DIF File

From Retro Arcade Guides

Not to be confused with MAME/MESS differential dat-files created by MAME_DFC.

MAME (and MESS) use CHDs for some games (or systems), which are compressed images of hard disks and optical media (CDs, DVDs etc).

Just like ROMs, CHD files are hashed, so that it is possible to find out if a CHD file has the expected data.

When a CHD has an image of writeable media (such as hard disks) the game being played may write data to the writeable media. This would change the CHD's hash, which would make it impossible to verify if the CHD being used is the correct one.

To avoid that, all data that should be written to the CHD file by the emulator is instead written to files with the .dif extension in the diff folder.

A .dif file created in a previous emulator version may not work properly. It's then advised to delete the .dif file. These files are created automatically by the emulator when necessary.