Advanced disk filing system
BBC microcomputer filing system that supported double density floppy diskettes (200K single sided, 400K double sided, twice as much with an 80 track drive, but still with double sided diskettes considered 2 separate drives) using the Western Digital 1770 controller chip. It supported a nice nested directory structure.

The original ADFS was made by Acorn; however, other manufacturers (such as Watford Electronics) soon made various compatible, nearly-compatible, and incompatible kits based on the 1770. This made a lot of sense, since the 8271 had already been discontinued, and its price grew steadily throughout the lifetime of the DFS.