Disc Filing System
   HOME

TheInfoList



OR:

The Disc Filing System (DFS) is a computer
file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
developed by
Acorn Computers Acorn Computers Ltd. was a British computer company established in Cambridge, England, in 1978. The company produced a number of computers which were especially popular in the United Kingdom, UK, including the Acorn Electron and the Acorn Archi ...
, initially as an add-on to the Eurocard-based
Acorn System 2 The Acorn Eurocard systems were a series of modular microcomputer systems based on rack-mounted Eurocards developed by Acorn Computers from 1979 to 1982, aimed primarily at industrial and laboratory use, but also home enthusiasts. The experience ...
. 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 Hermann Maria Hauser, KBE, FRS, FREng, FInstP, CPhys (born 1948) is an Austrian-born entrepreneur, venture capitalist and inventor who is primarily associated with the Cambridge technology community in England. Education and early life Whe ...
and
Chris Curry Christopher Curry (born 28 January 1946 in Cambridge) is the co-founder of Acorn Computers, with Hermann Hauser and Andy Hopper. He became a millionaire as a result of Acorn's success. In his early career days, he worked at Pye, Royal Radar ...
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 Acorn Electron (nicknamed the Elk inside Acorn and beyond) was a lower-cost alternative to the BBC Micro educational/ home computer, also developed by Acorn Computers Ltd, to provide many of the features of that more expensive machine at a ...
. The DFS shipped as a
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * R ...
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. 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 implementations included Cumana, Solidisk, Opus and
Watford Electronics Watford Electronics was a British computer electronics company. It was founded in 1972 in a bedroom belonging to brothers Nazir and Raza Jessa, and grew to become one of the best-known suppliers of microcomputers and micro peripherals during the 19 ...
. The Watford Electronics implementation is notable for supporting 62 files per disc instead of the usual 31, using a non-standard disc format. Beyond that, the Solidisk implementation introduced proprietary "chained" catalogues which allowed unlimited files per disc (only constrained by the disk size). 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 A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
. 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 Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...
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 The Advanced Disc Filing System (ADFS) is a computing file system unique to the 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 ...
, as this is present in all Acorn 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.


Single- and double-sided operation

The DFS does not directly support double-sided discs; instead, the two heads of a double-sided drive are treated as two separate logical drives. The DFS can support up to four volumes, numbered from 0 to 3. Drive 0 is the default with drive 1 representing a second drive attached to the cable. "Drive" 2 referred to the reverse side of drive 0, and "drive" 3 was the reverse of drive 1. There is no support for more than two physical drives. Due to the installed base of single-sided drives, commercial software was normally provided on single-sided discs, or as " flippy discs" that were manually reversed to access the other side.


40- and 80-track compatibility

Discs can be formatted using 40 or 80 tracks, using the *FORM40 or *FORM80 commands, and drives can be either 40 or 80 track. This is the most common compatibility issue for DFS users: 40-track discs were the norm for commercial software distribution, due to the installed base of 40-track drives, but 80-track drives became more common as prices dropped, allowing users to store more data. An 80-track drive will not automatically read 40-track discs. The disc capacity is stored as a sector count in the catalogue on track zero. Track zero is located in the same place on both 40- and 80-track discs, allowing a disc file system to set the motor stepping accordingly. However, the Intel 8271-based Acorn DFS does not do so, and so dual-format capability was addressed in a number of ways: *by simply attaching both a 40-track drive and an 80-track drive to the BBC Micro, although this was costly for the home user; *some disc drive resellers, notably UFD (User Friendly Devices) and Akhter Computer Group, offered drive assemblies fitted with switches to select 40- or 80-track operation; *magazines such as ''
The Micro User ''The Micro User'' (titled ''BBC Micro User'' in the first three issues) was a British specialist magazine catering to users of the BBC Microcomputer series, Acorn Electron, Acorn Archimedes and, to a limited extent, the Cambridge Z88. It had a ...
'' offered kits to build circuit boards that could be wired into the disc drive cable, optionally 'double-stepping' the attached drives; *''The Micro User'' also published an article on creating dual-format discs, with 21 tracks' worth of data stored in both formats so that either type of drive could access the contents; however these had limited capacity and once created were read-only; *''
Acorn User ''Acorn User'' magazine was founded by Acorn Computers in 1982, contract-published by Addison-Wesley, to coincide with the launch of the BBC Micro. It covered the range of Acorn home computers, the BBC Micro and Atom at first and later the Elect ...
'' magazine distributed 40-track cover discs with a small utility program on track zero, so that owners of 80-track drives could reformat them into 80-track discs with the original contents on the first 40 tracks; or *the user could upgrade to a WD1770 or similar controller. Acorn 1770 DFS and some third-party controller systems provided dual-format capability in software by reprogramming the controller during track seeks; as a bonus, third-party systems offered proprietary MFM (so-called "double-density") formats for even greater disc capacity. Failure to use the correct setting would result in errors from the DFS such as Disk fault 18 at 01/00,Acorn Disc Filing System User Guide, issue no.2, July 1983, page 84. "Disk" is spelled with a "k" in error messages, but "c" in the name of the filing system. or damage to the disc drive by trying to step the heads beyond the physical end of the disc surface. Switching to 80 tracks did not extend the catalogue in any way, leaving the user prone to running out of filename slots before running out of space on the disc. This situation resulted in a Cat full error.


