Floppy disk format

Main article: Floppy disk

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 oersteds, or in modern SI units in amperes 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 tracks 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 disks.

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.

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.^{[citation needed]} 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 disk 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.

Known disk logical formats

Main article: List of floppy disk formats

Throughout the 1970s and 1980s, many different logical disk formats were used, depending on the hardware platform.

See also

• Double-sided disk
• Double-sided RAM

References

• Floppy Controllers Jumper settings
• Floppy disk specifications at Disc Interchange
• Apple II Diskette FAQ and Apple II Drive FAQ at comp.sys.apple2 FAQ mirror
• "Anatomy of the 1541 Disk Drive" (1984) Abacus Software Inc ISBN 0916439011