Acorn System Disk File System / Atom DOS

Categories:

• bbc
• filesystem

Discs for the System series have an identical catalogue format to those for the BBC Micro, except all entries are in a directory " " (space), disks do not have a cycle number, and files do not have bits 16 and 17 of load and execution addresses.

The places they would occupy are reserved and must be cleared. A BASIC program named DCONV, on the Utility disc, performs this task.

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									Reserved
5									Number of files
6											Boot Opts				98
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									15							8
14									Reserved		17 16		Reserved		9 8
15	Lk.	Ascii Space (0x20, 32)							7	Start sector						0

Differences between Acorn DFS and System DFS

• The Reserved entries must be clear.
• The disk type in bits 2 and 3 of sector 1 byte 6 must be clear as with DFS.
• Lk. in byte 7 is the same as for Acorn DFS

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