Page 51 - DCAP101_BASIC_COMPUTER_SKILLS

Basic Computer Skills

Notes primary storage of today’s computers is not sufficient to store the large volume of data
handled by most data processing centers.
2. Volatile. Primary storage is volatile and the data stored in it is lost when power is
turned off or interrupted. However, computer systems need to store data on 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 system. Secondary storage is non-volatile and has lower cost per bit stored
but it generally has an operating speed far slower than that of primary storage. It is used
primarily to store large volume of data on permanent basis that can be partially transferred
to primary storage, whenever required for processing.
Over the years, several devices and media have been used as secondary storage in computer
systems. Many of them such as punched paper tape and punched cards have become obsolete
now. They are magnetic tape, magnetic disk, optical disk, and memory storage devices. In
this chapter, you will learn about the terminologies, principles of operation, uses, and trade-
offs of different types of secondary storage devices.
4.1.1 Sequential and Direct-Access Devices

Several different devices can be used as a secondary storage device but the one selected for
a particular application depends mainly upon how the stored information needs to be
accessed. 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
desired location may be preceded by sequencing through other location so that access time
varies according to location. That is, information on a sequential-access device can be
retrieved in the same sequence only in which it is stored. Sequential processing is suitable
for such applications like preparation of monthly pay slips, monthly electricity bills, etc.
where most, if not all, of the data records need to be processed one after another. In these
applications, data records for every employee or customer needs to be processed at scheduled
intervals (in this case monthly). However, while working with a sequential-access device for
if an address is required out of order, it can be reached only by searching through all the
addresses stored before. For instance, data stored at the last few locations cannot be accessed
until all preceding locations in the sequence have been traversed. This is analogous to a
music tape cassette. If 10 songs are recorded on a music tape and you want to listen to (or
access) the eighth song, you Must first listen to (or access) the seven songs coming before
your favorite song. Your player may “fast forward” the tape quickly past the first seven
songs but the first seven songs are still accessed, although, not fully played. fn the same way,
to access the 28" data record stored on a sequential-access device a computer must first access
(although not process) the first 27 data records. Magnetic tape is an example of a sequential-
access storage device. In many applications, we need to access information in a more direct
manner than sequential-access device allow. For example, in a computerized bank, at any
instance, it may be required to determine the exact balance in the savings account of a
customer. Similarly, in a computerized airline ticket booking system immediate access may
be required to reservation system records to find out if seats are currently available on a
particular flight in such applications, if we use a sequential-access device for data storage,
the time taken to access the desired information may be long enough to cause frustration
to the customer. Secondary storage devices exist that permit access to individual information
in a more direct or immediate manner. These direct-access devices are also called random-
access devices because out of all the information stored, any randomly chosen piece of
information can be accessed immediately. Hence, a random-access storage device is one in
which any location in the device may be selected at random, access to the information stored
is direct, and approximately equal access time is required for each location. This is analogous
to a music CD. If 10 songs are recorded on a music CD and you want listen (or access) the
sixth song, you need not listen to or fast-forward the five songs coming before your favorite
song. All you have to do is select track number six on the player and the player directly

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