A floppy disk is a data storage medium that is composed of a disk of thin, flexible ("floppy") magnetic storage medium encased in a square or rectangular plastic shell.

Floppy disks are read and written by a floppy disk drive or FDD, the initials of which should not be confused with "fixed disk drive", which is another term for a (nonremovable) type of hard disk drive. Invented by IBM, floppy disks in 8-inch (200 mm), 5¼-inch (133.35 mm), and 3½-inch (90 mm) formats enjoyed many years as a popular and ubiquitous form of data storage and exchange, from the mid-1970s to the late 1990s. While floppy disk drives still have some limited uses, especially with legacy industrial computer equipment, they have now been largely superseded by USB flash drives, external hard disk drives, CDs, DVDs, and memory cards.

Usage

The flexible magnetic disk, or diskette revolutionized computer disk storage in the 1970s. Diskettes, which were often called floppy disks or floppies, became ubiquitous in the 1980s and 1990s in their use with personal computers and home computers to distribute software, transfer data, and create backups.

Before hard disks became affordable, floppy disks were often also used to store a computer's operating system (OS), in addition to application software and data. Most home computers had a primary OS (and often BASIC) stored permanently in on-board ROM, with the option of loading a more advanced disk operating system from a floppy, whether it be a proprietary system, CP/M, or later, DOS.

By the early 1990s, the increasing size of software meant that many programs demanded multiple diskettes; a large package like Windows or Adobe Photoshop could use a dozen disks or more. Toward the end of the 1990s, distribution of larger packages therefore gradually switched to CD-ROM (or online distribution for smaller programs).

Mechanically incompatible higher-density formats were introduced (e.g. the Iomega Zip drive) and were briefly popular, but adoption was limited by the competition between proprietary formats, and the need to buy expensive drives for computers where the media would be used. In some cases, such as with the Zip drive, the failure in market penetration was exacerbated by the release of newer higher-capacity versions of the drive and media that were not forward-compatible with the original drives, thus fragmenting the user base between new users and early adopters who were unwilling to pay for an upgrade so soon. A chicken or the egg scenario ensued, with consumers wary of making costly investments into unproven and rapidly changing technologies, with the result that none of the technologies were able to prove themselves and stabilize their market presence. Soon, inexpensive recordable CDs with even greater capacity, which were also compatible with an existing infrastructure of CD-ROM drives, made the new floppy technologies redundant. The last advantage of floppy disks, reusability, was countered by re-writable CDs. Later, advancements in flash-based devices and widespread adoption of the USB interface provided another alternative that, in turn, made even optical storage obsolete for some purposes.

An attempt to continue the traditional diskette was the SuperDisk (LS-120) in the late 1990s, with a capacity of 120 MB which was backward compatible with standard 3½-inch floppies. For some time, PC manufacturers were reluctant to remove the floppy drive because many IT departments appreciated a built-in file transfer mechanism that always worked and required no device driver to operate properly. However, manufacturers and retailers have progressively reduced the availability of computers fitted with floppy drives and of the disks themselves. Widespread built-in operating system support for USB flash drives, and even BIOS boot support for such devices on most modern systems, has helped this process along.

External USB-based floppy disk drives are available for computers without floppy drives, and they work on any machine that supports USB Mass Storage Devices. Many modern systems even provide firmware support for booting to a USB-mounted floppy drive.However these drives can't handle anything but the common 80-track MFM format. Which means that formats used by C64, Amiga, Macintosh, etc. can't be read by these devices.

Windows XP still requires the use of floppy drives to install third-party RAID, SATA, and AHCI hard drives, unless the install CD is modified to include these drivers. Customized Windows XP install CDs can be made with programs such as nLite. This requirement was dropped only with the introduction of Windows Vista in 2007. Most PC motherboards will still attempt to boot from a floppy drive, depending on CMOS settings.

Disk formats

Floppy sizes are almost universally referred to in imperial measurements, even in countries where metric is the standard, and even when the size is in fact defined in metric (for instance the 3½-inch floppy, which is actually 90 mm). Formatted capacities are generally set in terms of binary kilobytes (as 1 sector is generally 512 bytes). For more information see below.

Historical sequence of floppy disk formats, including the last format to be generally adopted — the "High Density" 3½-inch HD floppy, introduced 1987.

