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Floppy disk

5 inch floppy disk; 8 inch floppy disk looks similar

3 inch (90mm) floppy disk

A floppy disk is a type of data storage device that comprises a circular piece of thin, flexible (hence the name) magnetic media encased in a square or rectangular plastic wallet. The fact that the exterior aspect is not circular confuses some novice users. 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 See also:
7 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 platforms such as the Apple II, Commodore 64, and IBM PC to distribute software, transfer data between computers, and create small backups. Before the advent of the hard drive, floppy disks were often used to store a computer's operating system (OS) and application software (although many home computers had their OS kernels stored permanently in on-board ROM chips).

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and USB "thumbdrives", the floppy was no longer necessary for data transfer either, and the floppy disk was essentially superseded.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.

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 eschew the floppy altogether with the release of their iMac model in 1998: it had no floppy disk drive. In March of 2003, Dell Computer 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.


Origins, the 8-inch disk

In 1967 IBM tasked their San Jose, California storage development center with a new task: develop a simple and inexpensive system for loading microcode into their 370 mainframes. The 370 was the first IBM machine to use semiconductor memory, and whenever the power was turned off it had to be re-loaded. Normally this task would be left to various tape drives which almost all 370 systems included, but tapes were large and slow. They wanted something more purpose-built that could 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" 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 256KB 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" 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" 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" format, eventually creating a new 800KB 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's employees, Jim Adkisson, was approached by An Wang of Wang Laboratories, who felt that the 8" 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 wide, and developed a new drive of this size storing 110KB.

The 5¼" drive was considerably less expensive than 8" drives from IBM, and soon started appearing on CP/M machines. At one point Shugart was producing 4000 drives a day. By 1978 there were more than 10 manufacturers producing 5¼" floppy drives, and the format quickly displaced the 8" from most applications. Tandon then introduced a double-sided drive, doubling the capacity, and a new "double density" format doubled it again, to 360KB.

For most of the 1970s and 80s 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 1984 along with the IBM PC/AT, the Quad Density disk appeared, providing 1.2 megabytes (MB) of storage.

The 3½-inch microfloppy

Throughout the early 1980s the limitations of the 5¼" format were starting to become clear as machines grew in power. A number of solutions were developed, with drives at 2", 2.5", 3" and 3.5" 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" 160KB single-sided disk drive into their CPC and PCW lines, which was later "inherited" by the ZX Spectrum 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 3.5" format (originally defined as 90mm) for their Macintosh computers, thereby forcing it to become the standard format in the US. By 1989 the 3.5" was outselling the 5¼", which then disappeared from the market over the next couple of years.

Like the 5¼", the 3.5" disk underwent an evolution of its own. They were originally offered in a 360KB single-sided and 720KB double-sided double-density format (the same as then-current 5¼" disks). A newer "high-density" format, displayed as "HD" on the disks themselves, was introduced on the Macintosh IIx series machines in the later half of the 1980s, storing 1.44MB of data. Another advance in the oxide coatings allowed for a new "extended-density" ("ED") format at 2.88MB 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 1.44, and never became widely used.


5¼ inch floppy disk drive
The 5¼" disk had a large circular hole in the centre 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 media 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 writeable, detected by a photo transistor above it. Another LED/phototransistor pair located near the centre 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 5¼" 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" disk is made of two pieces of rigid plastic, with the fabric-media-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 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. The write-protect and high-density holes on a 3½" 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. Macintosh computers typically contained "Automatic" floppy disk drives, which used a motorized mechanism to eject disks. This mechanism was triggered through software, rather than a control on the drive itself.

3½ inch (90mm) floppy disk drive with cover removed

The 3" disk bears a lot of similarity to the 3½" type, with some unique and somehow curious features. One example is the rectangular-shaped plastic casing, almost taller than a 3½" 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" 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" 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.


Obviously, the three physical sizes of floppy disks are incompatible, and disks can only be loaded on the correct size of drive. 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" 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" 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" diskettes. The head of a 1.2M drive is narrower than that of a 360K drive, but will format, read and write 360K diskettes with apparent success. A blank 360K disk formatted and written on a 1.2M drive can be taken to a 360K drive without problems, similarly a disk formatted on a 360K drive can be used on a 1.2M drive. But a disk written on a 360K drive and updated on a 1.2M drive becomes permanently unreadable on any 360K 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 360K single sided 5¼" 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, you could purchase a special tool called a "disk notcher" which would allow you to cut a second "write unprotect" notch in these diskettes and thus use them as "flippies" by putting them in the disk drive one side up and then the other – to get double the data storage capacity. 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

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. The HD format of 3½" floppy disks use 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 (various 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; Microsoft applications were often distributed on 'Microsoft distribution format' disks, a hack that allowed 1.68MB to be stored on a 1.44MB disk by formatting it with 21 sectors instead of 18). 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.

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 at the edge, from 18 to 30 for instance, thereby keeping the amount of physical disk space for storing each 512 byte sector constant. 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 400KB per side, amounting to an extra 80KB 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 Amiga computers used other kinds of floppy disk optimizations for extra storage. One was to not use sectors, and instead write an entire track as a single object with no stop or start information which normally takes up some 10% of the disk. They combined this with a variable sector format similar to the Mac, but without the complexity of the variable-speed drive (in retrospect, one wonders why Apple developed this solution at all). These changes add up to allowing considerably better storage capacity of about 880KB on a DD floppy, and 1.76MB on HD.

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

The Commodore 128 also used a special 3½" 800KB 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" 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 disk rotation speeds according to the track position. Eventually, however, Commodore had to give in to disk format standardization, and made its last 5¼" 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 MSDOS disks (using extra software), which was a crucial feature for some office work.

See also:

External links