DISK SECTOR

7/6/2002

Disk Sector: A disk sector is the smallest recognizable unit of data on a computer disk. A sector consists of a contiguous string of bits that includes data and possibly identification and quality verification information. Sectors are separated by blank areas that are delimited by permanent markings in "hard sectored" disks or by the blank areas themselves on soft sectored disks. Because larger physical sectors have fewer intersector gaps, large sectors permit more data per track on soft sectored disks. This is taken advantage of in some "soft sectored" formats that use very large sectors to maximize the amount of data stored. This applies primarily to floppy disks. PC hard disks universally use 512 byte sectors for their user interface and generally (always?) use 512 byte physical sectors as well although modern disk designs would allow other arrangements if a disk manufacturer chose to use them. At least one major manufacturer used 520 byte sectors with 512 data bytes until well into the 1990s.

Sectors are read and written as serial bit strings using various modulation schemes. Sector IDs that are not returned by the controller precede the data. A checksum or other quality check -- typically a Cyclic Redundancy Check -- is appended at the end. Some commonly used file systems group sectors into larger entries called clusters that contain a number of sectors.

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