Notes
Fundamentals Of Hardware And Software - Secondary Storage
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Fundamentals of Hardware and
Software
SECONDARY
STORAGE DEVICES/BACKING STORAGE/AUXILIARY STORAGE
This
is the storing of information for repeated or for future use. Secondary Storage refers also to long-term
storage of processed data. Secondary
Storage is Non-Volatile.
Secondary Storage is slower than Primary Memory where it takes a longer
time to locate information stored in Secondary Storage when compared to Primary
Storage and information is written to Secondary Storage slower than Primary
Memory Storage.
Why Secondary Storage?
Devices
of memory storage (RAM, ROM, BUFFERS and REGISTERS) are limited in the amount
of information and programs instructions they can store and these memory
storage devices (except ROM) stores information and program instructions
temporarily. Therefore, they are
considered to be volatile (except ROM).
Secondary Storage allows the storing of information permanently (for a
long time). Therefore, Secondary Storage
provides non-volatile storage.
Secondary Storage Devices
(Medias)
Secondary
Storage Devices are devices that stores and retains information for future or
repeated use. Secondary Storage Devices
may also be described as the physical components on which information and
programs are stored for a long period of time.
Examples of Secondary Storage Devices are Magnetic Disks (Magnetic Hard
Disk and Magnetic Floppy Disks), Magnetic Tapes and Optical (Lazer) Disks
(Compact Disk Read Only Memory CD-ROM, Re-Writable Optical Lazer Disks (CD-RW
disk), Write Once Read Many Optical Lazer Disks WORM Disks, Compact
Disk Recordable (CD-R disk), Digital Video Disk (DVD Disk) and
Magneto Optical Lazer Disks - MO Disks).
TYPES OF
SECONDARY STORAGE DEVICES
MAGNETIC DISKS
Magnetic
Disk Technology permits direct and immediate access to information and programs
stored. The computer can move directly
to a specific record or piece of information on the disk instead of having to
read through every record one by one until the desired information is
located. For this reason, Magnetic Disk
is an example of Direct Access Storage Devices (DASD). Magnetic Disks are of two (2) types. They are Magnetic Hard Disks and Magnetic
Floppy Disks.
MAGNETIC HARD DISKS
This
is a rigid metal (coated with iron oxide) placed into the computer by the
manufacturer used for storing information and programs magnetically. Its rigid (hard) construction allows for
higher storage densities. It is used
when an organization requires a greater amount of on-line storage
(storage area directly accessible by the computer) which Magnetic
Floppy Disks and Magnetic Tapes cannot provide. Magnetic Hard Disks are of three (3)
types. They are Fixed Disks (Winchester Disks),
Removable (Interchangeable, Exchangeable) Disks and the Moving Head Hard
disks.
TYPES OF
MAGNETIC HARD DISKS
Interchangeable
(Exchangeable/Removable) Magnetic Hard Disks
These
are Magnetic Hard Disks that are stored off-line (not
directly accessible by the computer) and they are loaded or placed into
the computer as they are needed to store information or when information stored
on them needs to be obtained or retrieved.
These disks therefore may be
removed from the computer (System Unit) and replaced with another when
one disk becomes full of information and may not be able to store any more
information or when information is needed from another Hard Disk.
Fixed Hard (Winchester Disks)
These
Magnetic Hard Disks are also made from a hard metal usually made of aluminum
with a surface coated with magnetized elements such as Iron Oxide. The manufacturer places this Hard Disk
permanently into the computer so that the user may not remove it. Therefore, if the Fixed Hard Disks become
filled, it may not be replaced with another Magnetic Hard Disk. If the user needs additional Hard Disk
storage, they will have to either erase some information stored on that Fixed
Hard Disk or install another hard disk in the computer. A Hard Disk could also be attached outside of
the computer at the back of System Unit as a Peripheral Device.
Moving Head Magnetic Hard Disks
Advantages of Magnetic Hard Disks
1.
Individual information may be accessed easily because such information
stored has a specific address (in the computer's memory).
2. Provides direct access to information
stored.
3. Made of a hard metal, which is not easily
damaged which protects the Disk.
4. Allows information to be stored in a
well-organized manner.
5. Able to store a large number of software on
the computer.
Disadvantages of Magnetic Hard
Disks
1.
Very expensive.
2.
Must be kept in a cool and dust free environment.
3.
Catches viruses easily (when infected Magnetic Floppy Disks are used in
a machine, the infected Magnetic Floppy Disk will infect the Magnetic Hard
Disk).
4.
This disk can crash and information damaged if the Read/Write Head
touches the surface when reading or writing information to this disk.
