MDL

MDL is the file format used by Microsoft Flight Simulator to store visual model data. It uses the .mdl file extension.

The MDL format can be broken down into 2 main types; the DLL-based type and the RIFF-based type.

The DLL-based format was used in legacy versions of MSFS, before Flight Simulator 2002. It was actually written as a windows DLL library, and simply used the .mdl extension. It is believed to have been coded in Assembly language. More information may be found at Legacy MDL Format.

The RIFF-based format MDL was introduced in FS2002, and has been updated in every subsequent version of FS. It can be broken into 3 main versions: MDL8, MDL9, and MDLX. The MDL8 file was introduced with FS2002 and was used in both aircraft and scenery models. It used a simple architecture, containing mostly BGL opcodes wrapped around a simple RIFF shell. In FS9 we were introduced to the MDL9 format. FS9 is especially interesting because the scenery models used the new MDL9 format, but the aircraft models still used the MDL8 format. The MDL9 format introduced some new fourCC tags, and was better organized, but still relied heavily upon BGL codes to actually draw the model. The MDLX format was introduced with FSX. It is a complete departure away from the old BGL drawing codes and relies solely on data structures defined within 57 different fourCC tags.