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{{
{{infobox filesystem
|name = ADFS
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|introduction_os = [[Acorn MOS]]
|introduction_date = {{Start date and age|1983}}
|partition_id = <
|directory_struct = Hierarchical fixed-length tables
|file_struct = One range per file plus table of free-space ranges (L), bitmap with embedded file IDs (E)
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|max_filename_size = 10 characters
|max_files_no = 47 per directory (L), 77 per directory (E)
|max_volume_size = 512 [[
|max_file_size = 512 [[
|filename_character_set = [[ASCII]] (Acorn MOS), {{nowraplinks|[[ISO 8859-1]]}} (RISC OS)
|dates_recorded = Modification
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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
The BBC Master Compact contained ADFS version 2.0, which provided the addition of format, verify and backup commands in ROM, but omitted support for hard discs.<ref name="acornuser198610a">{{cite news |url=https://archive.org/details/AcornUser051-Oct86/page/n16/mode/1up |work=Acorn User |title=Taking the lid off the Master Compact |date=October 1986 |access-date=5 September 2020 |pages=17}}</ref>
==8-bit usage==
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.{{
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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Support for [[Advanced Technology Attachment|IDE]]/[[ATAPI]] style drives has been added 'unofficially' by third parties in recent years.<ref>{{cite web|url=http://mdfs.net/Info/Comp/BBC/IDE |title=BBC IDE Interface - MDFS::Info.Comp.BBC.IDE |website=MDFS.net |access-date=2016-08-01}}</ref>
The ADFS file format used the
==32-bit usage (Arthur and RISC OS)==
On 32-bit systems, a WD 1770 or 1772 was initially used as a floppy controller on the early machines of the range. Later models utilised a PC style multi-I/O controller requiring slight changes to ADFS. In addition to legacy support for the 'L'-type format, [[Arthur (operating system)|Arthur]] and later [[RISC OS]] provided enhanced formats which overcome the limitations of the BBC Micro.
Arthur added ''D'' format with 77 entries per directory as opposed to the previous 47, also usable on hard discs and a new 800 KB double-density floppy format. A per-file "type" attribute was added in space previously used to store Load and Execute addresses. The 12 bits of type information is used to denote the contents or intended use of a file, typically presented as three [[hexadecimal]] digits. This is similar to the 32-bit type attributes stored in Apple's [[Hierarchical File System (Apple)|HFS]] file system, and conceptually comparable to the more general use of [[MIME media type|MIME Types]] by the [[BeOS|Be Operating System]] (BeOS), or [[Magic number (programming)#Format indicator|magic number]]s in Unix systems (though the latter is stored as part of the file, not as metadata).
RISC OS brought in ''E'' and ''F'' format for double-density discs and high-density discs/hard disc drives respectively. These formats support file fragmentation (with the so-called "new map"), storage of multiple files per fragment and storage of small files in directory tables. The allocation strategy is optimised to minimise fragmentation, and sometimes performs defragmentation as part of a file storage operation.<ref name="reeves-efmt">{{cite web|last=Reeves|first=Nick|date=26 October 1990|title=E format design document|url=http://www.chiark.greenend.org.uk/~theom/riscos/docs/ultimate/a252efmt.txt|access-date=24 May 2013}}<!-- see exception clause in WP:USENET --></ref> RISC OS 4 added ''E+/F+'' format which allowed for long filenames and more than 77 files per directory.{{Citation needed|date=June 2007}} More recent versions of RISC OS, including those for [[Iyonix]], continue to provide ADFS, and have further extended it to cope with larger hard disc sizes.
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{{Reflist}}
{{refbegin}}
* Watford Electronics, "The Advanced Reference Manual for the BBC Master Series", 1988 (p. 169)
* Acorn Computers Ltd, "The BBC Microcomputer System Master Series Reference Manual Part 1", Part No,
{{refend}}
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[[Category:Acorn Computers]]
[[Category:RISC OS]]
[[Category:Disk file systems]]
[[Category:Disk operating systems]]
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