5.
Cannot be easily moved from place to place especially in the case of the
Fixed Magnetic Hard Disk because it is placed into the computer
permanently. The Exchangeable Magnetic
Hard Disks may be transported and carried from one place to another easily.
6. If water and other liquids get into the
machine on this disk it may lead to its malfunction.
COST
The
Magnetic Hard Disks may cost between $4, 000 - $12, 000 Jamaica Dollars depending
on the amount of storage capacity available and the material used to make it.
Storage Capacity/Density
A
Magnetic Hard Disk can store as much as 1.2 Gigabyte (GB) to 8 or more Gigabyte
(GB) of information.
Features of the Magnetic Hard Disks
1.
Made of a very hard metal.
2.
The metal it is made of is coated with magnetized elements such as iron
oxide.
3.
Not easily damaged.
4.
Used to store a large amount of information and software.
USE
1.
To store a large amount of information and programs.
MAGNETIC FLOPPY DISKS
These
disks are made of a flexible plastic coated with magnetic material that can
store information on one side (single density) or on both sides (double
density). Magnetic Floppy Disks are
mainly used with Microcomputers. This
disk comes in 3 sizes: 3 ½", 5 ¼" and 8". The smallest size of Magnetic Floppy Disks (3
½“) is called Micro Floppy Disks. These disks like the Magnetic Hard Disks are
divided into two parts namely TRACKS and SECTORS. Information is stored on the parts of the Magnetic
Disks (Floppy and Hard Disks) in binary pattern in magnetized spots. These disks are also an example of Direct
Access Storage Devices (DASDs).
Advantages of Magnetic Floppy
Disks
1.
They are cheap.
2.
They are physically smaller in size, which permits them to be carried
around easily.
3.
They are exchangeable.
4.
They are lightweight.
5.
They are convenient (a user can store information on it and take the
disk elsewhere and complete their work).
6.
They are portable.
7.
They provide direct access to information stored.
Disadvantages of Magnetic Floppy
Disks
1.
They are flexible and so they are easily damaged.
2.
The disk drive in which they must be used is more expensive than the
disk itself.
3.
Accessing of information stored is slower than the accessing time taken
to access information stored on Magnetic Hard Disks.
4.
Must be kept in a cool and dust free environment and away from heat.
COST
Magnetic
Floppy disks can cost as much as $40 - $60 Jamaican Dollars.
Storage Capacity/Density
DISK SIZE
STORAGE CAPACITIES
3 1/2"
720 KB - 2.8 MB
5 1/4"
360 KB - 1.2 MB
8"
Not Available
Generally
speaking Magnetic Floppy Disks can store between 1.2/1.44 - 2 MB of
information!
Figure 1
Table showing storage capacities of the different sizes of Magnetic
Floppy Disks
Features of Magnetic Floppy Disks
1.
They posses a much smaller storage capacity than Magnetic Hard Disks.
2.
They are much slower to be access (it takes a longer time to find
information stored).
3.
They are easy to be transported.
4.
They are made from flexible plastic coated with magnetic material.
USE
1.
To store a small amount of information which may be transported easily.
Magnetic Tape
Magnetic
Tape is a sequential secondary storage device on which information is stored in
a sequential manner. Magnetic Tape is
made of nylon plastic coated with magnetic materials on which information is
stored. Examples of Magnetic Tapes are audiocassettes and videocassettes. Magnetic Tape stores information
sequentially. In order to find specific
information, the Magnetic Tape has to be searched/read randomly (fast forward
or rewind) in order for specific information to be located. Magnetic Tapes are
more times used to store music, in that songs are stored in a sequential order,
from beginning to end. Companies and
individuals to back up information stored on their Magnetic Hard Disk
periodically also use it. It is
relatively slow to locate specific information and processed data stored on
Magnetic Tape. Magnetic Tape is one of
the oldest secondary storage devices and one of the cheapest storage
devices. Magnetic Tapes may be used in
older computer systems, which has a Tape Drive (similar to a Disk Drive used to
read information from and write information to the Magnetic Tape). They may also be used when an organization
requires a large amount of information to be stored sequential (in a set order)
at low cost. Magnetic Tape in the only
example of a Sequential Access Storage Device (SASD).
Advantages of Magnetic Tape
1.
A standard Magnetic Tape can store about 200 - 250 MB of
information.
2.
Magnetic Tape is cheap. Cost a
little more than Magnetic Floppy Disks.
3.
Magnetic Tapes are convenient to be used to store information because it
is light, small and easy to store on storage racks.
4.
Magnetic Tapes are re-usable - what is stored on the Magnetic Tape may
be erased and new information may then be stored.
