Floppy disk

5¼ inch floppy disk; 8 inch floppy disk looks similar

3½ 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.

Floppy disks, also known as floppies or diskettes (to be similar to cassette), were ubiquitous in the 1980s and 1990s, being used on personal computer platforms such as the IBM PC to distribute software, transfer data between computers, and create small backups. Before the advent of the hard drive, floppy disks were used to store a computer's operating system and software.

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 Zip disk). With the arrival of mass Internet access and cheap ethernet, the floppy was no longer necessary for data transfer either, and the floppy disk was essentially superseded. Mass backups are made to high capacity tape drives (such as DAT) or written to compact disc using CD-Writers.

Nonetheless, manufacturers were reluctant to remove the floppy drive from their PCs, for backwards compatibility. Apple Computer was the first 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 widely hailed as the end of the floppy disk as a mainstream means of data storage and exchange.

History

The original floppy disk was the eight-inch. These large disks predate the desktop machine and were mainly used for transferring data between mainframe machines. Their capacity was extremely limited by today's standards, with 160 kilobytes available. The reason for the name floppy was that the exterior casing was only soft flexible plastic, leaving them vulnerable to manhandling.

Home computer manufacturers, who had previously been using magnetic tape (compact cassette) to save data, saw the possibilities for this medium.

The first floppy disk that was used in home machines was the 5¼". Small enough to fit in a desktop unit, the usual incarnation offered 360kB of storage space. Since these machines had no hard disk, the operating system would have to be loaded on one disk and then removed and replaced by another disk containing the application. Later machines using two disk drives enabled the user to leave the operating system disk in one drive and only change the application disk. These disks were also flexible and were usually contained within individual paper envelopes. The much later Quad Density disk provided 1.2 megabytes of information on one disk.

Amstrad incorporated a 3-inch disk drive into their CPC664 and CPC6128 models, amongst other manufacturers. They remained expensive and did not become standardised. However, they had a small form factor and a rigid case with a slideable write-protect catch, features which would be seen later in the 3½" disk.This 3" disk drive was later "inherited" by the Spectrum ZX computer after Amstrad bought Sinclair.This format was also used on the SAM Coupe and mostly on Z80 based machines using CP/M as the operating system.Their capacity was roughly 160kB per side (most 3" formats were used in single-headed disk drives and were flippable,with two simmetric sides with independent file systems),just below the "standard" 360kB capacity of other floppy disk media of the time.

But by this time, the 90mm disk was arriving. American computing manufacturers called it a "3½ inch" disk (although 3.50 inches is actually 88.9mm), and the name has stuck even in countries that normally use the metric system. Borrowing from the advances made in the three-inch disks, as well as enhanced methods of manufacturing, they were able to contain 720kB of data in their first standard, Double Sided Double Density (when formatted to MS-DOS; Amiga floppies carried 880k, and early Apple floppies carried 800k). However, there was soon a new standard - High Density - represented by a human-readable stylised 'HD' in the top right of the disk and a machine-readable hole in the bottom left corner, with the write-protect catch remaining in the bottom-right corner. These were capable of 1.44MB (1.76MB on Amiga), and remain the standard in floppy disks, despite successful attempts to put 2.88MB on a disk via enhanced formatting techniques and use of special oxide coatings. 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. The most often used file system on floppy disks is FAT12.

Structure

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, allowing (write protected) floppies to be clipped into European ringbinders. 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.

The 3" disk bears a lot of similarity to the 3&frac12" type,with some unique and somehow curious features. One example is the rectangular-shaped plastic casing,almost taller than a 3&frac12" disk ,but narrower,and more than twice as thick,almost the size of a standard 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 weird protection and sealing mechanisms implemented on the disks,as described below. Such mechanisms were largely responsible for the thickness,length and high costs of the 3" disks.

The reason for this was that the metal hatch which protected the disk was INTERNAL to the plastic casing,and could only be opened by sliding a lever placed on the right side of the disk,depending on which front side of the disk one was facing.This lever was activated when pushing the disk into the drive,where the lever was caught and caused the hatch to slide aside as the disk was pushed all the way into the drive,thus exposing the media surface to the drive heads.This built-in mechanism required a non-trivial amount of force to be activated (in fact,inserting a 3" disk required some more effort than a 3&frac12" one),as it had a powerful return spring and of course was one of the factors that contributed to the high cost of the medium,as mentioned before.The mechanism itself proved reliable enough through the years, though.It also made it relatively difficult to "open" the disk by mistake when not inside the disk drive,thus protecting the disk from accidental dust exposure etc.

Another interesting feature of the 3" disks was that they were DOUBLE SIDED,and the two sides were entirely simmetrical.This was due to the fact that the drives used on the Amstrad CPC computers and most other 3" disk-using machines,had single heads,and required to "flip" the disk in the drive to access the "other side".Sides were labeled as 1 and 2,or as A and B,like audio cassettes. Needless to say,each side had its own,independent hatch door mechanism. Two-headed drives existed for this format,but were less common. A side effect was that while the total disk space was 320K,it was split into two 160K parts,and the whole system behaved like two separate smaller disks with separate file systems,thus making it impossible,for example,saving a continuum of small files with some being spread on both disk sides,or using the whole disk capacity "all at once".Unlike single-sided 3&frac12" or 5&frac14" disk drives however,the user had the opportunity to use both sides of the disk by simply flipping it,while with the other formats this was generally not possible,and thus half of the disk capacity was lost. Write protection was implemented by a push-pull switch place on the lower-right portion of each side of the disk.

The reliability of those 3" disks was almost as good,if not better,as that of the 3&frac12" disks,and the cumbersome hatchdoor mechanism proved reliable too. What caused most failures were,however,the 3" disk drives themselves,as they broke down very easily and proved unreliable through the years.

Compatibility

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.

Apple Macintosh computers can read, write and format IBM-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. More specifically,the Apple Macintosh computers employed,at least for the low capacity (800kB) disks,a Variable Sector Capacity format,as opposed to the Constant Angular Velocity,CAV and Constant Sector Capacity format used on the IBM and most others computer systems.In fact,spinning the disk at CAV while writing the same amount of data on each disk [[[sector]], implies that data density is greater in the inner sectors of the disk,closest to the ring,than the outer ones.

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 then be used on a 1.2M drive. However, a disk written on a 360K drive and then updated on a 1.2M drive then becomes permanently unreadable on any 360K drive, owing to the incompatibility of the track widths. There are several other scenarios that will not work.