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[ The PC Guide | Systems and Components Reference Guide | Floppy Disk Drives | Floppy Disk Media and Low-Level Data Structures ]

Media Density

The density of the disk surface refers to the amount of data that can be stored in a given amount of space. This is a function of two basic factors: how many tracks can be fit on the disk (track density), and how many bits can be fit on each track (bit density). The product of these two factors is called areal density, normally used to describe the capacity of hard disks; floppy disks are usually instead specified using the separate terms: track density (measured in tracks per inch or TPI) and bit density (measured in bits per inch or BPI).

Virtually every hard disk has a different set of density characteristics. With floppy disks, the densities are more standardized. In fact, there are two standard types of 5.25" disks, and three standard types of 3.5" disks. These vary solely in terms of their density, which are given specific names. This table shows the different density types and their characteristics:

Density Characteristic

360 KB 5.25"

1.2 MB 5.25"

720 KB 3.5"

1.44 MB 3.5"

2.88 MB 3.5"

Track Density (TPI)






Bit Density (BPI)






Density Name

Double Density (DD)

High Density (HD)

Double Density (DD)

High Density (HD)

Extra-High Density (ED)

Looking at these numbers you will immediately notice a few things. First, the lowest-density drives strangely use the term "double density". This is because they are successors to even lower-density media that stored even less than they do, many many years ago. Second, even though both 3.5" and 5.25" disks use the terms "double density" and "high density", they refer to entirely different density characteristics. Finally, some densities differ from others based on changes in track density, some based on changes in bit density, and some on both. In particular, all of the 3.5" drives use the same track density, 135 TPI. Double density disks often have the words "double density" printed on them, while high density disks use the familiar "HD" logo, usually stamped into the jacket of the diskette, right near the metal slider.

Some people incorrectly infer from the fact that both low and high density 3.5" disks have the same number of tracks per inch, that the physical disks are interchangeable and the only difference is that the HD disks are "higher quality". This is in fact not the case! As with hard disks, the tighter you pack the data together on a floppy disk, the more the chances of interference between adjacent tracks. Therefore, higher density disks use weaker write signals, and different magnetic coatings than lower density disks.

High density and double density media are not interchangeable. You should always use high-density disks in high-density drives. Considering that the low-density disk is obsolete and the high-density disks are dirt cheap these days, this isn't even something you should think twice about. It is possible to write data to double density disks in a high density drive, but you should always use the correct physical media. While high density drives are downward compatible with double density disks, the high density media is not. If you want to use the 720 KB format you must use double density disks, even if you are using a high density drive. Also, you must issue the correct parameters to the FORMAT command. Typing "FORMAT /?" will show you what they are.

Next: Data Encoding and Decoding

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