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{{infobox filesystem
|name = ADFS
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|OS = [[Acorn MOS]], [[RISC OS]]
}}
The '''Advanced Disc Filing System''' ('''ADFS''') is a computing [[file system]] unique to the [[Acorn Computers Ltd|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 disc]]s was added (using a [[Western Digital FD1771|WD1770 floppy disc controller]]) and on later [[32-bit]] systems a variant of a PC-style floppy controller.<ref>{{ cite book | url=http://chrisacorns.computinghistory.org.uk/docs/Acorn/Manuals/Acorn_ADFSUG.pdf
Acorn's original [[Disc Filing System]] was limited to 31 files per disk surface, 7 characters per file name and a single character for directory names, a format inherited from the earlier Atom and System 3–5 [[Eurocard (printed circuit board)|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. It retained some superficial attributes from DFS; the directory separator continued to be a dot and <code>$</code> now indicated the hierarchical root of the filesystem. <code>^</code> was used to refer to the parent directory, <code>@</code> the current directory, and <code>\</code> was the previously visited directory.
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ADFS on 8-bit systems required a WD1770 or later 1772-series floppy controller, owing to the inability of the original Intel 8271 chip to cope with the double-density format ADFS required. ADFS could however be used to support hard discs without a 1770 controller present; in development the use of hard discs was the primary goal, extension to handle floppies came later.{{citation needed|date=March 2021}} The 1770 floppy controller was directly incorporated into the design of the Master Series and B+ models,<ref name="acornuser198507">{{ cite news | url=https://archive.org/details/AcornUser036-Jul85/page/n161/mode/2up | title=B+ Grading | work=Acorn User | date=July 1985 | access-date=18 October 2020 | last1=Smith | first1=Bruce | pages=160–161 }}</ref> and was available as an upgrade board for the earlier Model B.<ref name="1770_disc_interface">{{ cite book | url=http://chrisacorns.computinghistory.org.uk/docs/Acorn/Misc/Acorn_1770DiscIF.pdf | title=1770 Disc Interface | publisher=Acorn Computers Limited | access-date=6 March 2021 }}</ref><ref name="1770_disc_upgrade_kit">{{ cite book | title=1770 Disc Interface Upgrade Kit Fitting Instructions | url=http://chrisacorns.computinghistory.org.uk/docs/Acorn/Tech/Acorn_1770DiscIFUpgradeKit.pdf | publisher=Acorn Computers Limited | date=August 1985 | issue=1 | access-date=11 March 2021 }}</ref> ADFS could be added to Model B and B+ systems with an additional upgrade.<ref name="adfs_upgrade">{{ cite book | url=http://chrisacorns.computinghistory.org.uk/docs/Acorn/Misc/Acorn_ADFSUpgrade.pdf | title=ADFS upgrade | publisher=Acorn Computers Limited | access-date=6 March 2021 }}</ref>
The [[Acorn Electron#Acorn Plus 3|Acorn Plus 3]], Acorn's official disc expansion for the Acorn Electron, was supplied with ADFS as standard, but this implementation featured various notable bugs. A file called {{mono|ZYSYSHELP}} was "required by the system" and created during formatting.<ref name="plus3ug">{{ cite book | url=http://chrisacorns.computinghistory.org.uk/docs/Acorn/Manuals/Acorn_Plus3UG.pdf | title=The Electron Plus 3 User Guide | publisher=Acorn Computers Limited | date=October 1984 | issue=1 | access-date=6 March 2021 }}</ref>{{rp|pages=7}} This was a kludge. Acorn's original ADFS implementation on the Electron was unreliable when writing to the first few tracks of a floppy disc, so this was a "fix" and simply involved writing a file full of garbage to the suspect part. The ADFS would then skip it.
On the Electron, disc formatting was done via the {{kbd|*EFORM}} command instead of the established {{kbd|*FORM40}}/{{kbd|*FORM80}} DFS commands. Note additionally that the {{kbd|*EFORM}} command differs from the equivalent {{kbd|*AFORM}} command for the 1770 ADFS on the BBC Microcomputer. This is possibly as a result of needing to create the {{mono|ZYSYSHELP}} file on the Electron. The {{kbd|*EFORM}} command was only supplied on the Welcome disc that was shipped with the Plus3, and was not included in the ROM.<ref name="plus3ug" />{{rp|pages=48}}
ADFS supported hard discs, and 5¼" and 3½" [[floppy disc]]s formatted up to 640 KB capacity using double density [[Modified Frequency Modulation|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 disc support in ADFS used the same format as ''L'' format floppies in terms of 256-byte
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