Watford Disk File System

Alternate DFS for the BBC Micro
Categories:

The Watford DFS uses the same structure as Acorn's DFS with some additional attributes:

In byte 6 of Sector 1, bit 2 is clear and 3 is clear for Acorn & WDFS for disks up to 256K, but set for WDFS and disks larger than 256K.

Watford DFS use 19-bit file lengths and 11-bit sector numbers making the largest possible disk being 512K and the largest file size of 512K-1.

Catalog format

The disk layout is identical to Acorn DFS with some differences shown below:

Byte Track 0 Sector 0 Track 0 Sector 1
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
0 First 8 characters of volume title padded with NULs or spaces Last 4 characters of volume title padded with NULs or spaces
1
2
3
4 BCD catalogue cycle number
5 Number of files
6 Boot Opts 0 108
7 7 Sectors in volume 0
8 File name padded with spaces 7 Load address 0 Last file
9 15 8
10 7 Execution address 0
11 15 8
12 7 File length 0
13 18Len 15 8
14 10Sect 17 16 Exec 17 16 17 16 Load 9 8
15 Lk. Directory 7 Start sector 0
16 File name padded with spaces 7 Load address 0 Second to last file
17 15 8
18 7 Execution address 0
19 15 8
20 7 File length 0
21 18Len 15 8
22 10Sect 17 16 Exec 17 16 17 16 Load 9 8
23 Lk. Directory 7 Start sector 0
   

Differences between Acorn DFS and WDFS

  • Sector 1 byte 6 bit 3 is always 0 like Acorn DFS
  • Sector 1 byte 6 bit 2 is bit 10 of the disk size / sectors in volume. 0=max 256K, 1=max 512K
  • Len in byte 5 of the filename is bit 18 of the File Length.
  • Len in byte 6 of the filename is bit 10 of the Start Sector.
  • Lk. in byte 7 is the same as for Acorn DFS

Sources

This section is based loosely on the following:

Last modified October 31, 2021: Changed how the layouts finish so only show the first two as catalog entries go down & not up as how I originally had them sort of hints at (d3d5a16)