Disk sector

In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user-accessible data, traditionally 512 bytes for hard disk drives (HDDs) and 2048 bytes for CD-ROMs and DVD-ROMs. Newer HDDs use 4096-byte (4 KiB) sectors, which are known as the Advanced Format (AF). The sector is the minimum storage unit of a hard drive. Most disk partitioning schemes are designed to have files occupy an integral number of sectors regardless of the file's actual size. Files that do not fill a whole sector will have the remainder of their last sector filled with zeroes. In practice, operating systems typically operate on blocks of data, which may span multiple sectors. Geometrically, the word sector means a portion of a disk between a center, two radii and a corresponding arc (see Figure 1, item B), which is shaped like a slice of a pie. Thus, the disk sector (Figure 1, item C) refers to the intersection of a track and geometrical sector. In modern disk drives, each physical sector is made up of two basic parts, the sector header area (typically called 'ID') and the data area. The sector header contains information used by the drive and controller; this information includes sync bytes, address identification, flaw flag and error detection and correction information. The header may also include an alternate address to be used if the data area is undependable. The address identification is used to ensure that the mechanics of the drive have positioned the read/write head over the correct location. The data area contains the sync bytes, user data and an error-correcting code (ECC) that is used to check and possibly correct errors that may have been introduced into the data. The first disk drive, the 1957 IBM 350 disk storage had ten 100 character sectors per track; each character was six bits and included a parity bit. The number of sectors per track was identical on all recording surfaces. There was no recorded identifier field (ID) associated with each sector. The 1961 IBM 1301 disk storage introduced variable length sectors, termed records by IBM, and added to each record a record address field separate from the data in a record (sector). All modern disk drives have sector address fields, called ID fields, separate from the data in a sector. Also in 1961 Bryant with its 4000 series introduced the concept of zoned recording which allowed the number of sectors per track to vary as a function of the track's diameter - there are more sectors on an outer track than on an inner track. This became industry practice in the 1990s and is standard today. The disk drives announced with the IBM System/360 in 1964 detected errors in all fields of their sectors (records) with a cyclic redundancy check (CRC) replacing parity per character detection of prior generations. IBM's sectors (records) at this time added a third field to the physical sector, a key field to aid in searching for data. These IBM physical sectors, called records, have three basic parts, a Count field which acts as an ID field, a Key field not present in most disk drive sectors and a Data field, frequently called the CKD format for a record.