Duggan HDFS Filesystem Structure
		    ================================
		http://mdfs.net/Docs/Comp/Disk/Format/HDFS

HDFS 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.

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

Note that Watford DFS also extends the DFS catalog structure, but puts its
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, usually double-density
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 DFS/HDFS catalog. The catalog
is always stored sorted by file sector start address, with highest sector
address at the start and lowest address at the end.

000-007 First eight bytes of the disk title
  byte 0,b7: Total number of sectors b10 (HDFS)
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:
  byte 0-6 Filename, padded with spaces
  byte 7   Directory
  byte 0,b7: start sector b10 (HDFS)
  byte 1,b7: length b18 (HDFS)
  byte 2,b7: unused (HDFS)
  byte 3,b7: entry is subdirectory (HDFS)
  byte 4,b7: not readable (HDFS)
  byte 5,b7: not writable (HDFS)
  byte 6,b7: not executable (HDFS)
  byte 7,b7: file locked

100-103 Last four bytes of disk title
104     Disk cycle number
105     (Number of catalogue entries)*8
106     b6-b7: Unused, set to zero
        b4-b5: !Boot option (*OPT 4 value)
        b3:    DFS: 0        HDFS: 1
        b2:    DFS: unused   HDFS: (number of sides)-1
        b0-b1: Total number of sectors b8-b9
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
    b6-b7: file execution address b16-b17
    b4-b5: file length b16-b17
    b2-b3: file load address b16-b17
    b0-b1: file start sector b8-b9
  byte 7   file start sector b0-b7