Floppy disk format
Encyclopedia
Floppy disk format and density refer to the logical and physical layout of data stored on a floppy disk
. Since their introduction, there have been many popular and rare floppy disk types, densities, and formats used in computing, leading to much confusion over their differences. In the early 2000s, most floppy disk types and formats became obsolete, leaving the 3½ inch disk, using an IBM PC
compatible format of 1440 KB, as the only remaining popular format.
Different floppy disk types had different recording characteristics, with varying magnetic coercivity
(measured in oersted
s, or in modern SI
units in ampere
s per meter), ferrite grain size, and tracks per inch (TPI). TPI was not a part of the physical manufacturing process; it was a certification of how closely track
s of data could be spaced on the medium safely.
The term density has a double meaning for floppy disks. Originally, single density and double density indicated a difference in logical encoding on the same type of physical media -- FM
for single, and MFM
for double. Future use of the term "density" referred to physical characteristics of the media, with MFM assumed to be the logical format used. GCR
was also used on some platforms, but typically in a "double" density form.
8 and 5¼ inch floppy disks were available with both soft sectoring and hard sectoring
. Because of the similarity in magnetic characteristics between some disk types, it was possible to use an incorrectly certified disk in a soft sectored drive. Quad density 5¼ inch disks were rare, so it was not uncommon to use higher quality double density disks, which were usually capable of sustaining the 96 TPI formatting of quad density, in drives such as the Commodore 8050
.
Disks were available in both single and double sided forms, with double sided formats providing twice the storage capacity. Like TPI, "double sided" was mostly a certification indicator, as the magnetic media was usually recordable on both sides. Many (but not all) certified "double sided" 8 and 5¼ inch floppies had an index hole on both sides of the disk sleeve to make them usable as flippy disk
s.
A combination floppy disk and optical disc
, known as a Floptical
disk exists. The size of a 90 mm (3.5 in) disk, they are capable of holding close to 20.8 MB, but need a special drive.
Most common floppy disks in use are formatted in the FAT12 file system format, though sometimes disks may use a more exotic file system and/or be superformatted to accommodate slightly more data. Some floppy-based Linux distributions utilize such techniques. The capacity numbers given in this section assume FAT12 formatting unless otherwise noted.
which could typically hold 35-40 tracks of nine 512-byte
(or 18 256-byte) sectors each. Only one side of the disc was used, although some users did discover that punching additional holes into the disc jacket would allow the creation of a "flippy" disc which could be manually turned over to store additional data on the reverse side.
Single-sided disks began to become "obsolete" soon after the introduction of the original IBM
5150 PC
in 1981, which used 360Kb double-sided double-density drives. Ironically this same year, Commodore released a floppy disk system that could store 1MB of data but it was not well received in part because their users felt that it was overkill.
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
. Since their introduction, there have been many popular and rare floppy disk types, densities, and formats used in computing, leading to much confusion over their differences. In the early 2000s, most floppy disk types and formats became obsolete, leaving the 3½ inch disk, using an IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
compatible format of 1440 KB, as the only remaining popular format.
Different floppy disk types had different recording characteristics, with varying magnetic coercivity
Coercivity
In materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...
(measured in oersted
Oersted
Oersted is the unit of magnetizing field in the CGS system of units.-Difference between cgs and SI systems:...
s, or in modern SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
units in ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s per meter), ferrite grain size, and tracks per inch (TPI). TPI was not a part of the physical manufacturing process; it was a certification of how closely track
Cylinder-head-sector
Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.Though CHS...
s of data could be spaced on the medium safely.
The term density has a double meaning for floppy disks. Originally, single density and double density indicated a difference in logical encoding on the same type of physical media -- FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
for single, and MFM
Modified Frequency Modulation
Modified Frequency Modulation, commonly MFM, is a line coding scheme used to encode the actual data-bits on most floppy disk formats, hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles. Early hard disk drives also used this coding.MFM is a modification to the original...
for double. Future use of the term "density" referred to physical characteristics of the media, with MFM assumed to be the logical format used. GCR
Group Code Recording
In computer science, group code recording refers to several distinct but related encoding methods for magnetic media. The first, used in 6250 cpi magnetic tape, is an error-correcting code combined with a run length limited encoding scheme...
was also used on some platforms, but typically in a "double" density form.
8 and 5¼ inch floppy disks were available with both soft sectoring and hard sectoring
Hard sectoring
Hard sectoring in a magnetic or optical data storage device is a form of sectoring which uses a physical mark or hole in the recording medium to reference sector locations....
. Because of the similarity in magnetic characteristics between some disk types, it was possible to use an incorrectly certified disk in a soft sectored drive. Quad density 5¼ inch disks were rare, so it was not uncommon to use higher quality double density disks, which were usually capable of sustaining the 96 TPI formatting of quad density, in drives such as the Commodore 8050
Commodore 8050
The Commodore 8050 and Commodore 8250 were dual unit 5¼" floppy disk drives for Commodore International computers. They used a wide rectangular steel case form similar to that of the Commodore 4040, and used the IEEE-488 interface common to Commodore PET/CBM computers.The 8050 was a single...
.
Disks were available in both single and double sided forms, with double sided formats providing twice the storage capacity. Like TPI, "double sided" was mostly a certification indicator, as the magnetic media was usually recordable on both sides. Many (but not all) certified "double sided" 8 and 5¼ inch floppies had an index hole on both sides of the disk sleeve to make them usable as flippy disk
Flippy disk
A flippy disk is a double-sided 5¼" floppy disk, specially modified so that the two sides can be used independently in single-sided drives...
s.
A combination floppy disk and optical disc
Optical disc
In computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...
, known as a Floptical
Floptical
Floptical refers to a type of disk drive that combines magnetic and optical technologies to store large amounts of data on media similar to 3½-inch floppy disks. The name is a portmanteau of the words 'floppy' and 'optical'...
disk exists. The size of a 90 mm (3.5 in) disk, they are capable of holding close to 20.8 MB, but need a special drive.
Logical formatting
Formatted disk capacity is always less than the nominal capacity provided for each type of disk. Leaving some space empty between sectors and tracks provides some more reliability by preventing bits from being stored too close together in the magnetic film.Most common floppy disks in use are formatted in the FAT12 file system format, though sometimes disks may use a more exotic file system and/or be superformatted to accommodate slightly more data. Some floppy-based Linux distributions utilize such techniques. The capacity numbers given in this section assume FAT12 formatting unless otherwise noted.
Single Sided, Double Density
SSDD originally referred to Single Sided, Double Density, a format of (usually 5¼") floppy diskFloppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
which could typically hold 35-40 tracks of nine 512-byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
(or 18 256-byte) sectors each. Only one side of the disc was used, although some users did discover that punching additional holes into the disc jacket would allow the creation of a "flippy" disc which could be manually turned over to store additional data on the reverse side.
Single-sided disks began to become "obsolete" soon after the introduction of the original IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
5150 PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
in 1981, which used 360Kb double-sided double-density drives. Ironically this same year, Commodore released a floppy disk system that could store 1MB of data but it was not well received in part because their users felt that it was overkill.