This is a double density format, but it is mentioned here as the volume catalogues are based on Acorn DFS.
DDOS discs are partitioned into between one and eight volumes. Track 0 is reserved for the DFS-style volume catalogues, plus a disc catalogue unique to DDOS.
In each volume catalogue:
- The start sector may be any number less than the disc size, as DDOS adds a partition offset and so the catalogue is not in the same space as the files.
- The disc size is the size of the current volume. It is an 11 bit quantity, the top bit being in bit 2 of
sector 1 byte 6.
If and only if this bit is set:
- The file length steals an extra bit from the execution address of each file (bit 6 of sector 1 byte 14+n.)
- The start sector steals an extra bit from the load address of each file (bit 2 of sector 1 byte 14+n.)
This section is based loosely on the following:
- Acorn DFS disk format - beebwiki.mdfs.net
- Acorn, Watford, Solidisk and Duggan DFS Filesystem Structure mdfs.net