Floppy disk variants

The floppy disk was a ubiquitous data storage and transfer device from the mid-1970s well into the 2000s.^[1] Besides the 3½-inch and 5¼-inch formats used in IBM PC compatible systems, many proprietary floppy disk formats were developed, either using a different disk design or special layout and encoding methods for the data held on the disk.

Commodore 64/128

Commodore started its tradition of special disk formats with the 5¼-inch disk drives accompanying its PET/CBM, VIC-20 and Commodore 64 home computers, the same as the 1540 and 1541 drives used with the later two machines. The standard Commodore Group Code Recording (GCR) scheme used in 1541 and compatibles employed four different data rates depending upon track position (see zone bit recording). Tracks 1 to 17 had 21 sectors, 18 to 24 had 19, 25 to 30 had 18, and 31 to 35 had 17, for a disk capacity of 170 kB (170.75 KB). Unique among personal computer architectures, the operating system on the computer itself was unaware of the details of the disk and filesystem; disk operations were handled by Commodore DOS instead, which was implemented with an extra MOS-6502 processor on the disk drive. Many programs such as GEOS bypassed Commodore's DOS completely, and replaced it with "fast loading" programs in the 1541 drive.

Eventually Commodore gave in to disk format standardization, and made its last 5¼-inch drives, the 1570 and 1571, compatible with Modified Frequency Modulation (MFM), to enable the Commodore 128 to work with CP/M disks from several vendors. Equipped with one of these drives, the C128 was able to access both C64 and CP/M disks, as it needed to, as well as MS-DOS disks (using third-party software), which was a crucial feature for some office work. At least one commercial program, Big Blue Reader by SOGWAP software was available to perform the task.

Commodore also developed a 3½-inch 800 kByte disk format for its 8-bit machines with the 1581 disk drive, which used only MFM.

The GEOS operating system used a disk format that was largely identical to the Commodore DOS format with a few minor extensions; while generally compatible with standard Commodore disks, certain disk maintenance operations could corrupt the filesystem without proper supervision from the GEOS Kernel.

Atari 8-bit line

The combination of DOS and hardware (810, 1050 and XF551 disk drives) for Atari 8-bit floppy usage allowed sectors numbered from 1 to 720. The DOS' 2.0 disk bitmap provides information on sector allocation, counts from 0 to 719. As a result, sector 720 could not be written to by the DOS. Some companies used a copy protection scheme where "hidden" data was put in sector 720 that could not be copied through the DOS copy option. Another more-common early copy-protected scheme simply did not record important sectors as "used" in the FAT, so the DOS Utility Package (DUP) did not duplicate them. All of these early techniques were thwarted by the first program that simply duplicated all 720 sectors.

Later DOS versions (3.0 and later 2.5) and DOS systems by third parties (i.e. OSS) accepted (and formatted) disks with up to 960 and 1020 sectors, resulting in 127KB storage capacity per disk side on drives equipped with double-density heads (i.e. not the Atari 810) vs. previous 90KB. That unusual 127K format allowed sectors 1-720 to still be read on a single-density 810 disk drive, and was introduced by Atari with the 1050 drive with the introduction of DOS 3.0 in 1983.

A true 180K double-density Atari floppy format used 128 byte sectors for sectors 1-3, then 256 byte sectors for 4-720. The first three sectors typically contain boot code as used by the onboard ROM OS; it is up to the resulting boot program (such as SpartaDOS) to recognize the density of the formatted disk structure. While this 180K format was developed by Atari for their DOS 2.0D and their (canceled) Atari 815 Floppy Drive, that double-density DOS was never widely released and the format was generally used by third-party DOS products. Under the Atari DOS scheme, sector 360 was the FAT sector map, and sectors 361-367 contained the file listing. The Atari-brand DOS versions and compatible used three bytes per sector for housekeeping and to link-list to the next sector.

Third-party DOS systems added features such as double-sided drives, subdirectories, and drive types such as 1.2 MByte and 8". Well-known 3rd party Atari DOS products included SmartDOS (distributed with the Rana disk drive), TopDos, MyDos and SpartaDOS.