Disk format	Year introduced	Formatted Storage capacity in KiB (1024 bytes) if not stated	Marketed capacity¹
8-inch - IBM 23FD (read-only)	1971	79.7	?
8-inch - Memorex 650	1972	175 kB	1.5 megabit [unformatted]
8-inch - SSSD IBM 33FD / Shugart 901	1973	237.25	3.1 Mbits unformatted
8-inch - DSSD IBM 43FD / Shugart 850	1976	500.5	6.2 Mbits unformatted
5¼-inch (35 track) Shugart SA 400	1976	89.6 kB	110 kB
8-inch DSDD IBM 53FD / Shugart 850	1977	980 (CP/M) - 1200 (MS-DOS FAT)	1.2 MB
5¼-inch DD	1978	360 or 800	360 KB
5¼-inch Apple Disk II (Pre-DOS 3.3)	1978	113.75 (256 byte sectors, 13 sectors/track, 35 tracks)	113 KB
5¼-inch Apple Disk II (DOS 3.3)	1980	140 (256 byte sectors, 16 sectors/track, 35 tracks)	140 KB
3½-inch HP single sided	1982	280	264 kB
3-inch	1982	360	125 kB (SS/SD), 500 kB (DS/DD)
3½-inch (DD at release)	1984	720 (400 SS, 800 DS on Macintosh, 880 DS on Amiga)	1 MB
5¼-inch QD		720	720 KB
5¼-inch HD	1982 YE Data YD380	1,182,720 bytes	1.2 MB
3-inch DD	1984	720	?
3-inch Mitsumi Quick Disk	1985	128 to 256	?
2-inch	1985	720	?
2½-inch	1986	?	?
5¼-inch Perpendicular	1986	10 MB	?
3½-inch HD	1987	1440	1.44 MB (2.0 MB unformatted)
3½-inch ED	1987	2880	2.88 MB
3½-inch Floptical (LS)	1991	21000	21 MB
3½-inch LS-120	1996	120.375 MB	120 MB
3½-inch LS-240	1997	240.75 MB	240 MB
3½-inch HiFD	1998/99	150/200 MB	150/200 MB
Abbreviations:
¹ The formatted capacities of floppy disks frequently corresponded only vaguely to their capacities as marketed by drive and media companies, due to differences between formatted and unformatted capacities and also due to the non-standard use of binary prefixes in labeling and advertising floppy media. The erroneous "1.44 MB" value for the 3½-inch HD floppies is the most widely known example. See Ultimate capacity and speed.
Dates and capacities marked ? are of unclear origin and need source information; other listed capacities refer to: Formatted Storage Capacity is total size of all sectors on the disk: For 8-inch see Table of 8-inch floppy formats IBM 8-inch formats. Note that spare, hidden and otherwise reserved sectors are included in this number. For 5¼- and 3½-inch capacities quoted are from subsystem or system vendor statements. Marketed Capacity is the capacity, typically unformatted, by the original media OEM vendor or in the case of IBM media, the first OEM thereafter.Other formats may get more or less capacity from the same drives and disks.

History

The earliest floppy disks, invented at IBM, were 8 inches in diameter. They became commercially available in 1971. Disks in this form factor were produced and improved upon by IBM and other companies such as Memorex, Shugart Associates, and Burroughs Corporation..

In 1976 Shugart Associates introduced the first 5¼-inch FDD and associated media. By 1978 there were more than 10 manufacturers producing 5¼-inch FDDs, in competing disk formats: hard or soft sectored with various encoding schemes such as FM, MFM and GCR. The 5¼-inch formats quickly displaced the 8-inch for most applications, and the 5¼-inch hard-sectored disk format eventually disappeared.

In 1984, IBM introduced the 1.2 megabyte dual sided floppy disk along with its AT model. Although often used as backup storage, the high density floppy was not often used by software manufacturers for interchangeability. In 1986, IBM began to use the 720 kB double density 3.5" microfloppy disk on its Convertible laptop computer. It introduced the so-called "1.44 MB" high density version with the PS/2 line. These disk drives could be added to existing older model PCs. In 1988 IBM introduced a drive for 2.88 MB "DSED" diskettes in its top-of-the-line PS/2 models; it was a commercial failure.

Throughout the early 1980s the limitations of the 5¼-inch format were starting to become clear. Originally designed to be smaller and more practical than the 8-inch format, the 5¼-inch system was itself too large, and as the quality of the recording media grew, the same amount of data could be placed on a smaller surface.

