To run the arcade version of MvC2 in MAME, you need two distinct components:
Contains the actual game data, graphics, and music stored on the Naomi hard drive.
file is a disk image used for games that originally had hard drives or CD-ROMs. need a CHD? Technically, the arcade version of MvC2 was released as a for the NAOMI system, so it typically uses a standard ROM rather than a CHD. Why do people look for a CHD? Many users confuse MAME with other emulators like