File storage


Filenames

DFS is case-preserving but not case-sensitive. The prevalence of all-capitals filenames is most likely due to the BBC Micro defaulting to caps lock being enabled after a hard or soft reset. The character set is quite permissive, and all printable characters of 7-bit
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
are allowed, including spaces, but excluding: *The single wildcard character #. *The multiple wildcard character *. *
Control codes In computing and telecommunication, a control character or non-printing character (NPC) is a code point (a number) in a character set, that does not represent a written symbol. They are used as in-band signaling to cause effects other than the ...
generated by the shell
escape character In computing and telecommunication, an escape character is a character (computing), character that invokes an alternative interpretation on the following characters in a character sequence. An escape character is a particular case of metacharac ...
, , although the sequence , , can be used to represent a single , character in the filename. *The drive specifier character : as the first character of a ''leaf name'' (the file's name proper). This causes a Bad drive or Bad name error. Where the colon is unambiguous, for example in FOO:BAR, then it is allowed as part of the leaf name. *The directory specifier character . as the first or second character of a leaf name. . cannot be used as a directory character. Where the dot is unambiguous, such as in PRG.BAS, then it is allowed as part of the leaf name, and is ''not'' treated as a directory specifier (whereas F.MONEY would be a file MONEY in directory F). For the sake of portability to third-party DFS implementations, it is best to avoid : and . in leaf names. Quotation marks are allowed, although
BBC BASIC BBC BASIC is a version of the BASIC programming language released in 1981 as the native programming language for the BBC Micro home/personal computer, providing a standardized language for a UK computer literacy project of the BBC. It was wri ...
requires them to be escaped twice: *SAVE """""""A""" passes the string """A" to the DFS, which then saves a file named "A. *Conversely SAVE "A""" saves a file named A". *The same technique is used to insert spaces: SAVE """B A R""" saves a file named B A R. A fully qualified filename, or "file specification" ("fsp" for short) contains a colon then the drive number, a dot, then the directory letter, another dot, and the name. For example, a file in the default directory of "drive" 2 called BOB would have a complete specification of :2.$.BOB. The drive and directory specifiers are both optional.


Directories

"Directories" in the DFS are single character prefixes on filenames - such as F in F.BankLtr - used to group files. The arrangement is flat and a default directory of $ is used instead of a root directory. On requesting a catalogue of the disc (with the *CAT or *. commands), files in the current directory are shown with no directory prefix in one block, and below that are listed all other files in a second block, with their directory prefixes visible. For example, (from Acorn DFS - third party DFS implementations may vary slightly):
PROGRAM (12)
Drive 0             Option 2 (RUN)
Dir. :0.$           Lib. :0.$

    !BOOT               HELLO
    SUMS                TABLE
    TEST                VECTORS
    ZOMBIE

  A.HELLO   L         B.SUMS
  F.BankLtr
The top seven files are all in the current directory which is $ on drive 0. Below that are all the files in other directories, in this case A, B and F. An L after a filename (as with A.HELLO, above) shows the file is locked against modification or deletion. The first line contains the disc title and the modification count. The DFS provides a working space, divided up into the directory and the library. The "directory" is the
working directory In computing, the working directory of a process is a directory of a hierarchical file system, if any, dynamically associated with each process. It is sometimes called the current working directory (CWD), e.g. the BSD getcwd function, or just cur ...
on the current volume, much like the working directory on any other command line system. The "library" is a second, alternative working directory that functions more like ''
PATH A path is a route for physical travel – see Trail. Path or PATH may also refer to: Physical paths of different types * Bicycle path * Bridle path, used by people on horseback * Course (navigation), the intended path of a vehicle * Desire p ...
'' and had the benefit of being able to be on any volume. Requests to open files with unqualified names, will first be searched for in the working directory; failing this, the library directory will also be searched. The directory and library both default to the same directory.


Disc structure

The catalogue (file table) occupies the first two disc sectors: one for the names and directories of each file, and a matching sector holding the file locations, sizes and metadata. Eight bytes of each sector are used for each file. With a further eight bytes from each sector reserved for the 12-byte disc title and the volume information, the total number of files on the disc (irrespective of which directory each file is in) is limited to 31. In the interests of saving space, the most significant bit of the directory letter for a file is used as the locked (read-only) flag.