A number of solutions were developed, with drives at 2-inch, 2½-inch, 3-inch and 3½-inch (50, 60, 75 and 90 mm) all being offered by various companies. They all shared a number of advantages over the older format, including a small form factor and a rigid case with a slidable write protection tab. The almost-universal use of the 5¼-inch format made it very difficult for any of these new formats to gain any significant market share.

Sony introduced its own small-format 90.0 mm × 94.0 mm disk.; however, this format suffered from a fate similar to the other new formats: the 5¼-inch format simply had too much market share. A variant on the Sony design, introduced in 1982 by a large number of manufacturers, was then rapidly adopted. By 1988 the 3½-inch was outselling the 5¼-inch.

By the end of the 1980s, the 5¼-inch disks had been superseded by the 3½-inch disks. Though 5¼-inch drives were still available, as were disks, they faded in popularity as the 1990s began. By the mid-1990s the 5¼-inch drives had virtually disappeared as the 3½-inch disk became the predominant floppy disk. One of the chief advantages of the 3½-inch disk, besides its smaller size which allows it to fit in a shirt pocket, is its plastic case, which gives it good protection from dust, liquids, fingerprints, scratches, sunlight, warping, and other environmental risks.

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. There were a number of times where it was felt that the existing floppy was just about to be replaced by one of these newer devices, but a variety of problems ensured this never took place. 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.

The main technological change 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 the tracks themselves with a feedback loop in order to center themselves. 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 (marketing, true size was 21,040 KB, 25 MB unformatted). Later the same year it introduced the BR3225, which doubled the capacity. This model could also read standard 3½-inch disks.

Apparently 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 licenced 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 was not true to the 3½-inch form factor (hence not compatible with the standard 1.44 MB floppies), 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). However, hardware vendors such as Hewlett Packard, Dell and Compaq had promoted the same at a very high level. Zip drive media were 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" drive, often seen with the brand names Matsushita (Panasonic) and Imation, had an initial capacity of 120 MB (120.375 MB) using even higher density "LS-120" disks.

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, 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. This belief, true or otherwise, crippled adoption. The BIOS of many motherboards even to this day supports LS-120 drives as boot options.

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.

Structure

The 5¼-inch disk had a large circular hole in the center for the spindle of the drive and a small oval aperture in both sides of the plastic to allow the heads of the drive to read and write the data. The magnetic medium could be spun by rotating it from the middle hole. A small notch on the right hand side of the disk would identify whether the disk was read-only or writable, detected by a mechanical switch or photo transistor above it. Another LED/phototransistor pair located near the center of the disk could detect a small hole once per rotation, called the index hole, in the magnetic disk. It was used to detect the start of each track, and whether or not the disk rotated at the correct speed; some operating systems, such as Apple DOS, did not use index sync, and often the drives designed for such systems lacked the index hole sensor. Disks of this type were said to be soft sector disks. Very early 8-inch and 5¼-inch disks also had physical holes for each sector, and were termed hard sector disks. Inside the disk were two layers of fabric designed to reduce friction between the medium and the outer casing, with the medium sandwiched in the middle. The outer casing was usually a one-part sheet, folded double with flaps glued or spot-welded together. A catch was lowered into position in front of the drive to prevent the disk from emerging, as well as to raise or lower the spindle (and, in two-sided drives, the upper read/write head).

The 8-inch disk was very similar in structure to the 5¼-inch disk, with the exception that the read-only logic was in reverse: the slot on the side had to be taped over to allow writing.

The 3½-inch disk is made of two pieces of rigid plastic, with the fabric-medium-fabric sandwich in the middle to remove dust and dirt. The front has only a label and a small aperture for reading and writing data, protected by a spring-loaded metal or plastic cover, which is pushed back on entry into the drive.





Newer 5¼-inch drives and all 3½-inch drives automatically engage when the user inserts a disk, and disengage and eject with the press of the eject button. On Apple Macintosh computers with built-in floppy drives, the disk is ejected by a motor (similar to a VCR) instead of manually; there is no eject button. The disk's desktop icon is dragged onto the Trash icon to eject a disk.

