Floppy disk

8-inch, 5¼-inch, and 3½-inch floppy disks

8-inch, 5¼-inch (full height), and 3½-inch drives

A 3½-inch floppy disk removed from its housing

A floppy disk, also called a floppy, diskette or just disk, is a type of disk storage composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic enclosure lined with fabric that removes dust particles. Floppy disks are read and written by a floppy disk drive (FDD).

Floppy disks, initially as 8-inch (200 mm) media^[1] and later in 5¼-inch (133 mm) and 3½-inch (90 mm) sizes, were a ubiquitous form of data storage and exchange from the mid-1970s into the late 2000s.^[2]

By 2007, computers were rarely manufactured with installed floppy disk drives; 3½-inch floppy disks can be used with an external USB floppy disk drive, but USB drives for 5¼-inch, 8-inch, and non-standard diskettes are rare to non-existent. These formats are usually handled by older equipment.

While floppy disk drives still have some limited uses, especially with legacy industrial computer equipment, they have been superseded by data storage methods with much greater capacity, such as USB flash drives, flash storage cards, portable external hard disk drives, optical discs, ROM cartridges and storage available through computer networks.

History

8-inch disk drive with diskette (3½-inch disk for comparison)

3½-inch, high-density diskettes affixed with adhesive labels

The first floppy disks, developed in the late 1960s, are 8 inches (200 mm) in diameter;^[1][2] they became commercially available in 1971 as a component of IBM products and then were sold separately beginning in 1972 by Memorex and others.^[3] These disks and associated drives were produced and improved upon by IBM and other companies such as Memorex, Shugart Associates, and Burroughs Corporation.^[4] The term "floppy disk" appeared in print as early as 1970,^[5] and although in 1973 IBM announced its first media as "Type 1 Diskette" the industry continued to use the terms "floppy disk" or "floppy".

In 1976, Shugart Associates introduced the first 5¼-inch FDD. By 1978 there were more than 10 manufacturers producing such FDDs.^[6] There were competing floppy disk formats, with hard- and soft-sector versions and encoding schemes such as FM, MFM and GCR. The 5¼-inch format displaced the 8-inch one for most applications, and the hard-sectored disk format disappeared. The most common capacity of the 5¼-inch format in DOS-based PCs was 360 KB and in 1984 IBM introduced the 1.2 MB dual-sided floppy disk along with its PC-AT model. IBM started using the 720 KB double-density 3½-inch microfloppy disk on its Convertible laptop computer in 1986 and the 1.44 MB high-density version with the PS/2 line in 1987. These disk drives could be added to older PC models. In 1988 IBM introduced a drive for 2.88 MB "DSED" diskettes in its top-of-the-line PS/2 models, but this was a commercial failure.

Throughout the early 1980s, limitations of the 5¼-inch format became clear. Originally designed to be more practical than the 8-inch format, it was itself too large; as the quality of recording media grew, data could be stored in a smaller area.^[7] A number of solutions were developed, with drives at 2, 2½, 3 and 3½ inches (and Sony's 90.0 mm × 94.0 mm disk) offered by various companies.^[7] They all shared a number of advantages over the old format, including a rigid case with a sliding metal cover over the head slot, which helped protect the delicate magnetic medium from dust and damage, and a sliding write protection tab, which was far more practical than the adhesive tabs used with earlier disks. The large market share of the 5¼-inch format made it difficult for these new formats to gain significant market share.^[7] 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.^[8]

By the end of the 1980s, 5¼-inch disks had been superseded by 3½-inch disks. By the mid-1990s, 5¼-inch drives had virtually disappeared, as the 3½-inch disk became the predominant floppy disk. The advantages of the 3½-inch disk were its smaller size and its rigid case, which provided better protection from dirt and other environmental risks, while the 5¼-inch disk was available cheaper per piece throughout its history, usually with a price in the range of a third to half that of a 3½-inch disk.

Ubiquity

Imation USB floppy drive, model 01946: an external drive that accepts high-density disks

Floppy disks became ubiquitous during the 1980s and 1990s in their use with personal computers to distribute software, transfer data, and create backups. Before hard disks became affordable to the general population, floppy disks were often used to store a computer's operating system (OS). Most home computers from that period have a primary OS and BASIC stored as ROM, with the option of loading a more advanced operating system from a floppy disk.

