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 KB, as the only remaining popular format.

Contents 1 Physical composition 2 Logical formatting 3 Known disk logical formats 4 Single Sided, Double Density 5 See also 6 Notes 7 References

[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 90 mm (3.5 in) 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 KB	360	FM	[2]
Apple II	5¼ in	double	256	13	35	1	113.75 KB	300	GCR	[3]
						2	227.50 KB
				16		1	140 KB
						2	280 KB
Apple II, Apple Macintosh	3½ in (90 mm)	double	512	Variable (8-12)	80	1	400 KB	CLV	GCR	[4]
						2	800 KB
		high	512	18	80	2	1440 KB	300	MFM	[5]
Commodore (8-bit)	5¼ in	double	256	Variable (17-21)	35	1	170 KB	300	GCR	[6]
						2	340 KB
Commodore Amiga	3½ in (90 mm)	double	512	11	80	2	880 KB	300	MFM	[7]
		high		22			1760 KB	150
IBM PC compatibles	5¼ in	double	512	8/9	40	1	160/180 KB	300	MFM
						2	320/360 KB
		high		15	80	2	1200 KB	360
	3½ in (90 mm)	double	512	9	80	2	720 KB	300	MFM
		high		18	80		1440 KB
				21	80		1680 KB			[8]
				21	82		1720 KB
		extended		36	80		2880 KB
NEC PC98	3½ in (90 mm)	high	512	15	80	2	1200 KB	360	MFM	[9]
			1024	8			1280 KB
Atari 8-bit	5¼ in	single	128	18	40	1	90 KB	288	FM	[10]
		enhanced	128	26			127 KB		MFM
		double	256	18			180 KB

[edit] 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.

[edit] See also

• List of floppy disk formats
• Double-sided disk
• Double-sided RAM

[edit] Notes

[edit] 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