Fundamental of Computers
& Information Technology
Unit-I
Computer: -A Computer is an
electronic device that can perform a variety of operations in accordance with a
set of instructions called program.
The
word “computer” comes from the word “compute” which means to calculate. So a
computer is normally considered to be a calculating device that can perform
arithmetic operations at enormous speed. In fact, the original objective for
inventing the computer was to create a fast-calculating machine. But more than
80% of the work done by computers today is of non-mathematical or non-numerical
nature. Hence to define a computer merely as calculating device is to ignore
over 80% of the work. More accurately, a computer may be defined as a device
that operates upon data. Data can be anything like Bio-data of various
applicants, marks obtained by various students in various subjects etc.
Data comes in various shapes and sizes depending upon the
type of computer application. A computer can store, process and retrieve data
as and when desired. The fact that computers process data is so fundamental
that many people have started calling it a data processor. The activity of
processing data using a computer is called data processing. Data processing
consists of three sub-activities: capturing the input data, manipulating the
data, and managing the output results. Computer can access and process data
millions of times faster than humans can. A computer can store data and
information in its memory, process them and produce the desired results.
Computers can do a lot of different tasks such as playing games, railway
reservation, weather forecasting etc.
Data: - Data in computer
terminology mean raw facts and figures.
For example, Mohan, 1977, A etc.
Information: - It means what
we get after processing data (meaningful data). Data are aggregated and
summarized in various meaningful ways to form information. For example, Mohan,
whose roll number is 1977 has got grade A.
Computer System concept: - A computer system
consists of a computer and supporting devices for input and output of data. The
data to be processed are supplied to the computer with the help of input
devices. The processing unit performs the desired operations on the information
and the result of calculations/processing is obtained on the output devices.
Several types of input/output devices can be attached to the computer. A
computer consists of electronic circuits only, while the input/output devices
have both electronic and mechanical components.
Application area :
Characteristics of computers: -
1.Speed: - A computer is
a very fast device. It can perform in a few seconds the amount of work that a
human being can do an entire year – if he worked day and night and did nothing
else. While talking about the speed of a computer, we do not talk in terms of
seconds or even milliseconds (10-3). Our units of speed are the
microseconds (10-6), the nanoseconds (10-9) and even the
Pico seconds (10-12). A powerful computer is capable of performing
several billion (109) simple arithmetic operations per second.
2.Accuracy: - In addition to
being very fast, computers are very accurate.
The accuracy of a computer is consistently high and the degree of
accuracy of a particular computer depends upon its design. But for a particular
computer, each and every calculation is performed with the same accuracy.
Errors can
occur in a computer, but these are mainly due to human rather than
technological weakness, that is, due to imprecise thinking by the programmer or
due to incorrect input data. Computer errors caused due to incorrect input data
or unreliable programs is often referred to as Garbage-in-Garbage-out or
GIGO.
3.Diligence: - Unlike human beings, a
computer is free from monotony, tiredness, lack of concentration etc and hence
can work for hours together without creating any error and without grumbling.
Due to this property, computers obviously score over human beings in doing
routine type of jobs, which require great accuracy. If ten million calculations have to be performed,
a computer will perform the ten millionth calculation with exactly the same
accuracy and speed as the first one.
4.Versatility: - Versatility
is one of the most wonderful things about the computer. One moment, it is
preparing the results of particular examination, the next moment it is busy
preparing electricity bills and in between it may be helping an office
secretary to trace an important letter in seconds. All this work is possible by
changing the program (sequence of instructions for computers). Briefly, a
computer is capable of performing almost any task provided that the task can be
reduced to a series of logical steps.
5.Power of
remembering: - As a human being acquires new knowledge, the brain subconsciously
selects what it feels to be important and worth retaining in its memory, and
relegates unimportant details to the back of the mind or just forgets them.
With computers, this is not the case.
A computer can store and recall any amount of information because of its
secondary storage capability. Every piece of information can be retained as
long as desired by the user and can be recalled as and when required. Even
after several years, the information recalled would be as accurate as on the
day when it was fed to the computer. A computer forgets or losses certain
information only when it is asked to do so. So it is entirely up to the user to
make a computer retain or forget particular information.
In order to explain, the idea of storage capacity, we can take an
example of CD-ROM and a hard disk. A single CD-ROM can store up to 700 MB of
data while a hard disk can have a capacity of around 80 GB (1 GB=1024MB).
6. Automatic: - A machine is
said to be automatic if it works by itself without human intervention.
