Acorn's Australian computer distributor, Barson Computers, convinced Joint Managing Directors Hermann Hauser and Chris Curry to allow the soon to be released Acorn BBC Microcomputer to be offered with disk storage as part of the bundle.
The DFS shipped as a ROM and Disk Controller Chip fitted to the BBC Micro's motherboard.
The filing system was of extremely limited functionality and storage capability, using a flat directory structure.
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 implementations included Cumana, Solidisk, Opus and Watford Electronics.
Beyond that, the Solidisk implementation introduced proprietary "chained" catalogues which allowed unlimited files per disc (only constrained by the disk size).
[2] Other features in third-party implementations included being able to review free space, and built-in FORMAT and VERIFY commands, which were shipped on a utility disc with the original Acorn DFS.
FM encoding gives half the recording capacity of MFM for a given physical disc density.
As of 2009, 3½" drives are more commonly used with BBC Micros than in the past, including use with DFS, due to their greater availability and easier data interchange with more recent computers.
However, the Intel 8271-based Acorn DFS does not do so, and so dual-format capability was addressed in a number of ways: Failure to use the correct setting would result in errors from the DFS such as Disk fault 18 at 01/00,[5] or damage to the disc drive by trying to step the heads beyond the physical end of the disc surface.
The prevalence of all-capitals filenames is most likely due to the BBC Micro defaulting to caps lock being enabled after a hard or soft reset.
In the interests of saving space, the most significant bit of the directory letter for a file is used as the locked (read-only) flag.
SAVE is also unable to split a file to fit the available space, but as the failure occurs at the sector allocation stage, the error returned is Disk full.
SAVE deletes any existing file and copies the specified block of memory to wherever there is space on the disc.
Like the cassette filing system, the Acorn DFS supports the BBC Micro's standard file metadata: load address and execution address, required because Acorn MOS (the operating system used by the BBC Micro) does not support relocation of binary code.
DFS discs do not track any dates (because Acorn MOS prior to version 3 did not maintain a real-time clock) but instead offer a peculiar feature: a modification count.
If the shift key is held while the machine is soft or hard reset, the DFS checks drive 0 for a disc containing a positive boot flag.
These are not true shell scripts but simply a series of keys to be typed, like a recording to play back.
The DFS was superseded by the Advanced Disc Filing System (ADFS) which was fully hierarchical and was suitable for running hard drives on the BBC Micro.