By the early 1990s, the increasing software size meant large packages like Windows or Adobe Photoshop required a dozen disks or more. In 1996, there were an estimated five billion standard floppy disks in use.^[9] Then, distribution of larger packages was gradually replaced by CD-ROM and online distribution (for small programs).

An attempt to continue the existing 3½ inch form factor was the SuperDisk in the late 1990s, utilizing very narrow data tracks and a high precision head guidance mechanism with a capacity of 120 MB^[10] and backward-compatible with standard 3½-inch floppies; a format war briefly occurred between SuperDisk and other high-density floppy-disk products, although ultimately flash storage, recordable CDs/DVDs, and online storage would render the matter irrelevant. External USB-based floppy disk drives are still available; many modern systems provide firmware support for booting from such drives.

Decline

In the mid 1990's mechanically incompatible higher-density floppy disks were introduced, like the Iomega Zip disk. Adoption was limited by the competition between proprietary formats and the need to buy expensive drives for computers where the disks would be used. In some cases, failure in market penetration was exacerbated by release of higher-capacity versions of the drive and media not backward-compatible with the original drives, dividing the users between new and old adopters. A chicken-or-the-egg scenario ensued, with consumers wary of making costly investments into unproven and rapidly changing technologies, resulting in none of the technologies becoming an established standard.

Little-used floppy disk drives sitting unused for years in computers meanwhile can actually damage disks, due to computer case air cooling that sucks air through the drive openings. This pulls dust into the mechanism, which coats all surfaces including the exposed read/write head. Upon loading a disk on the rare occasion the drive is needed, the dust is stirred up when the disk is inserted into the mechanism, and is then deposited onto the disk through the open shutters, leading to nearly instant fouling of the media. The dust also gets into the drive mechanism lubricant, turning it into a sticky slime, which may jam the mechanism and prevent head movement. This causes the computer user to see read errors on every disk they try to use, and may also lead to data corruption through writing in incorrect locations.

Apple introduced the iMac in 1998 with a CD-ROM drive but no floppy drive; this made USB-connected floppy drives popular accessories, as the iMac came without any writable removable media device.

Recordable CDs with even greater capacity, compatible with existing infrastructure of CD-ROM drives, made the new floppy technologies obsolete. The floppy disk's remaining reusability advantage was then eliminated by re-writeable CDs. Networking, advancements in flash-based devices and widespread adoption of USB provided another alternative that in turn made both floppy disks and optical storage obsolete for some purposes. The rise of file sharing and multi-megapixel digital photography encouraged the use of files larger than most 3½-inch disks could hold. Floppy disks were commonly used as sneakernet carriers for file transfer, but the broad availability of LANs and fast Internet connections provided a simpler and faster method of transferring such files. Other removable storage devices have advantages in both capacity and performance when network connections are unavailable or when networks are inadequate.

Different types of data storage methods.

Use in the early 21st century

A floppy hardware emulator, same size as a 3½-inch drive, provides a USB interface to the user

Nonetheless, as of 2002 most manufacturers still provided floppy disk drives as standard equipment to meet user demand for file-transfer and an emergency boot device as well as the general secure feeling of having the familiar device.^[11] Subsequently, enabled by the widespread support for USB flash drives and BIOS boot, manufacturers and retailers progressively reduced the availability of floppy disk drives as standard equipment. In February 2003, Dell announced that floppy drives would no longer be pre-installed on Dell Dimension home computers, although they were still available as a selectable option and purchasable as an aftermarket OEM add-on.^[12] As of January 2007 only 2% of computers sold in stores contained built-in floppy disk drives.^[13]

Floppy disks are used for emergency boots in aging systems lacking support for other bootable media and for BIOS updates, since most BIOS and firmware programs can still be executed from bootable floppy disks. If BIOS updates fail or become corrupt, floppy drives can sometimes be used to perform a recovery. The music and theatre industries still use equipment requiring standard floppy disks (e.g. synthesizers, samplers, drum machines, sequencers, and lighting consoles). Industrial automation equipment such as programmable machinery and industrial robots may not have a USB interface; data and programs are then loaded from disks, damageable in industrial environments. This may not be replaced due to cost or requirement for continuous availability; existing software emulation and virtualization do not solve this problem because no operating system is present or a customized operating system is used that has no drivers for USB devices. Hardware floppy disk emulators can be made to interface floppy-disk controllers to a USB port that can be used for flash drives.

Corporate computer environments may still make use of floppy disks for older machines that do not support the current company networks and in the case of laptops where Wi-Fi is not considered secure. The floppy disk provides for a controlled means of file transfer by permitting only a few files to be transmitted. This is as USB ports on enterprise computer terminals/workstations are often disabled in order to prevent employees from using a flash memory drive to take large amounts of data for unauthorized use. In addition, the loss of a floppy disk has less consequence than the loss of any modern piece of removable flash storage.

The abundance of old floppy drives has enabled forensics investigators to repurpose older computers as "front-ends" for forensic access to old floppy disks that have only recently been unearthed from some crime scenes.

In May 2016 the United States Government Accountability Office released a report that covered the need to upgrade or replace legacy computer systems within Federal Agencies. According to this document, old IBM Series/1 minicomputers running on 8-inch floppy disks are still used to coordinate "the operational functions of the United States’ nuclear forces..." The government plans to update some of the technology by the end of the 2017 fiscal year.^[14][15]

Windows 10 no longer includes a generic driver for USB floppy drives.^[16][17]

Legacy

Screenshot of the toolbar in OpenOffice.org, highlighting the Save icon, which depicts a floppy disk

For more than two decades, the floppy disk was the primary external writable storage device used. Most computing environments before the 1990s were non-networked, and floppy disks were the primary means of transferring data between computers, a method known informally as sneakernet. Unlike hard disks, floppy disks are handled and seen; even a novice user can identify a floppy disk. Because of these factors, a picture of a 3½-inch floppy disk has become an interface metaphor for saving data. The floppy disk symbol is still used by software on user-interface elements related to saving files, such as the release of Microsoft Office 2013, even though the physical floppy disks are largely obsolete.^[18]

Design

A disk notcher used to convert single-sided 5¼-inch diskettes to double-sided by adding the opposite side's write-enable notch. Drives that used the start of track index hole also required a second pair of holes punched in the jacket.

Structure

The 5¼-inch disk has a large circular hole in the center for the drive's spindle and a small oval aperture in both sides of the plastic to allow the drive's heads to read and write data; the magnetic medium can be spun by rotating it from the middle hole. A small notch on the right of the disk identifies that it is writable, detected by a mechanical switch or phototransistor above it; if it is not present, the disk is read-only. Punch devices were sold to convert read-only disks to writable ones and enable writing on the unused side of single sided disks; such modified disks became known as flippy disks. Tape may be used over the notch to protect writable disks from unwanted writing. This arrangement was the inverse of the system used on 8-inch floppy disks where the notch had to be covered before the disk could be written to.

Another LED/photo-transistor pair located near the center of the disk detects the index hole once per rotation in the magnetic disk; it is used to detect the angular start of each track and whether or not the disk is rotating at the correct speed. Early 8‑inch and 5¼‑inch disks had physical holes for each sector and were termed hard sectored disks. Later soft sectored disks had only one index hole, and sector position was determined by the disk controller or low level software from patterns marking the start of a sector. Generally, the same drives were used to read and write both types of disks, with only the disks and disk controllers differing. Some operating systems utilizing soft sectors, such as Apple DOS, did not use the index hole; the drives designed for such systems often lacked the corresponding sensor; this was mainly a hardware cost-saving measure.

Inside the disk are two layers of fabric, with the medium sandwiched in the middle. The fabric is designed to reduce friction between the medium and the outer casing, and catch particles of debris abraded off the disk to keep them from accumulating on the heads. The outer casing is usually a one-part sheet, double-folded with flaps glued or spot-welded together. The 8-inch disk had read-only logic that was the reverse of the 5¼-inch disk, with the slot on the side having to be taped over to allow writing.

The core of the 3½-inch disk is the same as the other two disks, but the front has only a label and a small aperture for reading and writing data, protected by the slider — a spring-loaded metal or plastic cover, pushed to the side on entry into the drive. Rather than having a hole in the center, it has a metal hub which mates to the spindle of the drive. Typical 3½-inch disk magnetic coating materials are:^[19]

• DD: 2 µm magnetic iron oxide
• HD: 1.2 µm cobalt-doped iron oxide
• ED: 3 µm barium ferrite

Two holes at the bottom left and right indicate whether the disk is write-protected and whether it is high-density; these holes are spaced as far apart as the holes in punched A4 paper, allowing write-protected high-density floppies to be clipped into standard ring binders. A notch at top right ensures that the disk is in the correct orientation and an arrow at top left indicating direction of insertion. The drive usually has a button that when pressed ejects the disk with varying degrees of force, the discrepancy due to the ejection force provided by the spring of the slider cover. In IBM PC compatibles, Commodores, Apple II/IIIs, and other non-Apple-Macintosh machines with standard floppy disk drives, a disk may be inserted or ejected manually at any time. The drive has a disk-change switch that detects when a disk is ejected or inserted. Failure of this mechanical switch is a common source of disk corruption if a disk is changed and the drive (and hence the operating system) fails to notice.

One of the chief usability problems of the floppy disk is its vulnerability; even inside a closed plastic housing, the disk medium is highly sensitive to dust, condensation and temperature extremes. As with all magnetic storage, it is vulnerable to magnetic fields. Blank disks have been distributed with an extensive set of warnings, cautioning the user not to expose it to dangerous conditions. Disks must not be roughly treated or removed from the drive while the magnetic media is still spinning, since doing so is likely to cause damage to the disk, drive head, or stored data. On the other hand, the 3½‑inch floppy has been lauded for its mechanical usability by HCI expert Donald Norman:^[20]

"A simple example of a good design is the 3½-inch magnetic diskette for computers, a small circle of "floppy" magnetic material encased in hard plastic. Earlier types of floppy disks did not have this plastic case, which protects the magnetic material from abuse and damage. A sliding metal cover protects the delicate magnetic surface when the diskette is not in use and automatically opens when the diskette is inserted into the computer. The diskette has a square shape: there are apparently eight possible ways to insert it into the machine, only one of which is correct. What happens if I do it wrong? I try inserting the disk sideways. Ah, the designer thought of that. A little study shows that the case really isn't square: it's rectangular, so you can't insert a longer side. I try backward. The diskette goes in only part of the way. Small protrusions, indentations, and cutouts, prevent the diskette from being inserted backward or upside down: of the eight ways one might try to insert the diskette, only one is correct, and only that one will fit. An excellent design."

The spindle motor from a 3½‑inch unit

A read-write head from a 3½‑inch unit

Operation

How the read-write head is applied on the floppy

Visualization of magnetic information on floppy disk (image recorded with CMOS-MagView)

A spindle motor in the drive rotates the magnetic medium at a certain speed, while a stepper motor-operated mechanism moves the magnetic read/write head(s) radially along the surface of the disk. Both read and write operations require the media to be rotating and the head to contact the disk media, an action originally accomplished by a "disk load" solenoid.^[21] Later drives held the heads out of contact until a front-panel lever was rotated (5¼") or disk insertion was complete (3½"). To write data, current is sent through a coil in the head as the media rotates. The head's magnetic field aligns the magnetic particles directly below the head on the media. When the current is reversed the particles align in the opposite direction encoding the data digitally. To read data, the magnetic particles in the media induce a tiny voltage in the head coil as they pass under it. This small signal is amplified and sent to the floppy disk controller, which converts the streams of pulses from the media into data, checks it for errors, and sends it to the host computer system.

A blank unformatted diskette has a coating of magnetic oxide with no magnetic order to the particles. During formatting, the particles are aligned forming a pattern of magnetized tracks, each broken up into sectors, enabling the controller to properly read and write data. The tracks are concentric rings around the center, with spaces between tracks where no data is written; gaps with padding bytes are provided between the sectors and at the end of the track to allow for slight speed variations in the disk drive, and to permit better interoperability with disk drives connected to other similar systems. Each sector of data has a header that identifies the sector location on the disk. A cyclic redundancy check (CRC) is written into the sector headers and at the end of the user data so that the disk controller can detect potential errors. Some errors are soft and can be resolved by automatically re-trying the read operation; other errors are permanent and the disk controller will signal a failure to the operating system if multiple attempts to read the data still fail.

After a disk is inserted, a catch or lever at the front of the drive is manually lowered to prevent the disk from accidentally emerging, engage the spindle clamping hub, and in two-sided drives, engage the second read/write head with the media. In some 5¼-inch drives, insertion of the disk compresses and locks an ejection spring which partially ejects the disk upon opening the catch or lever. This enables a smaller concave area for the thumb and fingers to grasp the disk during removal. Newer 5¼-inch drives and all 3½-inch drives automatically engage the spindle and heads when a disk is inserted, doing the opposite with the press of the eject button. On Apple Macintosh computers with built-in floppy drives, the ejection button is replaced by software controlling an ejection motor which only does so when the operating system no longer needs to access the drive. The user could drag the image of the floppy drive to the trash can on the desktop to eject the disk. In the case of a power failure or drive malfunction, a loaded disk can be removed manually by inserting a straightened paper clip into a small hole at the drive's front panel, just as one would do with a CD-ROM drive in a similar situation.

Before a disk can be accessed, the drive needs to synchronize its head position with the disk tracks. In some drives, this is accomplished with a Track Zero Sensor, while for others it involves the drive head striking an immobile reference surface. In either case, the head is moved so that it is approaching track zero position of the disk. When a drive with the sensor has reached track zero, the head stops moving immediately and is correctly aligned. For a drive without the sensor, the mechanism attempts to move the head the maximum possible number of positions needed to reach track zero, knowing that once this motion is complete, the head will be positioned over track zero.

Some drive mechanisms such as the Apple II 5.25 inch drive without a track zero sensor, produce very characteristic mechanical noises when trying to move the heads past the reference surface. This physical striking is responsible for the 5.25 inch drive clicking during the boot of an Apple II, and the loud rattles of its DOS and ProDOS when disk errors occurred and track zero synchronization was attempted.

Sizes

Different sizes of floppy disks are mechanically incompatible, and disks can fit only one size of drive. Drive assemblies with both 3½-inch and 5¼-inch slots were available during the transition period between the sizes, but they contained two separate drive mechanisms. In addition, there are many subtle, usually software-driven incompatibilities between the two. 5¼-inch disks formatted for use with Apple II computers would be unreadable and treated as unformatted on a Commodore. As computer platforms began to form, attempts were made at interchangeability. For example, the "Superdrive" included from the Macintosh SE to the Power Macintosh G3 could read, write and format IBM PC format 3½-inch disks, but few IBM-compatible computers had drives that did the reverse. 8-inch, 5¼-inch and 3½-inch drives were manufactured in a variety of sizes, most to fit standardized drive bays. Alongside the common disk sizes were non-classical sizes for specialized systems.

8-inch floppy disk

8-inch floppy disk

The first floppy disk was 8 inches in diameter,^[1] was protected by a flexible plastic jacket and was a read-only device used by IBM as a way of loading microcode.^[22] Read/Write floppy disks and their drives became available in 1972 but it was IBM's 1973 introduction of the 3740 data entry system^[23] that began the establishment of floppy disks, called by IBM the "Diskette 1," as an industry standard for information interchange. Early microcomputers used for engineering, business, or word processing often used one or more 8-inch disk drives for removable storage; the CP/M operating system was developed for microcomputers with 8-inch drives.

The family of 8-inch disks and drives increased over time and later versions could store up to 1.2 MB;^[24] many microcomputer applications did not need that much capacity on one disk, so a smaller size disk with lower-cost media and drives was feasible. The 5¼-inch drive succeeded the 8-inch size in many applications, and developed to about the same storage capacity as the original 8-inch size, using higher-density media and recording techniques.

5¼-inch floppy disk

Uncovered 5¼‑inch disk mechanism with disk inserted. The edge of the disk with the opening for the medium was inserted first, then the lever was turned to close the mechanism and engage the drive motor and heads.

The head gap of an 80‑track high-density (1.2 MB in the MFM format) 5¼‑inch drive (a.k.a. Mini diskette, Mini disk, or Minifloppy) is smaller than that of a 40‑track double-density (360 KB) drive but can format, read and write 40‑track disks well provided the controller supports double stepping or has a switch to do such a process. A blank 40‑track disk formatted and written on an 80‑track drive can be taken to its native drive without problems, and a disk formatted on a 40‑track drive can be used on an 80‑track drive. Disks written on a 40‑track drive and then updated on an 80 track drive become unreadable on any 40‑track drives due to track width incompatibility.

Single sided disks were coated on both sides, despite the availability of more expensive double sided disks. The reason usually given for the higher cost was that double sided disks were certified error-free on both sides of the media. Double-sided disks could be used in some drives for single-sided disks, as long as an index signal was not needed. This was done one side at a time, by turning them over (flippy disks); more expensive dual-head drives which could read both sides without turning over were later produced, and eventually became used universally.

3½-inch floppy disk

Internal parts of a 3½‑inch floppy disk.
1) A hole that indicates a high-capacity disk.
2) The hub that engages with the drive motor.
3) A shutter that protects the surface when removed from the drive.
4) The plastic housing.
5) A polyester sheet reducing friction against the disk media as it rotates within the housing.
6) The magnetic coated plastic disk.
7) A schematic representation of one sector of data on the disk; the tracks and sectors are not visible on actual disks.
8) The write protection tab (unlabeled) is upper left.

A 3½-inch floppy disk drive.

In the early 1980s, a number of manufacturers introduced smaller floppy drives and media in various formats. A consortium of 21 companies eventually settled on a 3½-inch floppy disk (actually 90 mm wide) a.k.a. Micro diskette, Micro disk, or Micro floppy, similar to a Sony design, but improved to support both single-sided and double-sided media, with formatted capacities generally of 360 KB and 720 KB respectively. Single-sided drives shipped in 1983,^[25] and double sided in 1984. What became the most common format, the double-sided, high-density (HD) 1.44 MB disk drive, shipped in 1986.

The first Macintosh computers used single-sided 3½-inch floppy disks, but with 400 KB formatted capacity. These were followed in 1986 by double-sided 800 KB floppies. The higher capacity was achieved at the same recording density by varying the disk rotation speed with head position so that the linear speed of the disk was closer to constant. Later Macs could also read and write 1.44 MB HD disks in PC format with fixed rotation speed.

All 3½-inch disks have a rectangular hole in one corner which, if obstructed, write-enabled the disk. A sliding detented piece can be moved to block or reveal the part of the rectangular hole that is sensed by the drive. The HD 1.44 MB disks have a second, unobstructed hole in the opposite corner which identifies them as being of that capacity.

In IBM-compatible PCs, the three densities of 3½-inch floppy disks are backwards-compatible: higher density drives can read, write and format lower density media. It is also possible to format a disk at a lower density than it was intended for, but only if the disk is first thoroughly demagnetized with a bulk eraser, as the high density format is magnetically stronger and will prevent the disk from working in lower density modes.

Writing at different densities than disks were intended for, sometimes by altering or drilling holes, was possible but deprecated. The holes on the right side of a 3½‑inch disk can be altered as to make some disk drives and operating systems treat the disk as one of higher or lower density, for bidirectional compatibility or economical reasons.^[26][27] Some computers, such as the PS/2 and Acorn Archimedes, ignored these holes altogether.^[28]

It is possible to make a 3½-inch floppy disk drive be recognized by a system as a 5¼‑inch 360 KB or 1200 KB drive, and to read and write disks with the same number of tracks and sectors as those disks; this had some application in data exchange with obsolete CP/M systems.

Other sizes

Other smaller floppy size were proposed, especially for portable or pocket-sized devices that needed a smaller storage device. 3-inch disks similar in construction to 3½-inch were manufactured and used for a time, particularly by Amstrad computers and word processors. A 2-inch nominal size was introduced by Sony for compact pocket computers and was used with some electronic musical instrument controllers. Neither of these sizes achieved much market success.^[29]

Sizes, performance and capacity

Floppy disk size is often referred to in inches, even in countries using metric and though the size is defined in metric. The ANSI specification of 3½-inch disks is entitled in part "90 mm (3.5 in)" though 90 mm is closer to 3.54 inches.^[30] Formatted capacities are generally set in terms of kilobytes and megabytes.

A box of about 80 floppy disks together with one USB memory stick. The stick is capable of holding over 130 times as much data as the entire box of disks put together.

Data is generally written to floppy disks in sectors (angular blocks) and tracks (concentric rings at a constant radius). For example, 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.^[43] Some disk controllers can vary these parameters at the user's request, increasing storage on the disk, although they may not be able to be read on machines with other controllers. For example, Microsoft applications were often distributed on 3½-inch 1.68 MB DMF disks formatted with 21 sectors instead of 18; they could still be recognized by a standard controller. On the IBM PC, MSX and most other microcomputer platforms, disks were written using a constant angular velocity (CAV) format,^[42] with the disk spinning at a constant speed and the sectors holding the same amount of information on each track regardless of radial location.