The reverse has a similar covered aperture, as well as a hole to allow the spindle to connect into a metal plate glued to the medium. Two holes, bottom left and right, indicate the write-protect status and high-density disk correspondingly, a hole meaning protected or high density, and a covered gap meaning write-enabled or low density. (Incidentally, the write-protect and high-density holes on a 3½-inch disk are spaced exactly as far apart as the holes in punched A4 paper (8 cm), allowing write-protected floppies to be clipped into standard ring binders.) A notch top right ensures that the disk is inserted correctly, and an arrow top left indicates the direction of insertion. The drive usually has a button that, when pressed, will spring the disk out at varying degrees of force. Some will barely make it out of the disk drive; others will shoot out at a fairly high speed. In a majority of drives, the ejection force is provided by the spring that holds the cover shut, and therefore the ejection speed is dependent on this spring. In PC-type machines, a floppy disk can be inserted or ejected manually at any time (evoking an error message or even lost data in some cases), as the drive is not continuously monitored for status and so programs can make assumptions that do not match actual status (e.g., disk 123 is still in the drive and has not been altered by any other agency). With Apple Macintosh computers, disk drives are continuously monitored by the OS; a disk inserted is automatically searched for content, and one is ejected only when the software agrees the disk should be ejected. This kind of disk drive (starting with the slim "Twiggy" drives of the late Apple "Lisa") does not have an eject button, but uses a motorized mechanism to eject disks; this action is triggered by the OS software (e.g., the user dragged the "drive" icon to the "trash can" icon). Should this not work (as in the case of a power failure or drive malfunction), one can insert a straightened paper clip into a small hole at the drive's front, thereby forcing the disk to eject (similar to that found on CD–DVD drives). External 3.5" floppy drives from Apple were equipped with eject buttons. The button was ignored when the drive was plugged into a Mac, but would eject the disk if the drive was used with an Apple II, as ProDOS didn't support or implement software-controlled eject. Some other computer designs (such as the Commodore Amiga) monitor for a new disk continuously but still have push-button eject mechanisms.

The 3-inch disk, widely used on Amstrad CPC machines, bears much similarity to the 3½-inch type, with some unique and somewhat curious features. One example is the rectangular-shaped plastic casing, almost taller than a 3½-inch disk, but narrower, and more than twice as thick, almost the size of a standard compact audio cassette. This made the disk look more like a greatly oversized present day memory card or a standard PC card notebook expansion card rather than a floppy disk. Despite the size, 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. Such mechanisms were largely responsible for the thickness, length and high costs of the 3-inch disks. On the Amstrad machines the disks were typically flipped over to use both sides, as opposed to being truly double-sided. Double-sided mechanisms were available but rare.

Legacy

The advent of other portable storage options, such as USB storage devices, SD Cards, recordable CDs and DVDs, and the rise of multi-megapixel digital photography encouraged the creation and use of files larger than most 3½-inch disks could hold. Additionally, the increasing availability of broadband and wireless Internet connections decreased the overall utility of removable storage devices. While the 3½-inch floppy was in continuous use longer than any other format, they were considered almost completely obsolete by the early 21st century.

Floppies are still used for emergency boots in aging systems which lack support for other bootable media. They can also be used for BIOS updates since most BIOS and firmware programs can still be executed from bootable floppy disks. Furthermore, if a BIOS update fails or becomes corrupted somehow, floppy drives can be used to perform a recovery. The music and theatre industries still use equipment (ie. synthesizers, samplers, drum machines, sequencers, and lighting consoles) that requires standard floppy disks as a storage medium.

In 1991, Commodore introduced the CDTV, which used a CD-ROM drive in place of the floppy drive. The majority of AmigaOS was stored in read-only memory, making it easier to boot from a CD-ROM rather than floppy.

In 1998, Apple introduced the iMac which had no floppy drive. This made USB-connected floppy drives a popular accessory for the early iMacs, since the basic model of iMac at the time had only a CD-ROM drive, giving users no easy access to writable removable media. This transition away from standard floppies was relatively easy for Apple, since all Macintosh models that were originally designed to use a CD-ROM drive were able to boot and install their operating system from CD-ROM early on.

In February 2003, Dell, Inc. announced that they would no longer include standard floppy drives on their Dell Dimension home computers as standard equipment, although they are available as a selectable option for around 20 US$ and can be purchased as an aftermarket OEM add-on anywhere between 5 US$ and 25 US$.

