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Encyclopedia > Floppy disk format

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 KiB, as the only remaining popular format. A floppy disk is a data storage device that is composed of a disk of thin, flexible (floppy) magnetic storage medium encased in a square or rectangular plastic shell. ... IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ...

Contents

Physical composition

Floppy disk physical characteristics
(capacity and tracks are nominal, per side)
Size Density Tracks TPI Coercivity Capacity
3½ in single 40 67.5 600 Oe 250 KB
double 80 135 600 Oe 500 KB
high 80 135 750 Oe 1000 KB
extended 80 135 900 Oe 2000 KB
5¼ in single/double 40 48 300 Oe 250 KB
quad 80 96 300 Oe 500 KB
high 80 96 600 Oe 750 KB
8 in single/double 77 48 300 Oe 1000 KB

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 oersted is old CGS unit of magnetic field strength (or magnetic induction). ... Look up si, Si, SI in Wiktionary, the free dictionary. ... Current can be measured by a galvanometer, via the deflection of a magnetic needle in the magnetic field created by the current. ... The metre or meter is a measure of length. ... It has been suggested that Disk sector be merged into this article or section. ...


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. Frequency modulation (FM) is a form of modulation that represents information as variations in the instantaneous frequency of a carrier wave. ... Modified Frequency Modulation, commonly MFM, is a line coding scheme used to encode information on most floppy disk formats, which include the floppy disk formats used in most CP/M machines as well as PCs running DOS. MFM is a modification to the original FM (frequency modulation) scheme for encoding... Group Code Recording (GCR) is a floppy disk data encoding format used by the Apple II and Commodore Business Machines in the 5¼ disk drives for their 8-bit computers (the best-known drives being the Disk II for the Apple II family and the Commodore 1541, used with the...


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. Hard sectoring in a magnetic or optical data storage device is an archaic form of sectoring that uses a physical mark or hole in the recording medium, from which sector locations are referenced. ... The Commodore 8050 and Commodore 8250 were dual unit 5¼ floppy disk drives for Commodore International computers. ...


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 flippy disk is a double-sided 5 1/4 floppy disk, specially modified so that the two sides can be used independently (but not simultaneously) in single-sided drives. ...


A combination floppy disk and optical disc, known as a Floptical disk exists. The size of a 3.5 disk, they are capable of holding close to 20.8 MB[1], but need a special drive. The optical lens of a compact disc 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.


Known disk logical formats

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

Common floppy disk formats, logical characteristics by platform
Platform Size Density Bytes/ sector Sectors/ track Tracks/ side Sides Capacity RPM Encoding Note
IBM (3740 format) 8 in single 128 26 74 1 250.25 KiB 360 FM [2]
Apple II 3½ in double 512 Variable (8-12) 80 1 400 KiB CLV GCR [3]
2 800 KiB
high 512 18 80 2 1440 KiB 300 MFM [4]
5¼ in double 256 13 35 1 113.75 KiB  ??? GCR [5]
2 227.50 KiB
16 1 140 KiB
2 280 KiB
Commodore (8-bit) 5¼ in double 256 Varied 35 1 170 KiB 300 GCR [6]
2 340 KiB
Commodore Amiga 3½ in double 512 11 80 2 880 KiB 300 MFM [7]
high 22 1760 KiB 150
IBM PC compatibles 3½ in double 512 9 80 2 720 KiB 300 MFM
high 18 80 1440 KiB
21 80 1680 KiB [8]
21 82 1720 KiB
extended 36 80 2880 KiB
5¼ in double 512 8/9 40 1 160/180 KiB 300 MFM
2 320/360 KiB
high 15 80 2 1200 KiB 360
NEC PC98 3½ in high 512 15 80 2 1200 KiB 360 MFM [9]
1024 8 1280 KiB

International Business Machines Corporation (known as IBM or Big Blue; NYSE: IBM) is a multinational computer technology and consulting corporation headquartered in Armonk, New York, USA. The company is one of the few information technology companies with a continuous history dating back to the 19th century. ... The Apple II was one of the most popular personal computers of the 1980s. ... Constant Linear Velocity (CLV) refers to how information is written to or read from a rotating data disk. ... Commodore, the commonly used name for Commodore International, was an American electronics company based in West Chester, Pennsylvania which was a vital player in the home/personal computer field in the 1980s. ... The original Amiga 1000 (1985) with various peripherals The Amiga is a family of personal computers originally developed by Amiga Corporation. ... IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ... The NEC PC-9801 (or the PC-98 for short) is a Japanese microcomputer manufactured by NEC. It2 CPU, which could selectably run at a speed of either 5 or 8 MHz. ...

Notes

  1. ^ http://library.n0i.net/miscellanea/ul-pcbuy/ch14/222-225.html
  2. ^ They have 73 data tracks, 1 index track, 2 spare tracks, 1 reserved track
  3. ^ Apple II double-density 3½ inch drives use variable sectors sizes (tracks 00-15: 12 sectors, tracks 16-31: 11 sectors, tracks 32-47: 10 sectors, tracks 48-63: 9 sectors, tracks 64-79: 8 sectors)
  4. ^ Apple II high-density 3½ inch drives require a compatible disk controller and ProDOS 8.
  5. ^ 16 sector filesystems require a compatible disk controller (PROM update) and Apple DOS 3.3 or later
  6. ^ The Commodore floppy disk format used different numbers of sectors per track in order to store data on wider tracks more efficiently.
  7. ^ Though the Amiga used MFM, the format places sectors too close together for a standard IBM PC compatible floppy disk controller to read (appearing as one 5632-byte physical sector per track).
  8. ^ These variations are known as DMF diskettes, used for a time to pack more data on to each disk for software distribution.
  9. ^ The PC98 3½" formats are also known as "Mode 3" floppy disks, usable on IBM PC compatibles with a tri-mode floppy drive.

Distribution Media Format (DMF) is a format for floppy disks that Microsoft used to distribute software. ...

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

  Results from FactBites:
 
Floppy disk - Wikipedia, the free encyclopedia (10172 words)
Floppy disks are read and written by a floppy disk drive or FDD, the latter initialism not to be confused with "fixed disk drive", which is an old IBM term for a hard disk drive.
Floppy disk sizes are almost universally referred to in imperial measurements, even in countries where metric is the standard, and even when the size is in fact defined in metric (for instance the 3½-inch floppy which is actually 9 cm).
The 3½-inch disks had, by way of their rigid case's slide-in-place metal cover, the significant advantage of being much better protected against unintended physical contact with the disk surface than 5¼-inch disks when the disk was handled outside the disk drive.
  More results at FactBites »


 
 

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