Acorn, Watford, Solidisk and Duggan DFS Filesystem Structure
	============================================================
		 http://mdfs.net/Docs/Comp/Disk/Format/DFS

Acorn System/Atom DOS, Acorn DFS, Watford DFS, Solidisk DFS and Angus
Duggan's HDFS share the same basic file structure and each filing system
understands a large subset of each filestructure.

Single density DFS floppy disks have 10 physical sectors of 256 bytes on
each track, numbered from 0 to 9. Double density DFS floppy disks have 18
physical sectors of 256 bytes on each track, numbered from 0 to 17.

Tracks are sequential, that is the tracks start at track 0 on one disk of
a disk, and continue up the same side of the disk.

DFS uses 256-byte logical disk sectors. Logical sectors are counted with
10-bit or 11-bit numbers starting from &000 at the start of the
filesystem.

Acorn DFSs use 18-bit file lengths and 10-bit sector numbers, so the
largest possible DFS disk system is 256K and the largest possible file is
256K-1.

Watford DFS, Solidisk DFS and Duggan HDFS use 19-bit file lengths and
11-bit sector numbers, so the largest possible WDFS/SDFS/HDFS disk system
is 512K and the largest possible file is 512K-1.

Note that WDFS, SDFS and HDFS extend the DFS catalog structure, but put
their additional information in different locations. DFS, WDFS and HDFS
directories can be distinguished by looking at byte &106 in the catalog:

106:  b3=0 b2=0 DFS or WDFS disk up to 256K
106:  b3=0 b2=1 WDFS, disk larger than 256K
106:  b3=1 b2=0 HDFS, single-sided disk
106:  b3=1 b2=1 HDFS, double-sided disk


Sector 0,1
----------
The first two sectors contain the catalog. It contains 32 entries, a
header followed by 31 file entries. The entries are split across the two
sectors, with the first eight bytes holding the name in sector 0 and the
second eight bytes holding the information in sector 1.

The catalog is always stored sorted by file sector start address, with
highest sector address at the start and lowest address at the end, so the
catalog is sorted by decrementing sector number of all entries including
the header.

HDFS subdirectories and DFS62 multiple catalogs use the same layout.

Catalog header, entry 0:
             Sector 0                          Sector 1
 000 001 002 003 004 005 006 007   100 101 102 103 104 105 106 107
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
|          Disk Title           | |   DiskTitle   |Cyc|Num|Op|Sect|
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+

File entries, entries 1-31:
        Sector 0                  Sector 1
 000 001 002 003 004 005 006 007   100 101 102 103 104 105 106 107
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
|          Filename         |Dir| | Load  | Exec  | Size  |Op|Sect|
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+

Catalog header:
000-007    First eight bytes of the disk title, padded with spaces.
100-103    Last four bytes of disk title, padded with space.
           The disk title is the directory name in HDFS.
104        Disk cycle, HDFS: Key number
105        (Number of catalog entries)*8 - offset to end of directory
106 b7-b6: zero
    b5-b4: !Boot option (*OPT 4 value)
    b3:    0=DFS/WDFS, 1=HDFS
    b2:    Total number of sectors b10, HDFS: (number of sides)-1
    b1-b0: Total number of sectors b9-b8
107        Total number of sectors b7-b0
HDFS: The total number of sectors b10 is stored in b7 of byte 000.

File entries:
000-006    Filename and attributes
007        Directory and attributes
100-101    Load address b0-b15
102-103    Exec address b0-b15
104-105    File length b0-b15
106 b7-b6: Exec address b17-b16, SDDFS: also Load address b17-b16
    b5-b4: File length b17-b16
    b3-b2: Load address b17-b16
    b1-b0: Start sector b9-b8
107        Start sector b7-b0
Solidisk DDFS: byte 106 bit 2 = b10 of Start sector
               byte 106 bit 3 = b18 of File length

File attributes are stored in b7 of bytes 0-7:
 000,b7:   				HDFS: Start sector b10
 001,b7:   				HDFS: File length b18
 002,b7:   				HDFS: unused
 003,b7:   				HDFS: file/directory
 004,b7:   				HDFS: not readable
 005,b7:   WDFS: File length b18	HDFS: not writable
 006,b7:   WDFS: Start sector b10	HDFS: not executable
 007,b7:   All:  Locked

Sector 2,3
----------
Watford DFS allows a second 31-entry catalog in sectors 2 and 3. It is
identified by the first eight bytes of sector 2 containing &AA:

000-007 Eight bytes of &AA
008-0FF 31 file entries
100-104 Five bytes of &00
105     (Number of catalog entries)*8 - offset to end of directory
106-107 Copy of boot option and disk size in sector 1
108-1FF 31 file info entries

Disk Doctor second directory
----------------------------
An entry with name "!.!!!!!!!" which is locked is a Disk Doctor second
directory, with the same layout as the first directory. The Disk Doctor
*SWAP command swaps the two directories on the disk. Each directory has to
refer to its own section of the disk to avoid overwriting objects stored
in the other catalog.

Notes
-----
Some early DFSs allow the user to write disk titles or filenames with bit
7 set. This may cause these entries to appear to erroneously have HDFS or
WDFS attributes. Most DFSs disallow top-bit set filenames and disk titles.

The Solidisk DDFS extensions have been worked out from Andrew Benham's
FUSE DFS module at www.adsb.co.uk/bbc/linux in the absense of any extant
documentation, and may be incorrect.