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Floppy disk

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The 3½-inch floppy disk consists of a flexible recording medium inside a rigid plastic shellEnlarge


A floppy disk is a data storage device that comprises a circular piece of thin, flexible (hence the name) magnetic medium encased in a square or rectangular plastic wallet. Floppy disks are read and written by a floppy disk drive or FDD, not to be confused with "fixed disk drive", which is an old IBM term for a hard disk drive.

Table of contents
1 Background
2 History
3 Structure
4 Compatibility
5 More on floppy disk formats
6 The floppy as a metaphor
7 See also
8 External links


Floppy disks, also known as floppies or diskettes (a name chosen in order to be similar to the word "cassette"), were ubiquitous in the 1980s and 1990s, being used on home and personal computer ("PC") platforms such as the Apple II, Macintosh, Commodore 64, Amiga, and IBM PC to distribute software, transfer data between computers, and create small backups. Before the advent of the hard drive for PCs, floppy disks were often used to store a computer's operating system (OS), application software, and other data. Many home computers had their primary OS kernelss stored permanently in on-board ROM chips, but stored the disk operating system on a floppy. If it was not a proprietary one from the PC manufacturer, then it was often initially CP/M and later DOS.

By the early 1990s, the increasing size of software meant that many programs were distributed on sets of floppies. Toward the end of the 1990s, software distribution gradually switched to CD-ROM, and higher-density backup formats were introduced (e.g., the Iomega Zip disk). With the arrival of mass Internet access, cheap Ethernet, and USB "keydrives", the floppy was no longer necessary for data transfer either, and the floppy disk was essentially superseded. Mass backups were now made to high capacity tape drives such as DAT or streamers, or written to CDss or DVDs. One unsuccessful attempt in the late 1990s to continue the floppy was the SuperDisk (LS120) with a capacity of 120 MB while the drive was backward compatible with standard 3½-inch floppies.

Nonetheless, manufacturers were reluctant to remove the floppy drive from their PCs, for backward compatibility, and because many companies' IT departments appreciated a built-in file transfer mechanism that always worked and required no device driver to operate properly. Apple Computer was the first mass-market computer manufacturer to drop the floppy drive from a design altogether with the release of their iMac model in 1998. In March of 2003, Dell made a similar decision to make floppy drives optional on its higher-end computers, a move hailed by some as the end of the floppy disk as a mainstream means of data storage and exchange.


Floppy disks are almost universally referred to in imperial measurements, even in countries where metric is the standard.
[Note: Throughout this article, the "K" is used to indicate the "binary kilo" (1,024).]


Origins, the 8-inch disk

An 8-inch floppy disk looks exactly like a big 5¼-inch floppy disk (shown), with a partly exposed magnetic medium spun about a central hub for reading.  The flexible plastic cover contains a cloth inner liner to brush dust from the mediumEnlarge


In 1967 IBM gave their San Jose, California storage development center a new task: develop a simple and inexpensive system for loading microcode into their System/370 mainframess. The 370s were the first IBM machines to use semiconductor memory, and whenever the power was turned off the microcode had to be reloaded. Normally this task would be left to various tape drives which almost all 370 systems included, but tapes were large and slow. IBM wanted something faster and more purpose-built that could also be used to send out updates to customers for $5.

David Noble, working under the direction of Alan Shugart, tried a number of existing solutions to see if he could develop a new-style tape for the purpose, but eventually gave up and started over. The result was a read-only, 8-inch (20 cm) floppy they called the "memory disk", holding 80 kilobytes (KB). The original versions were simply the disk itself, but dirt became a serious problem and they enclosed it in a plastic envelope lined with fabric that would pick up the dirt. The new device became a standard part of the 370 in 1971.

In 1973 IBM released a new version of the floppy, this time on the 3740 Data Entry System. The new system used a different recording format that stored up to 256 KB on the same disks, and was read-write. These drives became common, and soon were being used to move smaller amounts of data around, almost completely replacing magnetic tapes.

When the first microcomputers were being developed in the 1970s, the 8-inch floppy found a place on them as one of the few "high speed" storage devices that could be afforded. The first microcomputer operating system, CP/M, originally shipped on 8-inch disks. However the drives were still very expensive, typically costing more than the computer they were attached to, so most machines of the era used cassette tape instead.

By this time Alan Shugart had left IBM, moved to Memorex for a brief time, and then again in 1973 to found Shugart Associates. They started working on improvements to the existing 8-inch format, eventually creating a new 800 KB system. However profits were hard to find, and in 1974 he was forced out of his own company.

The 5¼-inch minifloppy

In 1976 one of Shugart [Assoc.]'s employees, Jim Adkisson, was approached by An Wang of Wang Laboratories, who felt that the 8-inch format was simply too large for the desktop word processing machines he was developing at the time. After meeting in a bar in Boston, Adkisson asked Wang what size he thought the disks should be, and Wang pointed to a napkin and said "about that size". Adkisson took the napkin back to California, found it to be 5¼ inches (13 cm) wide, and developed a new drive of this size storing 110 KB.

The 5¼-inch drive was considerably less expensive than 8-inch drives from IBM, and soon started appearing on CP/M machines. At one point Shugart Assoc. was producing 4000 drives a day. By 1978 there were more than 10 manufacturers producing 5¼-inch floppy drives, and the format quickly displaced the 8-inch from most applications. These early drives read only one side of the disk, leading to the popular budget approach of cutting a second write-enable slot and index hole into the carrier envelope and flipping it over to use the other side for additional storage.

Tandon introduced a double-sided drive in 1978, doubling the capacity, and a new "double density" format doubled it again, to 360 KB.

For most of the 1970s and 1980s the floppy drive was the primary storage device for microcomputers. Since these micros had no hard drive, the OS would have to be loaded from one floppy disk, which was then removed and replaced by another one containing the application. Some machines using two disk drives (or one dual drive) enabled the user to leave the OS disk in place and simply change the application disks as needed. In the early 1980s, 96 track-per-inch drives appeared, increasing the capacity from 360 to 720 KB. These did not see widespread use. In 1984, along with the IBM PC/AT, the Quad Density disk appeared, which used 96 tracks per inch combined with a higher density on each track to provide 1.2 megabytes (MB) of storage. At a time when the average hard disk held 10–20 megabytes, this was considered quite spacious.

By the end of the 1980s, the 5¼-inch disks had been superseded by the 3½-inch disks. Even though 5¼-inch drives were still available, as were disks, they faded in popularity as the 1990s began. On most new computers the 5¼-inch drives were optional equipment. By the mid-1990s the drives had virtually disappeared as the 3½-inch disk became the pre-eminent floppy disk.

The 3½-inch microfloppy

The non-ferrous metal sliding door protects the 3½-inch floppy disk's recording medium allowing it to store as much as 1.44MBEnlarge


Throughout the early 1980s the limitations of the 5¼-inch format were starting to become clear as machines grew in power. 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 slideable write protect catch. Amstrad incorporated a 3-inch 160 KB single-sided disk drive into their CPC and PCW lines, and this format and the drive mechanism was later "inherited" by the ZX Spectrum +3 computer after Amstrad bought Sinclair Research. Media in this format remained expensive and it never caught on.

Things changed dramatically in 1984 when Apple Computer selected the Sony 90.0 × 94.0 mm format for their Macintosh computers, thereby forcing it to become the standard format in the United States. [This is yet another example of the "silent" change from imperial to metric units; this product was advertised and became popularly known as the 3½-inch disk, emphasizing the fact that it was smaller than the existing 5¼-inch.] — By 1989 the 3½-inch was outselling the 5¼-inch.

The 3½-inch disks had, by way of their rigid case's slide-in-place metal cover, the significant advantage of being quite well protected against unintended physical user contact with the disk surface whenever the disk was handled outside the disk drive. When the disk was inserted, however, a part inside the drive took care of moving the disk's metal cover aside, thus giving the drive's read/write heads their necessary access to the magnetic recording surfaces. [Adding the slide mechanism resulted in a slight departure from the previous square outline. The rectangular shape had the additional merit that it made it impossible to insert the disk sideways by mistake, as had indeed been possible with the earlier ones.]

Like the 5¼-inch, the 3½-inch disk underwent an evolution of its own. They were originally offered in a 360 KB single-sided and 720 KB double-sided double-density format (the same as then-current 5¼-inch disks). A newer "high-density" format, displayed as "HD" on the disks themselves and storing 1.4 MB of data, was introduced in the mid-80s. IBM used it on their PS/2 series introduced in 1987. Apple started using "HD" in 1988, on the Macintosh IIx. Another advance in the oxide coatings allowed for a new "extended-density" ("ED") format at 2.88 MB introduced on the second generation NeXT Computers in 1991, but by the time it was available it was already too small to be a useful advance over 1.4 MB, and never became widely used. The 3½-inch drives sold more than a decade later still used the same format that was standardized in 1989, in ISO 9529-1,2.

The 3½-inch disks are still widely available. The 3½-inch drives are still standard equipment on most new computers. On others, they are either optional equipment, or can be purchased on an after-market basis. However, with the advent of other portable storage options, such as ZIP disks, USB storage devices, and CD-R and CD-RW, the 3½-inch disk is becoming increasingly obsolete. Some manufactures have even stopped offering 3½-inch drives on new computers as standard equipment.

The formatted capacity of 3½-inch high-density floppies was originally 1440 kibibytes (KiB), or 1,474,560 bytes. This is equivalent to 1.41 MiB (1.47 MB decimal). However, their capacity is usually reported as 1.44 MB by diskette manufacturers.

In some places, especially South Africa, 3½-inch floppy disks have commonly been called stiffy disks, because of their "stiff" (rigid) cases, which are contrasted with the flexible "floppy" cases of 5¼-inch floppies.




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. Disks of this type were said to be soft sector disks. Very early 8-inch and 5¼-inch disks also had holes for each sector, and were termed hard sector disks. Inside the disk were two layers of fabric designed to reduce friction between the media and the outer casing, with the media sandwiched in the middle. The outer casing was usually a one-part sheet, folded double with flaps glued or spot-melted 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.

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

The 3½-inch floppy disk drive automatically engages when the user inserts a disk, and disengages and ejects with the press of a button, or by motor on the [[Apple MacintoshEnlarge



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 media. 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 European ring binders.] A notch top right ensures that the disk is not inserted incorrectly, 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 would barely make it out of the disk drive; others would 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. — With a PC, a floppy disk can be inserted or ejected manually at any time (at most resulting in an error message), as the drive is only monitored as required during read or write operations. With an Apple Macintosh computer, disk drives are continuously monitored by the OS; a disk inserted is automatically searched for content. These kind of disk drives (starting with the slim "Twiggy" drives of the late Apple "Lisa") do not have an eject button, but use 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 (like in the case of a power failure or drive malfunction), one can insert a straight-bent paper clip into a small hole at the drive's front, thereby forcing the disk to eject (similar to what is provided with CD/DVD drives).

The 3-inch disk bears a lot of similarity to the 3½-inch type, with some unique and somehow 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 PCMCIA notebook expansion card, rather than a floppy disk. Despite the size, the actual 3-inch magnetic-coated disk occupied less than 50 per cent 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.


The three physical sizes of floppy disks are incompatible, and disks can only be loaded 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. Consider, for example the following Apple/IBM 'schism': Apple Macintosh computers can read, write and format IBM PC-format 3½-inch diskettes, provided suitable software is installed. However, many IBM-compatible computers use floppy disk drives that are physically unable to use Apple-format disks. For the details on this, see the section "More on floppy disk formats".

Within the world of IBM-compatible computers, the three densities of 3½-inch floppy disks are partly compatible. Higher density drives are built to read, write and even format lower density media without problems, provided the correct media is used for the density selected. However, if by whatever means a diskette is formatted at the wrong density, the result is magnetically unstable with a risk of long-term data loss.

The situation was even more complex with 5¼-inch diskettes. The head of a 1.2 MB drive is narrower than that of a 360 KB drive, but will format, read and write 360 KB diskettes with apparent success. A blank 360 KB disk formatted and written on a 1.2 MB drive can be taken to a 360 KB drive without problems, similarly a disk formatted on a 360 KB drive can be used on a 1.2 MB drive. But a disk written on a 360 KB drive and updated on a 1.2 MB drive becomes permanently unreadable on any 360 KB drive, owing to the incompatibility of the track widths. 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 360 KB single sided 5¼-inch disks, and both sold disks labeled "single sided" and 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.

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 practically doubled. For re-protecting a disk side, one would simply place a piece of opaque tape over the notch/hole in question. These "flippy disk procedures" were followed by owners of practically all home computer single sided disk drives.

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, 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 'Microsoft distribution format' disks, a hack that allowed 1.68 MB 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 but also on the MSX, Atari ST, Amstrad CPC, and most other microcomputer platforms, disks are written using a Constant Angular Velocity (CAV) – 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 each.

However, this is not an efficient way to use the disk surface. The sectors having a constant angular size, this means that the 512 bytes packed into a small space near the disk's center is spread out across much more space near the edge. A better technique would be to increase the number of sectors toward the edge, from 18 to 30 for instance, thereby keeping constant the amount of physical disk space used for storing each 512 byte sector. 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 80 KB on a double-sided disk. This higher capacity came with a serious disadvantage, though; the format required a special mechanism that was 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 standard HD drives on their later machines.

The Commodore 128

The Commodore 128 used a special 3½-inch 800 KB disk format with its 1581 disk drive (which was compatible with all CBM 8-bit serial-bus based machines). Commodore actually started its tradition of special disk formats with the 5¼-inch disk drives accompanying its PET/CBM, VIC-20 and C64 home computers, like the 1540 and (better-known) 1541 drives used with the latter two machines. These disk drives used Commodore's in-house developed Group Code Recording, based on up to four different data rates according to the track position.

Eventually, however, Commodore had to give in to disk format standardization, and made its last 5¼-inch drives, the 1570 and 1571, compatible with Modified Frequency Modulation (MFM), to enable the C128 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 extra third-party software), which was a crucial feature for some office work. A typical usage would be to copy MS-DOS text files off PCs at one's workplace and take the files home to edit on a C128 (and then copy the files back).

The Commodore Amiga

The Commodore Amiga computers used other kinds of floppy disk optimizations for extra storage, mainly the use of smaller sector gaps, made possible by custom control of the floppy drive rather than using the IBM PC standard disk controller. This allowed 11 (512-byte) sectors per track instead of 9; a total of 880 KB on a DD floppy, and 1.76 MB on HD. Further tricks used by third-party developers, such as writing an entire track at once and removal of the generally unused "sector label" headers, allowed for 12 sectors per track and thus 960 KB on a standard DD floppy or 1.87 MB on HD.

The Acorn Archimedes

Another machine using a similar "advanced" disk format was the British Acorn Archimedes, which stored 800 KB on a 3½-inch DD floppy.

12-inch floppy disks

In the late 1970s some IBM mainframes also used a 12-inch (30 cm) floppy disk, but little information is currently available about their internal format or capacity.

What is the ultimate data capacity and speed of a floppy disk?

It is not easy to provide an answer for data capacity, as there are many factors involved, starting with the particular disk format used. The differences between formats and encoding methods can result in data capacities ranging from 720 kilobytes (KB) or less up to 1.72 megabytes (MB) or even more on a standard 3½-inch high-density floppy, just from using special floppy disk software, such as the fdformat utility which enables "standard" 3½-inch HD floppy drives to format HD disks at 1.62, 1.68 or 1.72 MB, though reading them back on any machine is another story.

Sometimes however, manufacturers provide an "unformatted capacity" figure, which is roughly 2.0 MB for a standard 3½-inch HD floppy, and should imply that data density can't (or shouldn't) exceed a certain amount. There are however some special hardware/software tools, such as the CatWeasel floppy disk controller and software, which claim up to 2.23 MB of formatted capacity on a HD floppy. Such formats however are not standard, hard to read and probably not very reliable. It's probably true that floppy disks can surely hold an extra 10–20% formatted capacity versus their "nominal" values, but at the expense of reliability or hardware complexity.

3½-inch HD floppy drives typically have a transfer rate of 500 kilobaud. While this 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 in each revolution.

This happens because sectors aren't written exactly in a sequential manner but are somehow scattered around the disk, which is a way to introduce yet another delay for older machines and controllers to cope with the data flow from the disk without having to actually stop it. Such delays however become unnecessary burdens when technology advances.

By changing this parameter, the actual sector sequence may become more adequate for the machine's speed. For example, an IBM format 1.4 MB disk formatted with a sector:shift ratio of 3:2 has a sequential reading time (for reading ALL of the disk in one go) of just 1 minute, versus 1 minute and 20 seconds or more of a "normally" formatted disk. It's interesting to note that the "specially" formatted disk is very—if not completely—compatible with all standard controllers and BIOS, and generally requires no extra software drivers, as the BIOS generally "adapts" well to the slightly modified format.

The floppy as a metaphor

For more than two decades now, the floppy disk has been the primary external writable storage device used. Also, in a non-network environment, floppies have been the primary means of transferring data between computers (sometimes jokingly referred to as Sneakernet or Frisbeenet). 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 on buttons and other user interface elements related to saving files.

See also

External links