Secondary Storage

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Secondary storage provides cheap, non-volatile high capacity storage. There Are basically two types of secondary storage: * Magnetic devices
* Optical devices


Magnetic Disc

The Magnetic Disk is Flat, circular platter with metallic coating that is rotated beneath read/write heads. It is a direct access device; read/write head can be moved to any location on the platter. Magnetic disks provide storage for large amounts of data and instructions that can be rapidly accessed. For data recording, the surface of a disk is divided into a number of invisible concentric circles called tracks. The tracks are numbered consecutively from outmost to innermost starting from zero. The number of tracks varies greatly between disks, from as few as 40 on some small, low capacity disks to several thousand on large, high capacity disks. Each track is further subdivided into sectors. For this, in addition to the concentric circles, the disc surface is also divided into invisible pie-shapes segments. Thus if there are eight such pie shaped segments, each track will get divided into eight parts, and each of these eight portions of a track is called a sector.

Storage capacity depends on
• Number of recording surfaces
• Numbers of tracks per surface
• Number of sectors per track
• Number of byes per sector

Magnetic Tape
A magnetically coated strip of plastic on which data can be encoded is called a magnetic tape. Tapes for computers are similar to tapes used to store music. Storing data on tapes is considerably cheaper than storing data on disks. Tapes also have large storage capacities, ranging from a few hundred kilobytes to several gigabytes. Accessing data on tapes, however, is much slower than accessing data on disks. Tapes are sequential-access media, which means that to get to a particular point on the tape, the tape must go through all the preceding points. In contrast, disks are random-access media because a disk...
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