5.
Large number of records of information may be stored and processed in a
sequential manner.
6.
A cheap storage device that can be used to back up a large amount of
information.
Disadvantages of Magnetic Tape
1.
Only can store information sequentially.
For one to find specific information, the entire tape has to be
searched.
2.
Relatively stable storage device (can last a long time but may begin to
age, crack after a while).
3.
After extensive use, the Magnetic Tape becomes stretched and so
information becomes partly damaged.
4.
Must be kept in a cool environment because it is made of plastic.
COST
Magnetic Tapes can cost between $60 - $300
Jamaican Dollars.
Storage Capacity/Density
Magnetic Tapes can store as much as 200 -
250 MB of information.
Features
1.
Made of flexible nylon plastic coated with magnetic material such as
iron oxide.
2.
Examples of Magnetic Tapes are audiocassettes and videocassettes.
Uses
1.
Used for off-line (not accessible by the computer) storage of large (sequential)
data files. Tapes are kept in storage
racks outside of the computer and are loaded to the computer Tape Drive when
needed to store or when information needs to be retrieved from them.
2.
May be used for back-up storage for on-line information stored on
Magnetic Hard Disk to ensure that a second copy of information is available in
case virus destroys information stored on the Magnetic Hard Disk.
3.
To store a large amount on information in sequential order. E.g. Bill payment information belonging to
Cable and Wireless Jamaica (CWJ), National Water Commission, Jamaica Public
Service, Paymasters, etc.
4. Protection
against lost of valuable data. Magnetic
Tape is used primarily as a back up medium for
Magnetic Disk Storage.
5. Archiving Files. Important files no longer needed for active
processing can be archived to Magnetic
Tape.
For example, banks archive old transactions (cheques, deposit slips,
etc.) for a number of years.
6. File Portability between computers. Large amount of information can be
transferred between
computers by writing to Magnetic Tape
at the source site and reading from the Magnetic Tape at the
destination site.
Optical (Lazer) Disk
This
is a secondary storage device that uses lazer technology to store information
to and read information stored. Optical
(Lazer) Disks are made from fiber plastic material. Optical (Lazer) Disks is an alternative storage
technology being used now widely in today's computer society over the
traditional storage technology of Magnetic Technology commonly used with
Magnetic Tape, Magnetic Floppy and Magnetic Hard Disks. Optical (Lazer) disks comes in many types
namely Compact Disk Read Only Memory Optical (Lazer) Disk (CD-ROM), Re-Writable
Optical (Lazer) Disk (CD-R Disk), Write Once Read Many Optical (Lazer)
Disks) - WORM Disks, Digital Video Disk (DVD), Compact Disk
Recordable (CD-R Disk), etc.
Optical (Lazer) Disk provides the user with Multimedia
possibilities. Multimedia is
the integration of two or more types or medias such as text, animation,
graphics, sound and full motion video into computer based application. This secondary storage device is popularly
used where enormous storage space is needed cheaply and in a compact form
providing easy retrieval. Optical
(Lazer) Disks are examples of Direct Access Storage Devices (DASDs).
Compact Disk Read Only Memory
(CD-ROM) Optical (Lazer) Disk
This
is a read only storage device, which means that no new data can be written to,
or stored on it by the user. The
information stored on this storage device may only be read from and cannot be
changed by the user. Unlike Compact
Disk Re-Writable Optical Disk, where the user may read and write (change)
information stored, with CD-ROM and WORM Optical Disks, the user can
only access/read information stored and not change what is stored or add new
information to it. CD-ROM disks
are widely used to store reference materials where large amount of information
such as an encyclopedia may be stored on a single disk. CD-ROM is now widely being used to
store images (graphics, animations, drawings, symbols, etc.), in the storing of
interactive entertainment (games) and in the storing of educational software
where multi-media is now being used to stimulate interest.
Re-Writable Optical (Lazer) Disk/Compact
Disk Rewritable (CD-RW)
These
Optical (Lazer) Disks uses a combination of magnetic and optical technologies
to read and store information. On this
storage device, the user may read information stored from the disk and the user
may change or add additional information if necessary. The user may change information stored on
these disks.
Write Once Read Only (WORM)
Optical (Lazer) Disks
These
disks allow information to be written to once and read from many times. The user may not change information stored on
these disks. WORM Optical Disks are used
by people to store and retrieve archival information, historical data, proprietary
data (such as the text for a book), etc.
