Date   : Tue, 30 Jun 2009 22:39:22 +0100
From   : mfirth@... (Michael Firth)
Subject: DSD format and 'short' images

Does anyone here know where the 'DSD' disk format originated? - It seems 
a strange layout for a BBC format, as the sector ordering is rather PC-centric
(being interleaved due to having the head before the cylinder), rather than
BBC-centric, which would be like the 'large SSD' that I've seen occasionally.

Also, is there a good algorithm for detecting a DSD - presumably looking
to see if sectors 0,1 and 10,11 look like DFS catalogues will catch most of them?

The BeebWiki definition of how to detect a DFS catalogue seems a bit too 
strict - certainly on early Acorn DFSes you could get away with putting 
control characters into the disk title or filenames with a sector editor 
(e.g. using a VDU21 / VDU6 combination to hide some files, or characters 
129-134 to give coloured titles in mode 7), which is listed as illegal on 
the Wiki. Is there any code I can crib anywhere that will do a 'first pass'
sanity check on a DFS catalogue - i.e. as a minimum ensuring that the files
don't overlap, and that the starting sectors are in descending order?

Finally, is there anything that can create 'short' DSD files like several
things do for SSD files (i.e. only containing the amount of sectors to cover
the highest occupied sector on either side), or are DSDs always 400k?

Thanks

Michael