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[ The PC Guide | Systems and Components Reference Guide | Floppy Disk Drives | Floppy Disk Formats and Logical Structures ]

File System and Floppy Disk Structures

Generally speaking, floppies use the FAT file system. This is the basic file system used by DOS, Windows 3.x, Windows 95 and optionally on Windows NT. The FAT file system is described in great detail in the file system section on hard disks; you will probably want to refer there if you want to learn more about FAT and how it works. I'll talk a bit here about some floppy-specific aspects of how FAT is used.

Floppy disks use the same basic structures as hard disks, only they are less complicated. Since floppies need not (and cannot) be partitioned, they do not have partition tables or a master boot record. Conceptually, a floppy disk has the same structures as a single hard disk volume. The disk has a single volume boot sector for the disk, and this is what is used when a floppy is booted (assuming the floppy is bootable).

All floppy disks are formatted in the FAT12 version of the file system; this is the oldest flavor of FAT and is sufficient for the small capacity of floppies. The cluster size of floppy disks is either one or two sectors, depending on the disk type. I am not really sure why, but the larger disks use smaller clusters; go figure. Either way, the clusters are small which means that the space on the disk is effectively utilized; there is little slack on a floppy disk, since even a two-sector cluster is only 1,024 bytes in size.

Floppies also have serious limitations on the number of entries in their root directory, far less than the 512 entries in the root directory of a hard disk. Again, this depends on the type of disk:

File System Parameter

360 KB 5.25"

1.2 MB 5.25"

720 KB 3.5"

1.44 MB 3.5"

2.88 MB 3.5"

Cluster Size

2 sectors

1 sector

2 sectors

1 sector

2 sectors

Maximum Number of Root Directory Entries

112

224

112

224

448

Next: Formatted and Unformatted Capacity


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