Advanced Disc Filing System: Difference between revisions

Revision as of 19:26, 5 November 2012

Revision as of 20:27, 8 January 2013

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Acorn's original [[Disc Filing System]] was rather limited in that few files could be stored on a disk, and directory and file names were restricted to a minimum of 1 and a maximum of 7 characters. The Disc Filing System's limitations were in part due to its basis on the disc firmware used in the earlier Acorn 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>" (minus the quotes) was used to refer to the parent directory and "<code>\</code>" was the previously-visited directory.

The BBC Master Compact contained ADFS Version 2.0, which provided the addition of format, verify and backup commands in ROM.<ref>[Acorn User October 1986 - Review - Page 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 a hard disc without a 1770 controller present. The 1770 floppy controller was directly incorporated into the design of the Master Series and B+ models{{FactCitation needed|date=June 2007}}, and was available as an 'upgrade' board for the earlier Model B. The Acorn Electron's floppy interface (Acorn Plus 3) was an add-on unit, initially available through Acorn and later Pres (aka Advanced Computer Products). The ACP implementation of ADFS fixed a flaw in the Acorn version v1.0, that required the use of a file named ZYSYSHELP. On the Electron, Disk corruption could also occur if attempting to use the <tt>*COMPACT</tt> command without disabling the blinking cursor. This was due to the fact that the <tt>*COMPACT</tt> command used screen memory as working space during the operation, and the blinking cursor corrupted that memory space.<ref>http://www.portices.fr/formation/Res/Info/Dimet/Ordinateurs/AcornBBC/www.stairwaytohell.com/Ressources/r-EU-ACP-E00ADFS.html</ref>.

ADFS supported hard discs, and 3½" [[floppy disc]]s formatted up to 640&nbsp;KB capacity using double density [[Modified Frequency Modulation|MFM]] encoding (''L'' format; single-sided disks were supported with the ''S'' format (160&nbsp;KB) and ''M'' format (320&nbsp;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 a modified format, and interfaced to a SCSI-based Winchester unit via the BBC Micro's 1&nbsp;MHz Bus.{{FactCitation needed|date=June 2007}} Support for [[Advanced Technology Attachment|IDE]]/[[ATAPI]] style drives has been added 'unofficially' by third parties in recent years.<ref>[http://mdfs.net/Info/Comp/BBC/IDE BBC IDE Interface]</ref>

== 32-bit usage (Arthur and RISC OS) ==

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Arthur added ''D'' format with 77 entries per directory as opposed to the previous 47, also usable on hard discs and a new 800&nbsp;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|HFS]] file system, and conceptually comparable to the more general use of [[MIME#Content-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 later ''F'') format for double and high-density discs respectively. These formats support file fragmentation (with the so-called "new map"). RISC OS 4 added ''E+'' format which allowed for long filenames and more than 77 files per directory.{{FactCitation 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.

Unlike the 8-bit implementation, ADFS as implemented on RISC OS is not monolithic. A system module called "ADFS" provides no more than the block driver and user interfaces, where the "FileCore" module contains the actual file system implementation, and FileSwitch contains the VFS and high-level file-access API implementations. This allows for other hardware to use the ADFS format easily, such as IDEFS (commonly used for IDE add-on cards), SCSIFS, and the network-aware AppFS. FileCore and FileSwitch's functions are in some ways similar to the IFS and IO system managers in Windows NT. This flexibility has allowed other filing systems to be implemented into RISC OS relatively easily.

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== Notes ==

{{reflist}}

== References ==

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