Alternate DFS for the BBC Micro

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: