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.
Background
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essentially superseded.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 CDs 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.
History
Origins, the 8-inch disk
In 1967 IBM tasked their San Jose 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"Êfloppy they called the "memory disk", holding 80Êkilobytes (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"Êfloppy 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"Êdisks. 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"Êformat, eventually creating a new 800KB
system. However profits were hard to find, and in 1974 he was forced out of
his own company.
The 5¹-inch minifloppy
In 1976 one of Shugart's employees, Jim Adkisson, was approached by An Wang
of Wang Laboratories, who felt that the 8"Êformat 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 5¹Êinches wide,
and developed a new drive of this size storing 110KB.
The 5¹"Êdrive was considerably less expensive than 8"Êdrives 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
5¹"Êfloppy 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 3¸-inch microfloppy
Throughout the early 1980s the limitations of the 5¹"Êformat 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"Ê160KB 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"Êformat (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 5¹", which then disappeared
from the market over the next couple of years.
Like the 5¹", the 3.5"Êdisk underwent an evolution of it's own. They were
originally offered in a 360KB single-sided and 720KB double-sided
double-density format (the same as then-current 5¹"Êdisks). 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.
Structure
The 5¹" 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 5¹"
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.
[3½ inch (90mm) floppy disk drive with cover removed]
3¸ inch (90mm) floppy disk drive with cover removed
The 3¸" 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
3¸" 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.
The 3" disk bears a lot of similarity to the 3¸" type,with some unique and
somehow curious features. One example is the rectangular-shaped plastic
casing,almost taller than a 3¸" 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.
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 within each form factor. Consider, for
example the following Apple/IBM 'schism': Apple Macintosh computers can
read, write and format IBM PC-format 3¸" 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 3¸"
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 5¹" 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 5¹"
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 3¸" 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 3¸" DD floppy.
The Commodore 128 also used a special 3¸" 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 5¹" 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 5¹" drives, the 1570 and 1571,
compatible with the MFM format, 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
(with extra software), which was a crucial feature for some office work.
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