EXTERNAL MEMORY
A tape drive is a sequential-access device. If the tape head is positioned at record 1. Then to read record N. It is necessary to read physical records 1 through N-1.if the head is currently positioned beyond the desired record. It is necessary to rewind the tape a certain distance and begin reading forward.
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IN contrast to the tape. The disk drive is referred to as a direct-access device. A disk drive need not read all the sectors on a disk sequentially to get to the desired one. It must only wait for the intervening sector within one track and can make successive accesses to any track.
Magnetic tape was the first kind of secondary memory. It has lowest-cost , slowest-speed member of the memory hierarchy.
The typical recording technique used in serial tapes is referred to as serpentine recording. In this technique, when data are being recorded, the first set of bits is recorded along the whole length of the tape. When the end of the tape is reached and the tape is again recorded in its whole length. This time in the opposite direction. The process continues, back and forth until the tape is full. Data are still recorded serially along individual tracks but blocks in sequence are stored on adjacent tracks.
MAGNETIC TAPE
Tape system use the same reading and recording techniques as disk system. The medium is flexible polyester tape coated with magnetizable material. The coating may consist of particle of pure metal in special binders or vapor-plated metal films. The tape and the tape driver are analogous to a home tape recorder system.
Data on the tape are structured as a number of parallel tracks running lengthwise. Earlier tape systems typically used nine tracks. The recording of data in this form is referred to as parallel recording. Most modern systems instead use serial recording. As is done with magnetic disks. Data are read and written in contiguous blocks on a tape. Block on the tape are separated by gaps.