Catalog

The catalog is located in sectors 0 & 1 on Track 0. It is capable of holding just 31 entries.

Formatting a disk on the real hardware only needs to write to these two sectors (via `*FORM40` or `*FORM80`) but a blank catalogue simply needs these two sectors blanked and the disk size field initialised.

Catalog format

This is the structure of the catalogue header:

All unused bits must be kept clear.

Disc title

Up to 12 printable ASCII characters, padded with NULs (&00) or spaces. The first eight characters are placed in sector 0 bytes 0 to 7, the last four characters in sector 1 bytes 0 to 3.

Disc size

The total number of sectors on this side of the disc. This is a 10-bit field with the low 8 bits in sector 1 byte 7, and the others in the low two bits of sector 1 byte 6.

File descriptor fields

There is room for 31 file entries in the catalogue. Each entry takes up eight bytes in each of the two catalogue sectors.

Files must be listed in descending order of start sector with no gaps in the catalogue. Empty files should be given a start sector of 2. Files cannot be fragmented but there may be free sectors between files.

The rest of each sector represents the files. Each file is represented by a block of 8 bytes in each sector offset by 8. So the last file starts at offset 8, second to last at 16 & so on.

Identifying and validating a DFS disc image

Unlike its successor ADFS, the DFS format contains no self-identification strings or checksums, so recognising a disc or disc image as Acorn DFS is a complex and unreliable process.

If one has a floppy disc in hand it is often enough to confirm that the physical format is the same (see above) as Acorn DFS is the majority producer. On the other hand DFS disc image files are normally given the extension .ssd or .dsd, indicating a single or double sided disc image respectively.

Often the task is to identify which format variant a disc or image contains. This is more a matter of finding valid catalogues in the appropriate locations.

To check that a pair of sectors contains a valid standard DFS catalogue

• Firstly confirm that all bits marked white or cream in the above diagram are zeroes; if not, the catalogue can be rejected immediately.
• Check also that the file offset is a multiple of 8.
• The disc size must be between 2 and 800 sectors inclusive, although the format allows up to 1023 sectors, and disk images may contain up to this many.
• The disc title consists of printable ASCII characters, padded with NULs or spaces.
• For each file entry in use:
  • The file name field must contain one to seven valid characters and be padded with spaces. Valid characters are the printable ASCII characters between &20 and &7E inclusive, except . : " # * and space.
  • The directory character (without the attribute bit) must be a valid file name character.
  • The file name (taking the directory character into account) must be unique.
  • The start sector must be more than 1 and less than the disc size.
• Skipping over zero-length files:
  • The start sectors must run in strictly descending order.
  • No file may overlap the one before it in the catalogue. That is, start_sector_2 + ((length_2 + 255) DIV 256) ≤ start_sector_1.
  • No file may overshoot the end of the disc. That is, start_sector + ((length + 255) DIV 256) ≤ disc_size. Equivalently, the first file in the catalogue must not overlap the imaginary file whose start sector field is the disc size field.

Further heuristics

Any catalogue complying with the above is a valid DFS catalogue; however, only a tiny subset of these look like catalogues seen in real life. As an informal test a pair of sectors that match many of the following is more likely to be a catalogue:

• The disc size is 400 sectors (40 tracks) or 800 sectors (80 tracks.)
• There is a file named !BOOT in directory $.
• The 16th and 17th bits of the load and execution (exec) addresses are all clear or all set in each file entry.
• There are few directories, or a few unique names in several directories.
• Most directory characters are $ (the default.)
• Some files have load addresses of the form &3nn00 (nn=0E, 11, 19, 1B, 1D, 1F or 21) and exec addresses of &3801F or &38023. (BASIC programs.)
• Some files have load and exec addresses the same, usually &xx00 or &3xx00, and often have length &yy00. (Screen dumps and machine code images.)
• Some files have load and exec addresses of 0, &30000 or &3FFFF (Text and sequential data.)
• Many files are less than 2K; few files more than 20K; no files more than 64K except text and sequential data.
• The file names have very few lowercase letters.
• The file names contain commonly-occurring pairs of letters.

Sources

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

Catalog format

This is the structure of the catalogue header:

Differences between Acorn DFS and HDFS

• For the volume header, bytes 0…7 of both sectors:
  • Track 0 sector 0 byte 0 bit 7 is bit 10 of the number of sectors in the volume.
  • Track 0 sector 1 byte 6 bit 3 is set. Bit 2 is the number of sides-1, 0=single sided, 1=double sided.
• For file entries:
  • Sect is bit 10 of the start sector.
  • Len is bit 18 of the file length.
  • IsDir defines if the entry is a file or a subdirectory.
  • If the Read, Write or Exec attributes are set then the file can not be read, written or executed respectively.
• For a directory entry, the directory has the same layout as the main Catalog, except the volume title becomes the directory name^{[Citation needed]}.