Commodore Amiga

The Commodore Amiga computers used an 880 kByte format (11×512-byte sectors per track, times 80 tracks, times two sides) on a 3½-inch floppy. Because the entire track is written at once, inter-sector gaps could be eliminated, saving space. The Amiga floppy controller was basic but much more flexible than the one on the PC: it was free of arbitrary format restrictions, encoding such as MFM and GCR could be done in software, and developers were able to create their own proprietary disc formats. Because of this, foreign formats such as the IBM PC-compatible could be handled with ease (by use of CrossDOS, which was included with later versions of AmigaOS). With the correct filesystem driver, an Amiga could theoretically read any arbitrary format on the 3½-inch floppy, including those recorded at a slightly different rotation rate. On the PC, however, there is no way to read an Amiga disk without special hardware, such as a CatWeasel, and a second floppy drive.^[2]

Commodore never upgraded the Amiga chip set to support high-density floppies, but sold a custom drive (made by Chinon) that spun at half speed (150 RPM) when a high-density floppy was inserted, enabling the existing floppy controller to be used. This drive was introduced with the launch of the Amiga 3000, although the later Amiga 1200 was only fitted with the standard DD drive. The Amiga HD disks could handle 1760 kByte, but using special software programs it could hold even more data. A company named Kolff Computer Supplies also made an external HD floppy drive (KCS Dual HD Drive) available which could handle HD format diskettes on all Amiga computer systems.^[3]

Because of storage reasons, the use of emulators and preserving data, many disks were packed into disk-images. Currently popular formats are .ADF (Amiga Disk File), .DMS (DiskMasher) and .IPF (Interchangeable Preservation Format) files. The DiskMasher format is copyright-protected and has problems storing particular sequences of bits due to bugs in the compression algorithm, but was widely used in the pirate and demo scenes. ADF has been around for almost as long as the Amiga itself though it was not initially called by that name. Only with the advent of the Internet and Amiga emulators has it become a popular way of distributing disk images. The proprietary IPF files were created to allow preservation of commercial games which have copy protection, which is something that ADF and DMS unfortunately cannot do.

Acorn Electron, BBC Micro, and Acorn Archimedes

The British company Acorn Computers used non-standard disk formats in their 8-bit BBC Micro and Acorn Electron, and their successor the 32-bit Acorn Archimedes. Acorn however, used standard disk controllers: initially FM, though they quickly transitioned to MFM. The original disk implementation for the BBC Micro stored 100 KB (40 track) or 200 KB (80 track) per side on 5¼-inch disks in a custom format using the Disc Filing System (DFS).

Due to the incompatibility between 40 and 80 track drives, much software was distributed on combined 40/80 track discs. These worked by writing the same data in pairs of consecutive tracks in 80 track format, and including a small loader program on track 1 (which is in the same physical position in either format). The loader program detected which type of drive was in use, and loaded the main software program straight from disc bypassing the DFS, double-stepping for 80 track drives and single-stepping for 40 track. This effectively achieved downgraded capacity to 100 KB from either disk format, but enabled distributed software to be effectively compatible with either drive.

For their Electron floppy disk add-on added, Acorn picked 3½-inch disks and developed the Advanced Disc Filing System (ADFS). It used double-density recording and added the ability to treat both sides of the disk as a single drive. This offered three formats:

• S (small): 160 KB, 40-track single-sided;
• M (medium): 320 KB, 80-track single-sided;
• L (large): 640 KB, 80-track double-sided.

ADFS provided hierarchical directory structure, rather than the flat model of DFS. ADFS also stored some metadata about each file, notably a load address, an execution address, owner and public privileges, and a "lock" bit. Even on the eight-bit machines, load addresses were stored in 32-bit format, since those machines supported 16 and 32-bit coprocessors.

The ADFS format was later adopted into the BBC line upon release of the BBC Master. The BBC Master Compact marked the move to 3½-inch disks, using the same ADFS formats.

The Acorn Archimedes added D format, which increased the number of objects per directory from 44 to 77 and increased the storage space to 800 KB. The extra space was obtained by using 1024 byte sectors instead of the usual 512 bytes, thus reducing the space needed for inter-sector gaps. As a further enhancement, successive tracks were offset by a sector, giving time for the head to advance to the next track without missing the first sector, thus increasing bulk throughput. The Archimedes used special values in the ADFS load/execute address metadata to store a 12-bit filetype field and a 40-bit timestamp.

