How Data Is Logically Stored On A Disk

The term “sector” has two meanings: it describes the entire pie-shaped wedge on one side of a disk, as well as the single segment of one track or circle that falls within the wedge. In most of our discussions, sector means the segment of one track, unless we specify that we mean the entire wedge. A sector, or a segment of a track always holds 512 bytes of data. This is true for all floppy disks, no matter what size or density.

“Sector” refers to how data is physically stored on a disk, while “cluster” describes how data is logically organized. The BIOS manages the disk as physical sectors, but the operating system considers the disk only as a long list of clusters that can each hold a fixed amount of data, as shown in the figure below.

The operating system reads data from and writes data to a disk in fixed-length chunks called clusters—a cluster is therefore the smallest unit of data that can be read from or written to a disk at one time.

Because the operating system manages a file on the disk as a group of clusters, a cluster is also called a file allocation unit. The operating system sees a disk as a long list of clusters, or file allocation units, and keeps that list in a table called the file allocation table or FAT.

The Formatting Process

The formatting of all disks is similar, no matter what size or density. During formatting, the Windows 9x or DOS FORMAT command without added options performs the following steps:

Creates the tracks and sectors by writing tracks as a series of F6s in hex and, as necessary, writing the sector address mark to identify the beginning sector on a track Creates the master boot record

Creates two copies of the file allocation table (FAT)

Creates the root directory

Creating the Tracks and Sectors

The FORMAT command is a DOS and Windows 9x command that prepares a disk for use. The first step in the formatting process erases any data on the disk. In its simplest form, without adding any parameters, the FORMAT command always overwrites the data with the F6h character.

The Master Boot Record

During formatting, DOS or Windows 9x prepares the disk so that you can use it to read and write data. DOS and Windows 9x prepare a disk the same way. At the beginning of each floppy disk, the first sector contains basic information about how the disk is organized, including the number of sectors, the number of sectors per cluster, the number of bits in each FAT entry, and other basic information that an operating system or BIOS needs to read the data on the disk.

This information is collectively called the master boot record (MBR). At the end of the MBR is a small program that can be used to boot from the disk. The table below shows the layout of the MBR and its contents. The MBR, sometimes called the DOS boot record, indicates which version of DOS or Windows was used to format the disk and is always located at the beginning of the disk at track 0, sector 1 (bottom of the disk, outermost track). This uniformity of layout and content allows any version of DOS or Windows to read any disk. A floppy disk has only one boot record, but a hard drive has at least two. On a floppy disk, the master boot record and the DOS boot record are the same record. On a hard drive, they are two different records, each with a different purpose.

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