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

Floppy Disk Geometry

The term geometry refers to the organization of the disk's data structures. For hard disks this is a complicated issue due to the use of physical, logical and translated geometry. For floppy disks it is quite simple: the geometry refers to the number of disk surfaces (which is the same as the number of read/write heads), the number of tracks per surface, and the number of sectors per track. All floppy disks use the same number of sectors for each track, despite the fact that the inner tracks on a disk are much smaller than the outer tracks. This is not the case for hard drives.

Since all (modern) floppy disks use both sides of the disk and therefore always have two heads and surfaces, the only important parameters are the number of tracks and the number of sectors. While hard disks are always referred to as having cylinders, floppies are sometimes said to have cylinders and sometimes to have tracks. Really, they are just different ways of referring to the same thing. Since floppies always have two surfaces, cylinder #N simply refers to track #N on both sides of the disk.

Each floppy disk format (size) has a specific geometry, unlike hard disks which of course vary from disk to disk. Here are the geometry specifications for the different disks formats:

Geometry Specification

360 KB 5.25"

1.2 MB 5.25"

720 KB 3.5"

1.44 MB 3.5"

2.88 MB 3.5"

Tracks (Cylinders)

40

80

80

80

80

Sectors Per Track/Cylinder

9

15

9

18

36

Total Sectors Per Disk

720

2,400

1,440

2,880

5,760

All floppy disks use 512 bytes per sector, which is the standard for both floppies and hard disks. Note that not all of the sectors indicated by the total above are usable for actual user data, because some are used by the file system structures (FAT, root directory etc.).

Next: Floppy Disk Formats and Logical Structures


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