PC Hardware in a Nutshell, 2nd Edition

6.1 Diskette Types and Formats

Before a diskette can be used to store data, it must be prepared by formatting it, although many diskettes nowadays come preformatted. Formatting creates the physical tracks and sectors that the drive uses to store data (called low-level or physical formatting) and the logical structure used by the operating system to organize that data (called logical or DOS formatting). Unlike hard disks, which require two separate formatting passes, FDDs perform both physical and logical formatting in one step. Also unlike hard disks, diskettes do not need to be partitioned.

The Quick Format option available in Windows and later versions of DOS doesn't really format the diskette. It simply "zeros out" the File Allocation Tables and Root Directory entries, giving the appearance of a freshly formatted diskette, but using the original format. Because data on diskettes fades with time, your data will be much safer if you do an actual format, which does a surface test and refreshes the physical and logical format structure of the diskette. Use Quick Format only on diskettes that have recently had a full format done on them. Also, do not count on a Quick Format to wipe sensitive data from a diskette. It's trivially easy to recover such data. To wipe data, do a full format on the diskette. Better yet, use a wipe utility or bulk-erase the diskette. You have been warned.

Format a diskette in Windows by right-clicking the drive icon in My Computer or Explorer, choosing Format, and marking the appropriate options. At the command line, format a diskette by typing the command format a: /options, where a: is the drive letter of the FDD, and /options controls how the disk will be formatted. The available options and the required syntax vary according to the version of DOS or Windows you use. Type format /? to display available formatting options. Which options are usable depends upon both the FDD type and the diskette type. Some FDDs accept only one type of diskette, while others accept two or more.

For about a decade, the 3.5" high-density (HD) FDD has been standard. However, you may encounter older types of FDDs and diskettes when upgrading an old machine or salvaging data, so it's worth knowing something about these obsolescent and obsolete formats. Table 6-1 lists the various diskette formats that have been supported on the IBM platform over the years.

Table 6-1. Diskette characteristics

	5.25" formats			3.5" formats
	SSDD	DSDD	HD	DD	HD	ED
Formatted capacity (KB)	160 / 180	320 / 360	1,200	720	1,440	2,880
Media descriptor byte	0xFE / 0xFC	0xFF / 0xFD	0xF9	0xF9	0xF0	0xF0
Bytes/Sector	512	512	512	512	512	512
Sectors/Track	8 / 9	8 / 9	15	9	18	36
Tracks/Side	40	40	80	80	80	80
Sides	1	2	2	2	2	2
Sectors/Disk	320 / 360	640 / 720	2,400	1,440	2,880	5,760
Available sectors/Disk	313 / 351	630 / 708	2,371	1,426	2,847	5,726
Tracks/inch (TPI)	48 / 48	48 / 48	96	135	135	135
Track width (inch/mm)	.0118/.300	.0118 /.300	.0061/.155	.0045/.115	.0045/.115	.0045/.115
Bits/inch (BPI)	5,876	5,876	9,646	8,717	17,434	34,868
Media formulation	Ferrite	Ferrite	Cobalt	Cobalt	Cobalt	Barium
Coercivity (Oersteds)	300 / 300	300 / 300	600	600	720	750
Sectors/Cluster	1	2	1	2	1	2
FAT type	12-bit	12-bit	12-bit	12-bit	12-bit	12-bit
FAT length (sectors)	1 / 2	1 / 2	7	3	9	9
Root directory (sectors)	4 / 4	7 / 7	14	7	14	15
Root directory entries	64 / 64	112 / 112	224	112	224	240

In addition to the standard formats described in Table 6-1, Microsoft still uses the proprietary DMF (Distribution Media Format) for some of the distribution diskettes it supplies. DMF increases the capacity of a standard high-density 3.5" diskette by reducing the inter-sector gap to allow 21 sectors/track rather than the standard 18 sectors/track, thereby expanding capacity to a true 1.64 MB (usually called 1.68, 1.7, or 1.72 MB).

On most systems, you cannot read data from or write data to DMF diskettes directly, because DIR, DISKCOPY, and other standard disk utilities do not recognize DMF. In fact, attempting to use DISKCOPY to copy a DMF diskette not only yields an unreadable target diskette, but may actually damage the DMF source diskette. DMF diskettes are readable only by Setup and other Microsoft utilities designed to work with CAB files (the compressed Cabinet files used for software distribution), and by some third-party utilities such as WinZip (http://www.winzip.com), which allows you to extract data directly from compressed CAB files, and WinImage (http://www.winimage.com), which allows you to format and copy DMF diskettes directly.

Fortunately, most software is now distributed on CD, so DMF diskettes are less commonly used today than they were a few years ago. We say fortunately, because in our experience DMF diskettes are much more likely than standard 1.44 MB diskettes to generate read errors. We have frequently found DMF diskettes that were unreadable straight out of the box, and a DMF diskette that is several years old is very likely to be unreadable. A standard diskette was simply never intended to store that much data.

If you encounter an unreadable DMF diskette, we recommend using WinImage to attempt to extract the CAB files manually to the hard disk. If one FDD consistently generates read errors, the diskette may be readable on a different FDD, at least well enough to let you get the CAB files extracted.