This was not the most efficient way to use the disk surface with available drive electronics; because the sectors have constant angular size, the 512 bytes in each sector are compressed more near the disk's center. A more space-efficient technique would be to increase the number of sectors per track toward the outer edge of the disk, from 18 to 30 for instance, thereby keeping nearly constant the amount of physical disk space used for storing each sector; an example is zone bit recording. Apple implemented this in early Macintosh computers by spinning the disk more slowly when the head was at the edge, while maintaining the data rate, allowing 400 KB of storage per side and an extra 160 KB on a double-sided disk.^[44] This higher capacity came with a disadvantage: the format used a unique drive mechanism and control circuitry, meaning that Mac disks could not be read on other computers. Apple eventually reverted to constant angular velocity on HD floppy disks with their later machines, still unique to Apple as they supported the older variable-speed formats.

Disk formatting is usually done by a utility program supplied by the computer OS manufacturer; generally, it sets up a file storage directory system on the disk, and initializes its sectors and tracks. Areas of the disk unusable for storage due to flaws can be locked (marked as "bad sectors") so that the operating system does not attempt to use them. This was time consuming so many environments had quick formatting which skipped the error checking process. When floppy disks were often used, disks pre-formatted for popular computers were sold. The unformatted capacity of a floppy disk does not include the sector and track headings of a formatted disk; the difference in storage between them depends on the drive's application. Floppy disk drive and media manufacturers specify the unformatted capacity (for example, 2 MB for a standard 3½-inch HD floppy). It is implied that this should not be exceeded, since doing so will most likely result in performance problems. DMF was introduced permitting 1.68 MB to fit onto an otherwise standard 3½-inch disk; utilities then appeared allowing disks to be formatted as such.

Mixtures of decimal prefixes and binary sector sizes require care to properly calculate total capacity. Whereas semiconductor memory naturally favors powers of two (size doubles each time an address pin is added to the integrated circuit), the capacity of a disk drive is the product of sector size, sectors per track, tracks per side and sides (which in hard disk drives with multiple platters can be greater than 2). Although other sector sizes have been known in the past, formatted sector sizes are now almost always set to powers of two (256 bytes, 512 bytes, etc.), and, in some cases, disk capacity is calculated as multiples of the sector size rather than only in bytes, leading to a combination of decimal multiples of sectors and binary sector sizes. For example, 1.44 MB 3½-inch HD disks have the "M" prefix peculiar to their context, coming from their capacity of 2,880 512-byte sectors (1,440 KiB), inconsistent with either a decimal megabyte nor a binary mebibyte (MiB). Hence, these disks hold 1.47 MB or 1.41 MiB. Usable data capacity is a function of the disk format used, which in turn is determined by the FDD controller and its settings. Differences between such formats can result in capacities ranging from approximately 1300 to 1760 KiB (1.80 MB) on a "standard" 3½-inch high density floppy (and up to nearly 2 MB with utilities such as 2MGUI). The highest capacity techniques require much tighter matching of drive head geometry between drives, something not always possible and unreliable. For example, the LS-240 drive supports a 32 MB capacity on standard 3½-inch HD disks,^[45] but it is, however, a write-once technique, and requires its own drive.

The raw maximum transfer rate of 3½-inch HD floppy drives and interfaces, disregarding overheads, is as much as 1,000 kilobits/s, or approximately 83% that of single-speed CD‑ROM (71% of audio CD). This represents the speed of raw data bits moving under the read head; however, the effective speed is somewhat less due to space used for headers, gaps and other format fields.

See also

• dd (Unix)
• Disk image
• Don't Copy That Floppy
• Semi-virtual diskette
• Hard disk drive
• Evolution of storage devices

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 & Reston Publishing Company (Prentice-Hall). ISBN 0-8359-3091-2.
• Englisch, Lothar; Szczepanowski, Norbert (1984). The Anatomy of the 1541 Disk Drive. Grand Rapids, Michigan, USA, 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 number 09121-90000.

External links

Wikimedia Commons has media related to Floppy disk.

• 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
• The Floppy User Guide -historical technical material
• ATHANA International Inc - present day manufacturers of diskettes and other media