Because WORM disks do not allow information stored to be erased or
changed once the information has been burnt on, authors and publishers of books
are now placing their books on Compact Disks rather than distributing them only
in printed form. WORM disks are used
where security of information is vital, crucial and essential such as for the
storing of Legal Documents such as laws and information about countries,
information for encyclopedias, financial yearly report, and data which has been
compiled for government purposes such as a census, population study, research,
etc.
Digital Video (Versatile) Disk
(DVD Disk)
This
storage device looks similarly to a regular Compact Disk. The only difference is that the DVD disk is
smaller in size. The DVD disk can store
as much as 7 – 14 times as much information as a regular Compact Disk. Some can store as much as 10 GB. This disk is used to store full-length video
movies.
Advantages
1. Provides better quality sound and
graphics.
2. Provides the user with the
possibility of multi-media applications
3. May be used to store educational
programs and encyclopedias with various graphics, availability of sound,
pictures, music, etc therefore making learning fun and more interesting.
4. Allows users to store large
amount of information in a small space.
5. Not very expensive.
6. Not damaged when water or liquid
catches the surface.
Disadvantages
1. If the disk surface becomes scratched,
information stored becomes partly damaged.
2. These disks must be handled with extreme care
and kept in a cool environment - away from excessive
heat.
3. More expensive than Magnetic Floppy Disk and
Magnetic Tape.
4. Easily damaged due to its glass-like surface.
5. Accessing of information stored on the Optical
(Lazer) Disk is slower than the accessing of
information stored on Magnetic Hard
Disk as the computer is able to spin the Magnetic Hard Disk
several times faster than it is able to
spin an Optical (Lazer) Disk.
COST
Optical
(Lazer) Disks can cost between $550 - $900 Jamaican Dollars.
STORAGE CAPACITY/DENSITY
This
storage device can store 660 MB of information (as much as 300 High
Density Magnetic Floppy Disks).
Feature
1. Made of a plastic-like glass material such as
fiberglass.
2. The information stored on CD-ROM
and WORM disk may only be read and not changed while information stored on
Re-Writable Optical (Lazer) Disk may be read from and changed by the user.
Use
1. To store a large amount of information in a
compact form.
2. To store multimedia applications.
TERMS
ASSOCIATED WITH BACKING/SECONDARY STORAGE DEVICES
Read/Write Head This is a component of the
disk/tape drive of the computer that reads and writes information
to and from Magnetic Disks/Tapes.
Access Arm This is a disk drive mechanism
used to position the Read/Write Head over a particular Track on which
information is stored on the Magnetic Disk/Tape.
Tracks This is the portion of the
Magnetic Disk that can be accessed in any given setting by the Read/Write
Head.
Sector This is a pie-shaped division of
each track on which data is stored and may be retrieved subsequently.
Cylinders When 10 Tracks comes together
they form a Cylinder.
Buffer Intermediate Memory that
temporarily stores/holds data being taken from Main Memory and being sent to an
Output Device (example Printer, Monitor, etc.).
Direct Access Data on any sector can be
accessed directly by using its disk address.
The Read/Write Head is moved until it is over the specified Track
(without reading information on the way as it move) and then as the specified
Sector of the Track passes beneath the Read/Write Head, information is read
from the disk surface into Main Memory.
In other words, the ability to directly access information stored on a
storage device no matter the physical location stored.
Access Time This is the time taken to read
information from a secondary storage device into Main Memory by the Read/Write
Head.
Sequential Access This is when data is accessed sequentially
(in a set order as information is stored).
Seek Time This is the time taken by the Read/Write
Head to reach the Track on which information is stored.
Data Transfer Time (Rate) This is the time taken to read
information into Main Memory/Primary Storage on a secondary storage device.
Rotational Delay This is the time taken to bring a
particular section of the disk (Track or Sector) beneath the Read/Write Head
for information to be read from or written to it.
Direct Access Time This refers to the speed at which
information stored in Primary (Main) Memory
Storage or Secondary Storage may be located on a secondary storage device.
Single Density This is the ability to store information
on one side of the Magnetic Floppy Disk.
Double Density This is the ability to store information
on both sides of the Magnetic Floppy Disk.
Disk Density This refers to the number of
bits that can be stored per unit area (e.g. per inch) on the disk surface.
Additional Content:
File Compression
Zip Disk/Zip
Drive
Super Disk
Compact Disk Recordable Disk (CD-R)
Compact Disk Rewritable (CD-RW)
Disk Formatting/Disk Initializing
Compact Disk Writers
Holographic (Memory) Storage
Disk Care:
Do’s and Don’ts
WORM Disk
Partition
Directory
Files
Compact Disk Writers
WORM Disk
Partition
Directory
Files
File Allocation Table (FAT)