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Hard Disk Organization

  • All hard disks, or hard disk drives, have basically the same structure, however diverse they are. Inside the case there are several disks with magnetic coating set on a single axis (spindle). A special motor provides the necessary rotation speed to the spindle, e.g. 5400 rpm, 7200 rpm, or 10000 rpm.

    Information on disks resides on concentric tracks. Each track has its number. The outermost track is number 0, and the numbers grow inwards.

    Each of the tracks is divided into sectors that contain minimal information blocks that can be written to disk or read from it. Sectors also have numbers. On every disk there is a marker that indicates the beginning of sector enumeration. The sector that is the closest to this marker is number 1.

    Usually sector size is 571 bytes. At the beginning of a sector there is a header (prefix portion) that marks the beginning of the sector and its number. At the end of a sector in the suffix portion there is the checksum that is used to check data integrity. Data area between the prefix and suffix portions is 512 bytes large.

    Both upper and lower sides of each disk on the spindle can be used to store data. All tracks that have the same number on all the surfaces of all disks comprise a cylinder. For each work surface of a disk in the drive there is a head that enables reading and writing data from/ to the disk. Heads are assembled into a block and are enumerated, starting with 0.

    To perform an elementary read or write operation the head block should be positioned at the necessary cylinder. When the necessary sector (with the necessary number in the service area) of the rotating disks approaches the head, data is exchanged between the head and the electronic board of the drive.

    Sector structure of a hard disk is created via low-level formatting during which each of the tracks of the disk is marked up.

    Modern disk drives usually contain relatively few magnetic disks (1-2) to make the head block lighter and speed up access to sectors (a drive like this has 2-4 heads respectively).

    There can be up to several tens of thousands of cylinders per disk. The higher the write density on the disk, the more cylinders can be created on it and the larger the capacity of the disk.

    This design has a lot of technical implementation peculiarities, but we are not discussing them here.