The boom moves in and out to position a head on a desired track. All the read - write heads are attached to a single boom or moving arm assembly.Therefore, before data can be accessed, the portion of the disk surface form which the data is to be read or written must rotate until it is immediately under the read - write head. A read - write head can access only data immediately adjacent to it.The data is accessed read or written by a series of read - write heads, one head per disk surface.Thus each sector is uniquely identified by a triple (cylinder number, track number, sector number). Sectors are numbered starting with zero consecutively anti - clockwise starting from a reference position.The various terms that are associated with the access of a disk block are :.Tracks are numbered starting with zero consecutively from the top track to the bottom track.Cylinders are normally numbered starting with zero consecutively from the outer most cylinder to the innermost cylinder.A sector is identified by its cylinder number, track number within the cylinder and the ordinal position number within the track.In addition, it can have sector management information such as error detections and correction code. A typical sector usually stores 512 or 1024 bytes of user data. Sector store fixed amount of data as a sequence of bits.There may be thousand of concentric cylinders in a disk drive and each track may contain hundred of sectors.A cylinder contains one track per platter surface.The set of tracks that are at one arm position from cylinder. These tracks are further subdivided into sectors. Thus tracks are concentric rings that are centred on spindle.
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