On 29 January 2007 the British computer retail chain PC World issued a statement saying that only 2% of the computers that they sold contained a built-in floppy disk drive and, once present stocks were exhausted, no more standard floppies would be sold.

In 2009, Hewlett-Packard stopped supplying standard floppy drives on business desktops.

Compatibility

In general, different physical sizes of floppy disks are incompatible by definition, and disks can be loaded only on the correct size of drive. There were some drives available with both 3½-inch and 5¼-inch slots that were popular in the transition period between the sizes.

However, there are many more subtle incompatibilities within each form factor. For example, all but the earliest models of Apple Macintosh computers that have built-in floppy drives included a disk controller that can read, write and format IBM PC-format 3½-inch diskettes. However, few IBM-compatible computers use floppy disk drives that can read or write disks in Apple's variable speed format. For details on this, see the section More on floppy disk formats.

The 3½-inch floppy disk

Within the world of IBM-compatible computers, the three densities of 3½-inch floppy disks are partially compatible. Higher density drives are built to read, write and even format lower density media without problems, provided the correct media are used for the density selected. However, if by whatever means a diskette is formatted at the wrong density, the result is a substantial risk of data loss due to magnetic mismatch between oxide and the drive head's writing attempts. Still, a fresh diskette that has been manufactured for high density use can theoretically be formatted as double density, but only if no information has ever been written on the disk using high density mode (for example, HD diskettes that are pre-formatted at the factory are out of the question). The magnetic strength of a high density record is stronger and will "overrule" the weaker lower density, remaining on the diskette and causing problems. However, in practice there are people who use downformatted (ED to HD, HD to DD) or even overformatted (DD to HD) without apparent problems. Doing so always constitutes a data risk, so one should weigh out the benefits (e.g. increased space or interoperability) versus the risks (data loss, permanent disk damage).

The holes on the right side of a 3½-inch disk can be altered as to 'fool' some disk drives or operating systems (others such as the Acorn Archimedes simply do not care about the holes) into treating the disk as a higher or lower density one, for backward compatibility or economical reasons . Possible modifications include:

• Drilling or cutting an extra hole into the right-lower side of a 3½-inch DD disk (symmetrical to the write-protect hole) in order to format the DD disk into a HD one. This was a popular practice during the early 1990s, as most people switched to HD from DD during those days and some of them "converted" some or all of their DD disks into HD ones, for gaining an extra "free" 720 KB of disk space. There even was a special hole punch that was made to easily make this extra (square) hole in a floppy.
• Taping or otherwise covering the bottom right hole on a HD 3½-inch disk enables it to be 'downgraded' to DD format. This may be done for reasons such as compatibility issues with older computers, drives or devices that use DD floppies, like some electronic keyboard instruments and samplers where a 'downgraded' disk can be useful, as factory-made DD disks have become hard to find after the mid-1990s. See the section "Compatibility" above.
  • Note: By default, many older HD drives will recognize ED disks as DD ones, since they lack the HD-specific holes and the drives lack the sensors to detect the ED-specific hole. Most DD drives will also handle ED (and some even HD) disks as DD ones.
• Similarly, drilling an HD-like hole (under the ED one) into an ED (2880 kB) disk for 'downgrading' it to HD (1440 kB) format if there are many unusable ED disks due to the lack of a specific ED drive, which can now be used as normal HD disks.
• Even if such a format was hardly officially supported on any system, it is possible to "force" a 3½-inch floppy disk drive to be recognized by the system as a 5¼-inch 360 kB or 1200 kB one (on PC and compatible, this can be done by simply changing the CMOS BIOS settings) and thus format and read non-standard disk formats, such as a double sided 360 kB 3½-inch disk. Possible applications include data exchange with obsolete CP/M systems, for example with an Amstrad CPC.

The 5¼-inch floppy disk

The situation was even more complex with 5¼-inch diskettes. The head gap of a 80-track high-density (1.2 MB in the MFM format) drive is shorter than that of a 40-track double-density (360 kB) drive, but will format, read and write 40 track diskettes with apparent success provided the controller supports double stepping (or the manufacturer fitted a switch to do double stepping in hardware). A blank 40 track disk formatted and written on an 80 track drive can be taken to a 40 track drive without problems, similarly a disk formatted on a 40 track drive can be used on an 80 track drive. But a disk written on a 40 track drive and updated on an 80 track drive becomes permanently unreadable on any 360 kB drive, owing to the incompatibility of the track widths (special, very slow programs could have been used to overcome this problem). There are several other bad scenarios.