RISC OS 2 introduced E format, which retained the same physical layout as D format, but supported file fragmentation and auto-compaction. Post-1991 machines including the A5000 and Risc PC added support for high-density disks with F format, storing 1600 KB. However, the PC combo IO chips used were unable to format disks with sector skew, losing some performance. ADFS and the PC controllers also support extended-density disks as G format, storing 3200 KB, but ED drives were never fitted to production machines.

With RISC OS 3, the Archimedes could also read and write disk formats from other machines, for example the Atari ST and the IBM PC. With third party software it could even read the BBC Micro's original single density 5¼-inch DFS disks. The Amiga's disks could not be read as they used unusual sector gap markers.

The Acorn filesystem design was interesting because all ADFS-based storage devices connected to a module called FileCore which provided almost all the features required to implement an ADFS-compatible filesystem. Because of this modular design, it was easy in RISC OS 3 to add support for so-called image filing systems. These were used to implement completely transparent support for IBM PC format floppy disks, including the slightly different Atari ST format. Computer Concepts released a package that implemented an image filing system to allow access to high density Macintosh format disks.

3" diskettes: Amstrad CPC, PCW, Spectrum, etc.

The 3-inch disk format was widely used by Amstrad CPC and Amstrad PCW machines, became available for Spectrum systems once Amstrad took over their manufacture, and was also adopted by some other manufacturers/systems such as the Tatung Einstein. Three-inch diskettes bear much similarity to the 3½-inch type, with some unique features. One example is the rectangular-shaped plastic casing, almost taller than a 3½-inch disk, but less wide and thicker (i.e. with increased depth). The actual 3-inch magnetic-coated disk occupied less than 50% of the space inside the casing, the rest being used by the complex protection and sealing mechanisms implemented on the disks, which thus were largely responsible for the thickness, length, and relatively high costs of the 3-inch disks. On the early Amstrad machines (the CPC line and the PCW 8256), the disks were typically flipped over to change the side, as opposed to being truly double-sided. Double-sided mechanisms were introduced on the later PCW 8512 and PCW 9512, thus removing the need to remove, flip, and reinsert the disk.

IBM DemiDiskettes

In the early 80s, IBM Rochester developed a 4-inch floppy diskette, the DemiDiskette. This program was driven by aggressive cost goals, but missed the pulse of the industry. The prospective users, both inside and outside IBM, preferred standardization to what by release time were small cost reductions, and were unwilling to retool packaging, interface chips and applications for a proprietary design. The product never appeared in the light of day, and IBM wrote off several hundred million dollars of development and manufacturing facility. IBM obtained patent number U.S. Patent 4,482,929 on the media and the drive for the DemiDiskette. At trade shows, the drive and media were labeled "Brown" and "Tabor".^{[citation needed]}

Flippy disks

A flippy disk (sometimes known as a "flippy") is a double-sided 5¼" floppy disk, specially modified so that the two sides can be used independently (but not simultaneously) in single-sided drives. Compute! published an article on the topic in March 1981.^[4]

Generally, there were two levels of modifications:

• For Disk Operating Systems that did not use the index hole in the disk to mark the beginnings of tracks, the "flippy" modification required only a new write-enable notch to be cut. For this purpose, specially designed single-square-hole hole punchers, commonly known as disk doublers, were produced and sold by third-party computer accessory manufacturers. Many users, however, made do with a standard (round) hole puncher and/or an ordinary pair of scissors for this job.

• For disk operating systems that did use index sync, a second index hole window had to be punched in both sides of the jacket, and for hard sectored formats, an additional window must be punched for the sector holes. While cutting a second notch was relatively safe, cutting additional windows into the jacket was at great peril to the disk within.

A number of floppy disk manufacturers produced ready-made "flippy" media. As the cost of media went down, and double-sided drives became the standard, "flippies" became obsolete.

Auto-loaders

IBM developed, and several companies copied, an autoloader mechanism that could load a stack of floppies one at a time into a drive unit. These were very bulky systems, and suffered from media hangups and chew-ups more than standard drives, ^{[citation needed]} but they were a partial answer to replication and large removable storage needs. The smaller 5¼- and 3½-inch floppy made this a much easier technology to perfect.

Floppy mass storage

A number of companies, including IBM and Burroughs, experimented with using large numbers of unenclosed disks to create massive amounts of storage. The Burroughs system used a stack of 256 12-inch disks, spinning at a high speed. The disk to be accessed was selected by using air jets to part the stack, and then a pair of heads flew over the surface as in any standard hard disk drive. This approach in some ways anticipated the Bernoulli disk technology implemented in the Iomega Bernoulli Box, but head crashes or air failures were spectacularly messy. The program did not reach production.

