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

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

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

Note that WDFS and HDFS extends the DFS catalogue structure, but puts its
additional information in different locations. DFS, WDFS and HDFS
directories can be distinguished by looking at byte &106 in the catalogue:

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 31-entry catalogue. The catalogue is
always stored sorted by file sector start address, with highest sector
address at the start and lowest address at the end.

		DFS		   WDFS			HDFS
000-007    First eight bytes of the disk title, padded with spaces
           This is the directory name in HDFS
  byte 0,b7:	zero		   zero			total sectors b10
  byte 1-7,b7:	zero		   zero			zero
008-00F    First file entry
010-017    Second file entry
018-01F    Third file entry
...
0F8-0FF 31st file entry

A file entry is laid out as:
		DFS		   WDFS			HDFS
  byte 0-6 Filename, padded with spaces
  byte 7   Directory prefix character
  byte 0,b7:	zero		   zero			sector b10
  byte 1,b7:	zero		   zero			length b18
  byte 2,b7:	zero		   zero			unused
  byte 3,b7:	zero		   zero			file/directory
  byte 4,b7:	zero		   zero			not readable
  byte 5,b7:	zero		   length b18		not writable
  byte 6,b7:	zero		   sector b10		not executable
  byte 7,b7:	locked		   locked		locked

		DFS		   WDFS			HDFS
100-103    Last four bytes of space-padded disk title/HDFS directory name
104		Disk cycle	   Disk cycle		Key number
105        (Number of catalogue entries)*8
106 b0-b1: Total number of sectors b8-b9
    b2:		zero		   total sectors b10	(number of sides)-1
    b3:		zero		   zero			one
    b4-b5 !Boot option (*OPT 4 value)
    b6-b7:	zero		   zero			zero
107        Total number of sectors b0-b7
108-10F    First file's info
110-117    Second file's info
...
1F8-1FF    31st file's info

A file info entry is laid out as:
  byte 0-1 file load address b0-b15
  byte 2-3 file execution address b0-b15
  byte 4-5 file length b0-b15
  byte 6
    b0-b1: file start sector b8-b9
    b2-b3: file load address b16-b17
    b4-b5: file length b16-b17
    b6-b7: file execution address b16-b17
  byte 7   file start sector b0-b7

Sector 2,3
----------
Watford DFS allows a second 31-entry catalogue 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
100-103 Four bytes of &00
104     Copy of disk cycle number in sector 1
105     (Number of catalogue entries)*8
106-107 Copy of boot option and disk size in sector 1
108-1FF 31 file info entries

Notes
-----
Some early DFSs allow the user to write disk title or filenames with b7 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.