【正文】 stems can also perform this task. Thus, if the format information on a disk is damaged, the disk can be reformatted, although this process destroys all the information that was previously recorded on the disk. The capacity of a disk storage system depend on the number of number of disks used and the density in which the tracks and sectors are placed. Lowercapacity systems consist of a single plastic disk known as a diskette or, in those cases in which the disk is flexible, by the less prestigious title of floppy disk. (today’ s 3 1／ 2inch diameter floppy disks are housed in rigid plastic cases, which do not constitute as flexible a package as their older 5 1／ 4inch diameter cousins that were housed in paper sleeves.) Diskettes are easily inserted and removed from their corresponding read/write units and are easily stored. As a consequence, diskettes are often used for offline storage of information. The generic 3 1／ 2inch diskette is capable of holding of data but nongeneric diskettes are available with much higher capacities. An example is the Zip disk system from Iomega Corporation, which provides storage capacities up to several hundred MB on a single rigid diskette. Highcapacity disk systems, capable of holding many gigabytes, consist of perhaps five to ten rigid disks mounted on a mon spindle. The fact that the disks used in these systems are rigid leads them to be known as harddisk systems, in contrast to their floppy counterparts. To allow for faster rotation speeds, the read/write heads in these systems do not touch the disk but instead “float”just off the surface. The spacing is so close that even a single particle of dust could bee jammed between the head and disk surface, destroying both (a phenomenon known as a head crash), Thus harddisk systems are housed in cases that are sealed at the factory. Several measurements are used to evaluate a disk system’s performance:(1)seek tome (the time required to move the read/write heads from one rack to another)。 however, unlike oldfashioned records, the track on a CD spirals from the inside out .This track is divided into units called sectors, each with its own identifying markings and a capacity of 2KB of data, which equates to 1／ 75 of a second of music in the case of audio recording . Information is stored on a CD at a uniform linear density over the entire spiraled track, which means that more information is stored in a loop around the outer portion of the spiral than in a loop around the inner portion. In turn, more sectors will be read in a single revolution of the disk when the laser beam is scanning the outer portion of the spiraled track than when the beam is scanning the inner portion. Thus, to obtain a uniform rate of data transfer, CDDA players are designed to vary the rotation speed depending on the location of the laser beam. However, most CD drives used for puter data storage spin at a faster, constant speed and thus must acmodate variations in data transfer rates. As a consequence of such design decision, CD storage systems perform best when dealing with long, continuous string of data, as when reproducing music. In contrast, when an application requires access to items of data in a random manner, the approach used in magic disk storage (individual, concentric tracks) outperforms the spiral approach used in CDs. Traditional CDs have capacities in the range of 600 to 700MB. However. Newer DVD (Digital Versatile Disks),which are constructed from multiple, semitransparent layers that can be distinguished by a precisely focused laser, provide storage capacities on order of several GB. Such disks are capable of storing lengthy multime