Acorn's Disk Filing Systems
Acorn Disk Filing System (DFS)
The Disc Filing System (DFS) is a computer file system developed by Acorn Computers Ltd, and introduced in 1982 for the Acorn BBC Microcomputer. It was shipped as a ROM to be inserted onto the BBC Micro's motherboard. It has an extremely limited design, and uses a flat directory structure. Each filename can be up to 7 letters long, plus one letter for the directory in which the file is stored. It was based around the Intel FDC 8271 disk drive controller, the immediate predecessor on the 8272 design found in the IBM PC.
The BBC Model A and B didn't come as standard with disk support - it was a £120 option. This Acorn disk upgrade comprised the 8271 chip, a handful of standard TTL ICs, an Acorn DFS ROM, and the disk manual. The ICs plugged into unpopulated sockets on the motherboard. The upgrade could be performed on an Issue 4 or Issue 7 BBC Model B board without any soldering, however, on Issues 1-3 some soldering was necessary (Issues 5 and 6 were never commercially released).
Disk Usage and Directories
DFS conventionally uses one side of a double-density 5¼" floppy disc. Discs are formatted as either 40 or 80 track, giving a capacity of 100 or 200 KB per side (10 256-byte sectors per track, with FM encoding).
"Directories" in the DFS are single character prefixes on filenames – such as "F" in "F.BankLtr" – used to group files. The arrangement is flat and a default directory of "$" is used instead of a root directory. On requesting a catalogue of the disc, files in the current directory are shown with no directory prefix in one block, and below that are listed all other files in a second block, with their directory prefixes visible
DFS discs do not track any dates (because Acorn MOS prior to version 3 did not maintain a real-time clock) but instead offered a peculiar feature: a modification count. Every time the disc is written to, the count increments. The count is shown in parentheses after the title in the first line of the disc catalogue
The DFS also supports a means to start up disc software based on a key sequence. If the shift key is held while the machine is soft or hard reset, the DFS checks the active drive for a disc containing a positive boot flag. The boot flag is either 0 (ignore), 1 (load file), 2 (run machine code file) or 3 ("execute" script). If the boot flag is positive, a file called !BOOT is looked for and loaded into memory (1), loaded and executed as machine code (2) or fed into the keyboard buffer (3). Option 3 reads "EXEC" files, text files used as very primitive shell scripts. These are not true shell scripts but simply a series of keys to be typed, like a recording to play back. Thus, they cannot loop or branch unless they input such code into the BASIC interpreter. As well as being used during a reset, they can be executed at any time with the operating system's EXEC command. EXEC files are not DFS-dependent.
Acorn produced a variant of the DFS called the DNFS, or Disc/Network Filing System, that contained the Econet Network Filing System (NFS), standard Disc Filing System and Tube co-processor support software on a single ROM; this ROM installed two filing systems into the OS at once.
Unlike most filing systems, there was no single vendor or implementation. The original DFS was written by Acorn, who continued to maintain their own codebase, but various disc drive vendors wrote their own implementations. Companies who wrote their own DFS implementation included Cumana, Opus and Watford Electronics. The Watford Electronics implementation is notable for supporting 62 files per disc instead of the usual 31, using a non-standard disc format. Other features in third-party implementations included being able to review free space, and in-built FORMAT and VERIFY commands, which was shipped on a utility disc with the original Acorn DFS.
DFS 0.90 = Most common version utilising Intel 8271
DFS 2.10 = Most common version utilitising WD1770
Acorn Advanced Disk Filing System (ADFS)
The Advanced Disc Filing System (ADFS) is a computing file system particular to the Acorn computer range and RISC OS based successors. Initially based on the rare Acorn Winchester Filing System, it was renamed to the Advanced Disc Filing System when support for floppy disks was added (utilising a WD1770 floppy disc controller) and on later 32-bit systems a variant of a PC style floppy controller. Note: whilst ADFS was marketed as the successor to DFS, they are completely different filing systems. ADFS cannot read/write to DFS-formatted floppies, and DFS cannot read/write to ADFS-formatted floppies. Any model BBC that needs to use both must have both DFS and ADFS ROMs installed.
Acorn's original DFS was rather limited in that few files could be stored on a disk, and directory and file names were restricted to 1 and 7 characters respectively. The Disc Filing System's limitations were in part due to its basis on the disk firmware used in the earlier Acorn Atom and System 3–5 Eurocard computers. To overcome some of these restrictions Acorn developed ADFS. The most dramatic change was the introduction of a hierarchical directory structure. The filename length increased from 7 to 10 letters and the number of files in a directory expanded to 47.
ADFS required a WD1770 or later WD1772 series floppy controller, owing to the inability of the original Intel 8271 chip to cope with the double density format ADFS required. The 1770 floppy controller was directly incorporated into the design of the Master Series and B+ models, and was available as an 'upgrade' board for the earlier Model B. The Acorn Electron's floppy interface was an add-on unit, initially available through Acorn and later Pres (aka Advanced Computer Products).
Hard Disk and 3½" floppy Support
ADFS supported hard discs, and 3½" floppy discs formatted up to 640 KB capacity using double density MFM encoding (L format; single-sided disks were supported with the S format (160 KB) and M format (320 KB)). ADFS as implemented in the BBC microcomputer system (and later RISC OS) never had support for single density floppies.
Hard disk support in ADFS used a modified format, communicating with a SCSI-based Winchester unit via the BBC Micro's 1 MHz Bus. Support for IDE/ATAPI style drives has been added 'unofficially' by third parties in more recent years.
Acorn DFS and ADFS Publications
In the table below you can view or download some DFS- and ADFS-related publications from Acorn:-
|Acorn ADFS Upgrade brochure (in PDF format)|
|Acorn 1770 Disk Interface Upgrade Kit brochure (in PDF format)|
|Acorn BBC Micro Disk Storage brochure (in PDF format)|
|ADFS User Guide (in PDF format)|
|Acorn Disk System User Guide from 1982 (in PDF format)|
|Acorn Disk Filing System User Guide from 1983 with addendum for the 1770 DFS|
|Acorn Winchester Disc System brochure (in PDF format)|
|Acorn Winchester Disc Filing System User Guide (in PDF format)|
This page was last updated on 2nd February 2015.