Prior to the problems with head and track size, there was a period when just trying to figure out which side of a "single sided" diskette was the right side was a problem. Both Radio Shack and Apple used 180 kB single-sided 5¼-inch disks, and both sold disks labeled "single sided" that were certified for use on only one side, even though they in fact were coated in magnetic material on both sides. The irony was that the disks would work on both Radio Shack and Apple machines, yet the Radio Shack TRS-80 Model I computers used one side and the Apple II machines used the other, regardless of whether there was software available which could make sense of the other format.

For quite a while in the 1980s, users could purchase a special tool called a disk notcher which would allow them to cut a second write-unprotect notch in these diskettes and thus use them as "flippies" (either inserted as intended or upside down): both sides could now be written on and thereby the data storage capacity was doubled. Other users made do with a steady hand and a hole punch or scissors. For re-protecting a disk side, one would simply place a piece of opaque tape over the notch or hole in question. These "flippy disk procedures" were followed by owners of practically every home-computer single sided disk drives. Proper disk labels became quite important for such users.Flippies were eventually adopted by some manufacturers, with a few programs being sold in this medium (they were also widely used for software distribution on systems that could be used with both 40 track and 80 track drives but lacked the software to read a 40 track disk in an 80 track drive). The practice eventually faded with the increased use of double-sided drives capable of accessing both sides of the disk without the need for flipping.

More on floppy disk formats

Using the disk space efficiently

In general, data is written to floppy disks in a series of sectors, angular blocks of the disk, and in tracks, concentric rings at a constant radius, e.g. the HD format of 3½-inch floppy disks uses 512 bytes per sector, 18 sectors per track, 80 tracks per side and two sides, for a total of 1,474,560 bytes per disk. (Some disk controllers can vary these parameters at the user's request, increasing the amount of storage on the disk, although these formats may not be able to be read on machines with other controllers; e.g. Microsoft applications were often distributed on Distribution Media Format (DMF) disks, a hack that allowed 1.68 MB (1680 kB) to be stored on a 3½-inch floppy by formatting it with 21 sectors instead of 18, while these disks were still properly recognized by a standard controller.) On the IBM PC and also on the MSX, Atari ST, Amstrad CPC, and most other microcomputer platforms, disks are written using a Constant Angular Velocity —Constant Sector Capacity format. This means that the disk spins at a constant speed, and the sectors on the disk all hold the same amount of information on each track regardless of radial location.

However, this is not the most efficient way to use the disk surface, even with available drive electronics. Because the sectors have a constant angular size, the 512 bytes in each sector are packed into a smaller length near the disk's center than nearer the disk's edge. A better technique would be to increase the number of sectors/track toward the outer edge of the disk, from 18 to 30 for instance, thereby keeping constant the amount of physical disk space used for storing each 512 byte sector (see zone bit recording). Apple implemented this solution in the early Macintosh computers by spinning the disk slower when the head was at the edge while keeping the data rate the same, allowing them to store 400 kB per side, amounting to an extra 160 kB on a double-sided disk. This higher capacity came with a serious disadvantage, however: the format required a special drive mechanism and control circuitry not used by other manufacturers, meaning that Mac disks could not be read on any other computers. Apple eventually gave up on the format and used constant angular velocity with HD floppy disks on their later machines; these drives were still unique to Apple as they still supported the older variable-speed format.

The 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 removed 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 , 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.

Commodore also offered its 8-bit machines a 3½-inch 800 kByte disk format with its 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 Kernal.

The 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 table, 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 contain code that signals the drive to switch into double-density mode. 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 and 8". Well-known 3rd party Atari DOS products included SmartDOS (distributed with the Rana disk drive), TopDos, MyDos and SpartaDOS.

The Commodore Amiga

The Commodore Amiga computers used an 880 kByte format (11 * 512-byte sectors per track) 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, or a second floppy drive, which is also a crucial reason for an emulator being technically unable to access real Amiga disks inserted in a standard PC floppy disk drive.

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 .

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.

The Electron, BBC Micro and Acorn Archimedes

The British company Acorn 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).

Because of 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; and 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.

