Advanced Disk Filing System

Acorn's original Disk Filing System (DFS) was a ROM for the BBC Micro. It has an extremely limited design in order to make best use of the 100k floppy discs it was originally designed for. It uses a flat directory structure with one letter to represent the current directory ($ by default). Changing directory meant using a different letter as the prefix. You could refer to a file in a different directory by prefixing it with the letter and a dot, eg "A.LETTER". Names could be up to 7 letters long, plus one letter for the directory. The file table was one disc sector, limiting the number of files on the disc to 31.

Other companies, including Watford Electronics sold their own replacement DFS. They typically added extensions, such as permitting two sectors to be used for the directory, increasing the maximum number of files to 62.

The most dramatic change in the Advanced Disc Filing System, initially introduced for the BBC Master was the hierarchical structure. The filename length expanded from 7 to 10 letters and the number of files in a directory expanded to 77. It retained some superficial attributes from DFS; the directory separator continued to be a dot and $ now indicated the hierarchical root of the filesystem. ^ was used to refer to the parent directory and \\ was the previously visited directory. It supported 3.5" floppy discs, formatted up to 640k capacity.

Later, RISC OS would add a per-file "type" attribute; 12 bits of type information that was used to denote the contents or intended use of a file. This can be thought of as being similar to Apple's resource forks, only with much less information. Later editions of ADFS supported 800k double density floppies, and 1600k high density floppies. It was also used as the hard disc filesystem on the Acorn Archimedes and Risc PC models.