2-inch floppy disks

At least two mutually-incompatible floppy disks measuring two inches appeared in the 1980s.

One of these, officially referred to as a Video Floppy (or VF for short) was used to store video information for still video cameras such as the original Sony Mavica (not to be confused with later Digital Mavica models) and the Ion and Xapshot cameras from Canon. VF was not a digital data format; each track on the disk stored one video field in the analog interlaced composite video format in either the North American NTSC or European PAL standard. This yielded a capacity of 25 images per disk in frame mode and 50 in field mode.

Another one, the LT-1, was digitally formatted - 720 kB, 245TPI, 80 tracks/side, double-sided, double-density. They were used exclusively in the Zenith Minisport laptop computer circa 1989. Although the media exhibited nearly identical performance to the 3½-inch disks of the time, they were not successful. This was due in part to the scarcity of other devices using this drive making it impractical for software transfer, and high media cost which was much more than 3½-inch and 5¼-inch disks of the time.

Standard floppy replacements

Through the early 1990s a number of attempts were made by various companies to introduce newer floppy-like formats based on the now-universal 3½-inch physical format. Most of these systems provided the ability to read and write standard DD and HD disks, while at the same time introducing a much higher-capacity format as well. None of these ever reached the point where it could be assumed that every current PC would have one, and they have now largely been replaced by CD and DVD burners and USB flash drives. Nevertheless, the 5¼ and 3½-inch sizes remain to this day as the standard for drive bays in computer cases, the former used for CD and DVD (including Blu-ray), and the latter for hard disk drives.

The main technological change for the higher-capacity formats was the addition of tracking information on the disk surface to allow the read/write heads to be positioned more accurately. Normal disks have no such information, so the drives use feedforward (blind) positioning by a stepper motor in order to position their heads over the desired track. For good interoperability of disks among drives, this requires precise alignment of the drive heads to a reference standard, somewhat similar to the alignment required to get the best performance out of an audio tape deck. The newer systems generally used marks burned onto the surface of the disk to find the tracks, allowing the track width to be greatly reduced.

Flextra

As early as 1988, Brier Technology introduced the Flextra BR 3020, which boasted 21.4 MB (a value used for marketing: its true size was 21,040 KB, 2 sides × 526 cyl × 40 tracks × 512 bytes or 25 MB unformatted). Later the same year it introduced the BR3225, which doubled the capacity. This model could also read standard 3½-inch disks.

It used 3½-inch standard disks which had servo information embedded on them for use with the Twin Tier Tracking technology.

Original Floptical

In 1991, Insite Peripherals introduced the "Floptical", which used an infra-red LED to position the heads over marks in the disk surface. The original drive stored 21 MB, while also reading and writing standard DD and HD floppies. In order to improve data transfer speeds and make the high-capacity drive usefully quick as well, the drives were attached to the system using a SCSI connector instead of the normal floppy controller. This made them appear to the operating system as a hard drive instead of a floppy, meaning that most PCs were unable to boot from them. This again adversely affected pickup rates.

Insite licensed their technology to a number of companies, who introduced compatible devices as well as even larger-capacity formats. The most popular of these, by far, was the LS-120, mentioned below.

Zip drive

In 1994, Iomega introduced the Zip drive. Although it did not conform to the 3½-inch form factor and hence was not compatible with standard 1.44 MB drives, it still became the most popular of the "super floppies". It boasted 100 MB, later 250 MB, and then 750 MB of storage. Though Zip drives gained in popularity for several years they never reached the same market penetration as standard floppy drives, since only some new computers were sold with the drives. Eventually the falling prices of CD-R and CD-RW media and USB flash drives, along with notorious hardware failures (the so-called "click of death"), reduced the popularity of the Zip drive.

A major reason for the failure of the Zip Drives is also attributed to the higher pricing they carried (partly because of royalties that 3rd-party manufacturers of drives and disks had to pay). Zip drive media was primarily popular for the excellent storage density and drive speed they carried, but were always overshadowed by the price.

