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Old 29 July 2009, 21:34   #62
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Join Date: Nov 2008
Location: Monza, Italy
Posts: 2,524
Originally Posted by Harrison View Post
As mentioned, many roms are flagged as bad dumps, even though many of them do still run. People redump these as they find original hardware and so these get replaced in the next version, making the older bad dumped versions of the roms obsolete.

Another reason, and more the main reason for changes to the roms with each revision is the merging of identical rom code into a parent/child/clone rom set structure. Many games used the same underlying cabinets, and many games share a lot of the same roms (using the same libraries and other code on existing roms from other games, and just added a couple of additional roms for the specific game's code). Or other versions of the same games were released in slightly different forms for different countries or markets, and for these most of the roms were identical, so the games are classed as clones of the original and can use the majority of the same roms.

Therefore there isn't really any need to have duplicate copies of the same dumped roms held in the full MAME rom set for all these clones of the same games or games based on the same original game cabinet. These are referred to in MAME as child rom sets. And as new versions of MAME are developed more games that use identical rom sets are found, and so these games become child sets of the original games parent rom sets. So only one of these roms is needed to load and run a large number of games that shared it. This means that the actual rom set file structure is completely changed, and is the biggest reason why games that used to work on an older version of MAME might not work on a newer one. Because the rom set structure has changed.

I hope that made sense.
Thank you very much Harrison,
great explanation!!
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