I think if you're going to go that route, just replace FDS+1A with GDM+1A directly. That would better let old emulators know that it's not the kind of file they think it is, and have a better chance of them producing a meaningful user error than having them incorrectly assuming it's a valid FDS file and trying to run it.Great Hierophant wrote:Bytes 1-5 are the ASCII for FDS, the DOS end of File Character $1A and the Number of Disk Sides
Bytes 6-8 are the ASCII for GDM, which is intended to stand for Game Doctor/Magicard.
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Now you might observe that this limits you to four disk sides whereas the Game Doctor can use at least eight. What do you do then? You tack on another header after the end of the data on Disk Side 4 and continue with the data. The second header is identical to the first until Bytes 9-10, which represent the additional storage space required by Disk Side 5.
At that point, you can make the header whatever size it needs to be. There's no need to compromise to make it fit in 16 bytes, that's just seems a needless complication.
Get rid of the old "FDS" FOURCC, and make the rest of the header an appropriate size for the different format. You could still use the .FDS extension if you think that's important (though I don't, personally), but at least with a different FOURCC you don't have to make the cause of incompatibiliy so obtuse for people using older emulators. Leaving the old FOURCC on there will just "trick" them into trying to run something they can't.