Welcome to ROMdb, a Wiki to organize and collect meta-data for ROM databases provided by projects like No-Intro, MAME, Redump.org, and The Good Old Days:

Organize: ROMs (let's start calling them versions, right?) are grouped by their parent games. So "Mortal Kombat 2" for Arcade, Megadrive and Super Nintendo will be "siblings", descendants from the common game "Mortal Kombat 2", which is part of "Mortal Kombat" saga. To sum it up, the structure is saga > game > version.

Meta-data: Projects like MAME, No-Intro and Redump.org do an excellent job dumping and verifying versions but there is much more information to gather; what year was the version published? what languages (voice and text) does it contain? what about the genre, the perspective, the number of players...

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News regarding MAME

As you may know, each version in ROMdb is identified by its CRC32. To be more specific, by its "clean compound" CRC32: a) some files are not included (e.g. .cue files in optical disc dumps), and b) when versions are composed of multiple files, the sum of all relevant files' CRC32 is computed.

That approach wasn't good enough for MAME because BIOS ROM files are included in each game. Any new BIOS file in a new version of MAME would change the compound CRC32 of all the games including it. That situation would inevitably render all affected game entries in ROMdb obsolete and create new ones completely empty of metadata (parent game, release year, screen title, screenshots...). A good example of this issue were the dozens of Neo-Geo games.

The problem has been solved now by ignoring BIOs files and in the future, versions won't be affected by changes in them. Unfortunately and inevitably, in the process all versions with BIOS files have changed (for the last time) their CRC32. You will see many obsolete ROMs in arcade versions index; they will be fixed by hand. The process will be slow and painful but i'll pay off in the mid-long term.