Volume size

Although physical disks are usually formatted as either 100 KB or 200 KB, DFS supports volume sizes up to 256 KB. The largest DFS file size allowed is the volume size minus ½ KB for the catalogue, as file sizes are stored as an 18-bit quantity.


File allocation

The DFS does not support data fragmentation, meaning a file's data must be stored in a single run of consecutive sectors, but free space is prone to becoming fragmented. Random-access file writes fail when the end of the file reaches the beginning of the next, even though there may be free sectors elsewhere on the disc. In such cases the DFS aborts with a Can't extend error. SAVE is also unable to split a file to fit the available space, but as the failure occurs at the sector allocation stage, the error returned is Disk full. A *COMPACT command is provided to relocate all files on disc to a solid block, placing all the free space after it in a second block. This allows the next file created to fill the disc, but only the last existing file can be extended without being moved. SAVE deletes any existing file and copies the specified block of memory to wherever there is space on the disc. In contrast the *COMPACT command uses program memory as a buffer to relocate the files, overwriting any program and data in memory.


Metadata

Like the cassette filing system, the Acorn DFS supports the BBC Micro's standard file metadata: load address and execution address, required because
Acorn MOS The Machine Operating System (MOS) or OS is a discontinued computer operating system (OS) used in Acorn Computers' BBC computer range. It included support for four-channel sound, graphics, file system abstraction, and digital and analogue input/ ...
(the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
used by the BBC Micro) does not support relocation of binary code. A file should be loaded to the address the programmer intended, as the contents may refer to internal locations by absolute addresses. An execution address is also recorded as the entry point is not necessarily at the beginning, or even within the file. File attributes are limited to a single bit: Locked. When set, an L appears to the right of the file's name in the catalogue and the file may not be altered, overwritten or deleted.


Dates

DFS discs do not track any dates (because Acorn MOS prior to version 3 did not maintain a real-time clock) but instead offer a peculiar feature: a modification count. Every time the catalogue is updated, the count increments. The count is shown in parentheses after the title in the first line of the disc catalogue, such as the 12 in the catalogue listing shown earlier.


Other features

The DFS also supports a means to start up disc software based on a key sequence. If the shift key is held while the machine is soft or hard reset, the DFS checks drive 0 for a disc containing a positive boot flag. The boot flag is either 0 (ignore), 1 (load file), 2 (run machine code file) or 3 ("execute" script). If the boot flag is positive, a file called $.!BOOT is looked for and loaded into memory (1), loaded and executed as machine code (2) or fed into the keyboard buffer (3). Option 3 reads "EXEC" files, text macro files used as primitive
shell script A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be scripting languages. Typical operations performed by shell scripts include file manip ...
s. These are not true shell scripts but simply a series of keys to be typed, like a recording to play back. Thus, they cannot loop or branch unless they input such code into the BASIC interpreter. As well as being used during a reset, they can be executed at any time with the operating system's *EXEC command. EXEC files are file system independent.


Alternatives

There was a variant of the DFS called the DNFS, or Disc/Network Filing System, that contained the Econet Network Filing System (NFS), standard Disc Filing System and Tube co-processor support software on a single ROM; this ROM installed two filing systems into the OS at once. The initial design for the DFS was based around an Intel Corporation FDC 8271 disc drive controller, the immediate predecessor of the 8272 design found in the
IBM Personal Computer The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a team ...
. The 8271 controller was of limited functionality and obsolete, and later versions of the file system from various vendors including Solidisk, Acorn and Watford Electronics were based on the later WD1770 and WD1772 drive controllers. The 1770 controller quickly became the standard controller in the Acorn range following the advent of ADFS. The DFS was superseded by the Advanced Disc Filing System (ADFS) which was fully hierarchical and was suitable for running hard drives on the BBC Micro. The ADFS was the default filing system on the
BBC Master The BBC Master is a home computer released by Acorn Computers in early 1986. It was designed and built for the BBC, British Broadcasting Corporation (BBC) and was the successor to the BBC Micro, BBC Micro Model B. The Master 128 remained in produ ...
but most users are likely to have reverted to the DFS for compatibility reasons; ADFS could not read DFS discs. The ADFS was also chosen as the standard filing system for
RISC OS RISC OS is a computer operating system originally designed by Acorn Computers Ltd in Cambridge, England. First released in 1987, it was designed to run on the ARM chipset, which Acorn had designed concurrently for use in its new line of Archim ...
.


References


External links


Acorn, Watford and Duggan DFS file system structureWatford DFS (extended Acorn DFS) specification
- includes the original Acorn DFS specification
Acorn DFS manual
(RTF format) {{File systems Disk file systems Acorn Computers