LS-120

Announced in 1995, the "SuperDisk" marketed as the LS-120 drive, often seen with the brand names Matsushita (Panasonic) and Imation, had an initial capacity of 120 MB (120.375 MB)^[5] using even higher density "LS-120" disks.

LS in this case stands for Laser Servo,^[6] which used a very low power superluminescent LED that generated light with a small focal spot. This allowed the drive to align its rotation to precisely the same point each time, allowing far more data to be written due to the absence of conventional magnetic alignment marks. The alignment was based on hard coded optical alignment marks, which meant that a complete format could be done. This worked very well at the time and as a result failures associated with magnetic fields wiping the ZIP drive alignment Z tracks were less of a problem.

It was upgraded (as the "LS-240") to 240 MB (240.75 MB). Not only could the drive read and write 1440 kB disks, but the last versions of the drives could write 32 MB onto a normal 1440 kB disk (see note below). Unfortunately, popular opinion held the Super Disk disks to be quite unreliable,^{[citation needed]} though no more so than the Zip drives and SyQuest Technology offerings of the same period and there were also many reported problems moving standard floppies between LS-120 drives and normal floppy drives.^{[citation needed]} This belief, true or otherwise, crippled adoption. The BIOS of many motherboards even to this day supports LS-120 drives as boot options.

One suggested improvement to LS-240 was the addition of an optical reader based on low resolution B/W CCD technology, as this would be able to detect disk flaws before they could cause data loss and adjust write strategies accordingly. Also it would allow detection of damage caused by head misalignment before the data surface was compromised. Had this ever been implemented then it would have allowed the LS240 to store nearly 500MB of data. This strategy was later implemented on HP and Epson printers to allow photo quality printing on normal non-photo paper.

LS-120 compatible drives were available as options on many computers, including desktop and notebook computers from Compaq Computer Corporation. In the case of the Compaq notebooks, the LS-120 drive replaced the standard floppy drive in a multibay configuration.

Sony HiFD

Sony introduced its own floptical-like system in 1997 as the "150 MB Sony HiFD" which could hold 150 megabytes (157.3 actual megabytes) of data. Although by this time the LS-120 had already garnered some market penetration, industry observers nevertheless confidently predicted the HiFD would be the real standard-floppy-killer and finally replace standard floppies in all machines.

After only a short time on the market the product was pulled, as it was discovered there were a number of performance and reliability problems that made the system essentially unusable. Sony then re-engineered the device for a quick re-release, but then extended the delay well into 1998 instead, and increased the capacity to "200 MB" (approximately 210 megabytes) while they were at it. By this point the market was already saturated by the Zip disk, so it never gained much market share.

Caleb Technology’s UHD144

The UHD144 drive surfaced early in 1998 as the it drive, and provided 144 MB of storage while also being compatible with the standard 1.44 MB floppies. The drive was slower than its competitors but the media were cheaper, running about 8 US$ at introduction and 5 US$ soon after.

See also

• dd (Unix)
• Disk storage
• Don't Copy That Floppy
• Floppy disk controller
• Floppy disk format
• Floppy disk hardware emulator
• Group code recording
• History of the floppy disk
• RaWrite2
• Semi-virtual diskette
• Sneakernet

References

Bibliography

• Weyhrich, Steven (2005). "The Disk II": A detailed essay describing one of the first commercial floppy disk drives (from the Apple II History website)
• Immers, Richard; Neufeld, Gerald G. (1984). Inside Commodore DOS. The Complete Guide to the 1541 Disk Operating System. DATAMOST, Inc & Reston Publishing Company, Inc. (Prentice-Hall). ISBN 0-8359-3091-2.
• Englisch, Lothar; Szczepanowski, Norbert (1984). The Anatomy of the 1541 Disk Drive. Grand Rapids, MI: Abacus Software (translated from the original 1983 German edition, Düsseldorf: Data Becker GmbH). ISBN 0-916439-01-1.
• Hewlett Packard: 9121D/S Disc Memory Operator's Manual; Printed 1 September 1982; Part No. 09121-90000

External links

Wikimedia Commons has media related to Floppy disk variants.

• Programming Floppy Disk Controllers
• HowStuffWorks: How Floppy Disk Drives Work
• Computer Hope: Information about computer floppy drives
• NCITS (mention of ANSI X3.162 and X3.171 floppy standards)
• Floppy disk drives and media technical information
• War of the Microdisks Popular Science, Dec. 1983

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