2.1.4 Secondary Storage

Organizations need to store large amounts of data and instructions more permanently
than allowed by the main memory. Secondary storage can be used for this purpose. It is
also called the permanent storage. There are different types of secondary storage
mediums such as magnetic discs and tapes. We discuss these in detail in the following
sub-sections.

Compared to Memory, secondary storage is non-volatile (i.e. data is not lost when
electrical power is switched off) and less expensive. The disadvantage is, it is much
slower than the memory. When selecting a secondary storage media and devices, it is
important to consider the access method, capacity and portability. Secondary storage
media with faster access methods are generally more expensive than slower media. Lets
consider what is meant by access method.

Access Methods
There are two methods of accessing data in the storage. They are, sequential and direct
access methods.

Sequential access – This means that data must be accessed in the order in which it is
stored. For example, product data stored sequentially may be stored by product number
ranging from 10 to 30 in a secondary storage device. If you want to retrieve information
on product number 25, you have to read and discard data relating to product numbers
from 10 to 24.

Direct Access – According to this method data can be accessed directly, without the
need to bypass other data in the sequence. If you consider the earlier example, you can
directly access information relating to product number 25. Therefore, this access method
is much faster than sequential access.

2.1.4.1 Magnetic Tape

Magnetic tape is a thin plastic tape on which data can be represented with magnetized
spots. The magnetic tape used for computers, is made from the same material used for
audiotape and videotape. Magnetic tape is kept on a large open reel or in a small
cartridge or cassette. Although this is an old technology, it remains popular because it is
the cheapest storage medium and can handle enormous amount of data. The downside
is that it isthe slowest for retrieval of data, because all the data are placed on the tape sequentially.

This means as explained earlier, the system might have to run through a major part of
the tape, before it comes to the desired piece of data.

2.1.4.2 Magnetic Disks
Magnetic disks come in a variety of styles and are extremely popular because they allow
much more rapid access to data compared with magnetic tape. The magnetic disk is like
a phonograph record in that it has tracks, and like a pizza in that it is divided into wedges
that are called sectors. Every piece of data has an address attached to it corresponding
to a particular track and sector. Any piece of desired data can be retrieved in a nonsequential
manner, by direct access. A read-write head uses the data address to quickly
locate and read the data.

2.1.4.3 Disks

Hard Disks

Hard drives use stacks of rigid magnetic disks. These disks are permanently mounted as
a unit that may be internal or external to the computer. The data transferred to and from
the computer is handled by the read/write heads. The read/write heads are attached to
arms that hover over the disks moving in and out. They read the data when positioned
over the correct track and when the correct sector spins by. Because the head floats just
above the surface of the disk (at 25 millionths of a meter), any bit of dust or
contamination can disrupt the device.

For that reason, hard drives are hermetically sealed when manufactured. A modern
personal computer may have several gigabytes of storage capacity in its internal hard
drive. Data access is very fast. For these reasons, hard disk drives are extremely
popular and common.

Floppy Disks

Magnetic diskettes are commonly called floppy disks and function similarly to hard
drives, but with some key differences. A floppy disk is a removable round, flat piece of
plastic that stores data and programs as magnetized spots. This is enclosed in a
rectangular plastic case. The basic sizes of the diskettes are 3½ inch which is now more
popular in 5¼ inch. A floppy drive copies or reads data from disk and writes or records
data to the disk.

Diskettes are often called “floppy” because the disk with the case is flexible. They are
much slower than hard drives. They have much less capacity, ranging from 1.44
megabytes for a standard high density disk to 200 megabytes for a disk formatted for a
zip drive (on which the data are compressed). Further, although they are individually
inexpensive floppy disks are less cost-efficient than hard drive storage. However, the big
advantage of floppy disks is that they are portable.

Optical Disks

An Optical Disk is a removable disk on which data is written and read through the use of
a laser beam. Few types of optical technologies are used in computers. They are:

• Compact Disk Read -Only Memory (CD-ROM)
CD-ROM technology uses 12cm compact disks similar to those used in stereo music
systems. Compact Disk Read -Only Memory (CD-ROM) storage devices feature high
capacity (more than 600MB can be stored in a disk), low cost, and high durability.
However, as it is a read –only medium, the CD-ROM can only be read and not written
on. CD-ROM disk drives use a laser device to read the binary codes formed by those
pits.
• Write Once Read Many Disk (WORM)
Write Once Read Many Disk can be written or recorded onto once and then cannot be
erased. But it can be read many times.
• Rewritable Compact Disk
Rewritable Compact Disks allow user to rewrite onto the CD.