The game itself. Note that the official ROM name inside MAME is often mslug6.zip . However, unlike Neo Geo games, the Atomiswave version has a specific file structure. Do not confuse this with the later Metal Slug 6 port for PlayStation 2 or PSP—these will not work in MAME.
Emulating this hardware requires significantly more processing power than the Neo-Geo MVS. The MAME version 0.139u1 is a specific "milestone" build often used in optimized emulation frontends (such as certain builds of FinalBurn Alpha or older RetroArch cores) because it represents a stable point in Atomiswave driver development. metal slug 6 mame 0139u1 rom top
You might wonder, with modern MAME versions available, why obsess over version 0.139u1? The game itself