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The Disc Filing System (DFS) is a computer file system developed by Acorn Computers, initially as an add-on to the Eurocard-based Acorn System 2. In 1981, the Education Departments of Western Australia and South Australia announced joint tenders calling for the supply of personal computers to their schools. Acorn's Australian computer distributor, Barson Computers, convinced Joint Managing Directors Hermann Hauser and Chris Curry to allow the soon to be released Acorn BBC Microcomputer to be offered with disk storage as part of the bundle. They agreed on condition that Barson adapted the Acorn DFS from the System 2 without assistance from Acorn as they had no resources available. This required some minor hardware and software changes to make the DFS compatible with the BBC Micro. Barson won the tenders for both states, with the DFS fitted, a year ahead of the UK. It was this early initiative that resulted in the BBC Micro being more heavily focused on the education market in Australia, with very little penetration of the home computer market until the arrival of the Acorn Electron. The DFS shipped as a ROM and Disk Controller Chip fitted to the BBC Micro's motherboard. The filing system was of extremely limited functionality and storage capability, using a flat directory structure. Each filename can be up to seven letters long, plus one letter for the directory in which the file is stored.〔http://acorn.chriswhy.co.uk/docs/Acorn/Manuals/Acorn_DiscSystemUGI2.pdf〕 The DFS is remarkable in that unlike most filing systems, there was no single vendor or implementation. The original DFS was written by Acorn, who continued to maintain their own codebase, but various disc drive vendors wrote their own implementations. Companies who wrote their own DFS implementation included Cumana, Opus and Watford Electronics. The Watford Electronics implementation is notable for supporting 62 files per disc instead of the usual 31, using a non-standard disc format. Other features in third-party implementations included being able to review free space, and built-in FORMAT and VERIFY commands, which were shipped on a utility disc with the original Acorn DFS.Acorn followed up their original DFS series with the Acorn 1770 DFS, which used the same disc format as the earlier version but added a set of extra commands and supported the improved WD1770 floppy drive controller chip. ==Physical format== DFS conventionally uses one side of a double-density 5¼" floppy disc. Discs are formatted as either 40 or 80 track, giving a capacity of 100 or 200 KB per side (ten 256-byte sectors per track, with FM encoding). The capacity is limited by the choice of the Intel 8271 controller in the original BBC Micro, which only supports FM encoding, not the MFM encoding which was already in common use by the time of the BBC Micro's launch. FM encoding gives half the recording capacity of MFM for a given physical disc density. FM and MFM encoding are commonly referred to as "single density" and "double density", although the discs and drives are the same, unlike "high density", which uses different drives and discs. Double-density 3½" discs can be formatted and used with 1770 DFS (the Intel 8271-based DFS has problems with many 3½" drives), giving the same "single-density" capacity with FM encoding, but this was not originally standard practice. 3½" discs were normally formatted as MFM "double density" using the later Advanced Disc Filing System, as this is present in all machines supplied with 3½" drives. As of 2009, 3½" drives are more commonly used with BBC Micros than in the past, including use with DFS, due to their greater availability and easier data interchange with more recent computers. High-density 5¼" and 3½" discs are not supported by DFS. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Disc Filing System」の詳細全文を読む スポンサード リンク
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