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 4482929 on the media and the drive for the DemiDiskette. At trade shows, the drive and media were labeled "Brown" and "Tabor".

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, 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

A small floppy disk was also used in the late 1980s to store video information for still video cameras such as the Sony Mavica (not to be confused with current Digital Mavica models) and the Ion and Xapshot cameras from Canon. It was officially referred to as a Video Floppy (or VF for short).

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.

The same media were used digitally formatted - 720 kB, 245TPI, 80 tracks/side, double-sided, double-density - 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.

Ultimate capacity and speed

Floppy disk drive and floppy media manufacturers specify an unformatted capacity, which is, for example, 2.0 MB for a standard 3½-inch HD floppy. It is implied that this data capacity should not be exceeded since exceeding such limitations will most likely degrade the design margins of the floppy system and could result in performance problems such as inability to interchange or even loss of data.

The nominal formatted capacity printed on labels is "1.44 MB" which uses an incorrect definition of the megabyte that combines decimal (base 10) with binary (base 2) to yield 1.44×1000×1024 bytes (approximately 1.47 million bytes). This usage of the "Mega-" prefix is not compatible with the International System of Units prefixes. Using SI-compliant definitions, the capacity of a 3½-inch HD floppy is properly written as 1.47 MB (base 10) or 1.40 MiB (base 2).

User available data capacity is a function of the particular disk format used which in turn is determined by the FDD controller manufacturer and the settings applied to its controller. The differences between formats can result in user data capacities ranging from approximately 1300 KB up to 1760 KB (1.80 MB) on a "standard" 3½-inch High Density floppy (and even up to near 2 MB with utilities like 2MGUI). The highest capacity techniques require much tighter matching of drive head geometry between drives; this is not always possible and cannot be relied upon. The LS-240 drive supports a (rarely used) 32 MB capacity on standard 3½-inch HD floppies —it is, however, a write-once technique, and cannot be used in a read/write/read mode. All the data must be read off, changed as needed and rewritten to the disk. The format also requires an LS-240 drive to read.

Double-sided Extended-density (DSED) 3½″ floppy disks, introduced by Toshiba in 1987 and adopted by IBM on the PS/2 in 1994, operate at twice the data rate and have twice the capacity of DSHD 3½″ FDDs. The only serious attempt to speed up a 3½” floppy drive beyond 2x was the X10 accelerated floppy drive. It used a combination of RAM and 4x spindle speed to read a floppy in less than six seconds versus the more than one minute of a conventional drive.

3½-inch HD floppy drives typically have a maximum transfer rate of 1000 kilobits/second (minus overhead such as error correction and file handling). (For comparison, a 1x CD transfers at 1200 kilobits per second (maximum), and a 1x DVD transfers at approximately 11,000 kilobits per second.) While the floppy's data rate cannot be easily changed, overall performance can be improved by optimizing drive access times, shortening some BIOS introduced delays (especially on the IBM PC and compatible platforms), and by changing the sector:shift parameter of a disk, which is, roughly, the numbers of sectors that are skipped by the drive's head when moving to the next track. Because of overhead and these additional delays, the average sequential read speed is rather 30–70 KB/s than 125 KB/s.

This happens because sectors are not typically written exactly in a sequential manner but are scattered around the disk, which introduces yet another delay. Older machines and controllers may take advantage of these delays to cope with the data flow from the disk without having to actually stop.

Usability

One of the chief usability problems of the floppy disk is its vulnerability. Even inside a closed plastic housing, the disk medium is still highly sensitive to dust, condensation and temperature extremes. As with any magnetic storage, it is also vulnerable to magnetic fields. Blank floppies have usually been distributed with an extensive set of warnings, cautioning the user not to expose it to conditions which can endanger it.

Users damaging floppy disks (or their contents) were once a staple of "stupid user" folklore among computer technicians. These stories poked fun at users who stapled floppies to papers, made faxes or photocopies of them when asked to "copy a disk," or stored floppies by holding them with a magnet to a file cabinet. The flexible 5¼-inch disk could also (apocryphally) be abused by rolling it into a typewriter to type a label, or by removing the disk medium from the plastic enclosure, the same way a record is removed from its slipsleeve. Also, these same users were, conversely, often the victims of technicians' hoaxes. Stories of them being carried on Subway/Underground systems wrapped in tin-foil to protect them from the magnetic fields of the electric power supply were common (for an explanation of why this is plausible, see Faraday cage).

