... I hope that you are changing the DAT contents to match your set, and not the other way around, like that first sentence of yours indicated. The filesize in hex is after the filename and the after the first indexing number. When there are multiple C or V roms, the indexing number uses pretty much a predefined format, but I'm sure you have little trouble with those if you look at my DAT example. 2000000 = 32MB (33,553,432 800000 = 8MB (8,388,608 600000 = 6MB 400000 = 4MB (4,194,304 200000 = 2MB (2,097,152 100000 = 1MB 80000 = 512kB (524,288 40000 = 256kB (262,144 20000 = 128kB (131,072 10000 = 64kB Are you beginning to see a pattern again?