Floppy disk format

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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.

Contents

[edit] 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 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 3.5 disk, they are capable of holding close to 20.8 MB[1], but need a special drive.

[edit] 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.

[edit] 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 300 GCR [5]
2 227.50 KiB
16 1 140 KiB
2 280 KiB
Commodore (8-bit) 5¼ in double 256 Variable (17-21) 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
Atari 8-bit 5¼ in single 128 18 40 1 90 KiB 288 FM [10]
enhanced 128 26 127 KiB MFM
double 256 18 180 KiB

[edit] Notes

  1. ^ The Ultimate Computer Buyer's Guide:Storage Devices
  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.
  10. ^ Atari XF551 uses 360K, 300RPM, MFM, Double Side/Double Density.

[edit] References