On the other hand, the 3½-inch floppy has also been lauded for its mechanical usability by HCI expert Donald Norman:

The floppy as a metaphor

For more than two decades, the floppy disk was the primary external writable storage device used. Also, in a non-network environment, floppies were once the primary means of transferring data between computers (sometimes jokingly referred to as Sneakernet). Floppy disks are also, unlike hard disks, handled and seen; even a novice user can identify a floppy disk. Because of all these factors, the image of the floppy disk has become a metaphor for saving data, and the floppy disk symbol is often seen in programs on buttons and other user interface elements related to saving files, even though such disks are obsolete.

See also

• Floppy disk controller
• Floppy disk format
• RaWrite2 (a floppy disk image file writer/creator)
• On Unix or Unix-like systems the dd program can be used to write an image to a floppy.
• Don't Copy That Floppy
• Semi-virtual diskette

References

1. The Floppy Disk Drive Engineering Design Challenge SSD to FDD; see http://jimwarholic.com/2008/12/floppy-disk-drive-engineering-design.php
2. (actually 126,222,336 bytes, 6848 cylinders x 36 blocks/cylinder x 512 bytes/block; see http://linuxcommand.org/man_pages/floppy8.html
3. The IBM Diskette and Diskette Drive, James T. Engh, 1981 - "where k = 1000" ... "This increased the formatted disk capacity to 81.6 kbytes."
4. Memorex 650 Flexible Disc File - OEM Manual
5. The IBM Diskette and Diskette Drive, James T. Engh, 1981 - "The user capacity of the diskette was established at 242 944 bytes on 73 tracks with 26 sectors on each track."
6. The Evolution of Magnetic Storage, L.D. Stevens, 1981 - "This drive, with a capacity of 243 Kbytes"
7. The IBM Diskette and Diskette Drive, James T. Engh, 1981 - "This would double the capacity to approximately 0.5 megabytes (Mbytes)."
8. "In September, 1976, the first minifloppy disk drive was introduced by Shugart Associates."
9. Shugart SA 400 DatasheetFormatted with 256 byte sectors and 10 sectors per track the capacity is 89.6 Kbytes (256 x 10 x 35 = 89,600)
10. December 1982: Amdek releases the Amdisk-3 Micro-Floppy-disk Cartridge system.
11. per 1986 Disk/Trend Report, Flexible Disk Drives
12. http://www.atarimagazines.com/compute/issue70/054_1_THE_FUTURE_OF_MASS_STORAGE.php
13. Mueller, S: "Upgrading and Repairing PCs," p.656, Que Publishing, 2002.
14. IBM Archives: 20th century disk storage chronology
15. http://www.disktrend.com/5decades2.htm Five decades of disk drive industry firsts
16. 1991 Disk/Trend Report, Flexible Disk Drives, Figure 2
17. 2 sides × 526 cyl × 40 tracks × 512 bytes
18. 6848 cylinders × 36 blocks/cylinder × 512 bytes
19. "R.I.P. Floppy Disk", BBC News, 1 April 2003
20. "Dell Drops Floppy Drive on New Machine", Lisa Bruce, University of Missouri-Columbia, March 2003
21. "So farewell then, floppy disk", Richi Jennings, Computerworld, January 31, 2007
22. "PC World says farewell to floppy", BBC News, January 30, 2007
23. "Floppy disks ejected as demand slumps", David Derbyshire, Daily Telegraph, 30 January 2007
24. KCS Dual HD Drive for Amiga computers
25. Since USB floppy drives include their own controllers, it's possible use their buffer to exceed the speed of a standard floppy drive without confusing the host. Such USB 3½″ FDDs are available from SmartDisk, Y-E Data, Sony, and Apricorn; however, their internal drives are DSHD FDD and their sustained speed is 1x.

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

• Programming Floppy Disk Controllers
• HowStuffWorks: How Floppy Disk Drives Work – By Gary Brown.
• Computer Hope: Information about computer floppy drives – Including abbreviated history, physical parameters and cable pin specifications.
• NCITS (mention of ANSI X3.162 (5¼-inch) and X3.171 (90 mm) floppy standards)
• Tandon Floppy Disk Drive Manuals
• Persci Floppy Disk Drive Manuals
• The 3″ Bible
• Floppy Drive Tech Info