best case, optimal for accuracy and minimizing the effort. Occasionally, as was the case with some of the engines/game cores in ScummVM, the original developers have helped the projects by supplying the original source code (those can be then called source ports). See also: Source port and List of commercial video games with available source code MAME is an example of a video game engine emulation project which also follows this philosophy for accurate representation of the games. Most often, the result is also not called 'game engine' but 'game recreation' or 'game clone'. Also on the negative side, the resulting code is very specifically tied to this single game, often ugly ('pseudo- assembly code' ), and can hardly be reused as general game engine.

In the development phase this has the disadvantage that for a long time no running prototype exists. In these cases, the game core is recreated bottom up with reverse engineering of the original dis-assembled binary executable, CPU instruction for instruction. Unlike Top down game engine recreations, bottom up dis-assembled/decompiled versions for a specific game are often able to replicate the behaviour of the original exactly. See also: List of commercial video games with available source code § Games with reconstructed source code On the positive side, running code exists faster, and the finally resulting source code is less specifically tied to a specific, single game and can be reused as a general game engine for other games. Typically, this approach results in an approximation of the original behaviour only and not a ' clock cycle wise' identical behaviour.

Another example is the Total Annihilation engine remake Spring Engine, which resulted in being used for many more games. An example is OpenRA based on specifications contributed by the community by clean-room re-implementations without dis-assembling the original executable, which result in game engines whose behavior differs from the original. Then, in later steps, the resulting engine is adapted to the specific detail behaviour of the original game, often by reverse engineering, debugging and profiling the original. When game engine recreations are made in a top down development methodology, in the first step the general game's functionality is programmed and the structure is defined.

Generals game.dat file