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electric power is turned off or interrupted. However computer systems need to store data
on a permanent basis for several days, several months, or even several years.
As a result, additional memory, called auxiliary memory or secondary storage, is used
with most computer systems. This section of the computer's memory is non-volatile and
has low cost per bit stored, but it generally has an operating speed far slower than that of
the primary storage. This section of the memory is used to store large volume of data on a
permanent basis which can be partially transferred to the primary storage as and when
required for processing. Data is stored in secondary storage in the same binary codes as in
main storage and is made available to main storage as needed.
Over the years, a wide range of devices and media have been developed for use as
secondary storage in computer systems. Many of them, such as punched paper tape and
punched cards, have become obsolete. As shown in Figure 8.1, the popular ones used in
today’s computer systems are magnetic tape, floppy disk, zip disk, disk pack, Winchester
disk, CD-ROM, WORM disk and magneto-optical disk. In this chapter you will learn
about the terminologies, principles of operation, uses, and trade-offs of many different
types of secondary storage devices.
SEQUENTIAL AND DIRECT-ACCESS DEVICES
Several different devices can be used as a secondary storage device, but the one selected
for a particular application mainly depends upon how the stored information needs to be
accessed. Basically, there are two methods of accessing information - sequential or serial
access and direct or random access. A sequential-access storage device is one in which
the arrival at the location desired may be preceded by sequencing through other locations,
so that access time varies according to location. In other words, information on a sequential-access device can only be retrieved in the same sequence in which it is stored.
Sequential processing is quite suitable for such applications like prepa...
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This document was uploaded on 04/07/2014.
- Spring '14