Computers are automatic machines because once started on a job, they carry on
until the job is finished, normally without any human assistance. However,
computers being machines cannot start themselves. They cannot go out and fine
their own problems and solutions. They have to be instructed. That is, a
computer works from a program of coded instruction that specify exactly how a
particular job is to be done. While the job is in progress, the program is
stored in the computer, and the parts of the instruction are obeyed. As soon as
one instruction is completed, the next is obeyed automatically. Some of the
other characteristics of computers such as speed and accuracy are due to the
fact that they are automatic and work on a problem without any human
intervention.
7.No I.Q.: - A computer is not a magical device. It can only perform tasks that
a human being can. The difference is
that it performs these tasks with unthinkable speed and accuracy. It possesses
no intelligence of its own. Its I.Q. is zero, at least till today. It has to be
told what to do and in what sequence. Hence, only the user can determine what
tasks a computer will perform. A computer cannot take its own decision in this
regard.
8.No Feeling: - Computer is
devoid of emotions. They have no feelings and no instincts because they are
machine. Although men have succeeded in building a memory for the computer, but
no computer possess the equivalent of a human heart and soul. Based on our
feelings, taste, knowledge and experience, we often make certain judgments in
our day-to-day life. But computers cannot make such judgments on their own.
Their judgment is based on the instructions given to them in the form of
programs that are written by us. They are only as good as man makes and uses
them.
Basic Components of a
Computer System: - A computer can be viewed as a system that comprises several units.
The term computer system refers to the whole of computer hardware, components,
peripherals and data communications equipment’s. It consists of a number of
interrelated components that work together with the aim of converting data into
information. A computer system essentially has following components:
The Central Processing Unit
(CPU): - The CPU is the brain of a computer system. All major calculations and
comparisons performed by a computer are carried out inside its CPU. The CPU is
also responsible for activating and controlling the operations of other units
of the computer system.
Control Unit: - We know that
the two basic components of a CPU are the
Control Unit and the Arithmetic Logic Unit. The CU controls and guides the
interpretation, flow and manipulation of all data and information. The CU sends
control signals until the required operations are done properly by ALU and
memory. Another important function of CU is the program execution i.e. carrying
out all the instruction stored in the program. The CU gets program instruction
from memory and executes them one after the other. After getting the
instruction form memory in CU, the instruction is decoded and interpreted i.e.,
which operation is to be performed. The control unit even controls the flow of
data from input devices to memory and from memory to output devices.
Although, the control unit does not perform any actual
processing on the data, it acts as a central nervous system for the other
components of the computer.
Arithmetic Logic Unit: - The
arithmetic logic unit (ALU) of a computer system is the place where the actual
execution of the instructions takes place during the processing operation. All
calculation is performed and all comparisons (decision) are made in the ALU.
The data and instruction stored in the primary storage prior to processing are
transferred as and when needed to the ALU where processing takes place. No
processing done in the primary storage unit. The ALU perform all the four
arithmetical (+, -, *, /) and some logical (<, >, =, <=, >=,
<>) operations. When two numbers are required to be added, these numbers
are sent from memory to ALU where addition takes place and the result is put
back in the memory. The same way other arithmetic operations are performed. For
logical operation also, the numbers to be compared are sent from memory to ALU
where the comparisons takes place and the result is returned to the memory. The
result of a logical operation is either TRUE or FALSE. These operations provide
the capability of decision-making to the computer.
Input/Output functions: - A computer
system also incorporates input and output devices, which are a communication
medium between a human and the computer. An input unit accepts instructions and
data from the user and communicate them to the computer. The basic task of this
unit is to gather the data and converts it into the form that the computer can
understand. Some of the input devices are keyboard, mouse, light pen and so on.
Just as humans communicate with computers with input
devices, the computer can communicate with human beings with the help of output
devices. Like input units, output units are instruments of interpretations and
communications between humans and computers. These devices take the machine
coded output results from the processor and convert them into a form that can
be used by the people or as a machine input in another processing cycle. Some
of the commonly used output devices are printers, monitor and plotters.
In addition to these, a computer also employs secondary
storage devices, which are extensively used for storing data or instructions.
The physical components or materials on which data is stored permanently are
called storage units or devices. It supplies the stored information to the
other units of computer as and when required. Some common storage devices are
floppy disks, hard disks and tape drives.
Memory: Memory also known as
the primary storage or main memory. Memory is a part of microcomputers that
holds data for processing, instructions for processing the data and
information. Part of the contents of the memory is held only temporarily, that
is, it is stored only as long as the microcomputer is turned on. When you turn
the machine off, the contents are lost. The capacity of the memory to hold data
and program instructions varies in different computers. The original IBM PC could
hold approximately 640000 characters of data or instructions only. But modern
microcomputers can hold millions, even billions of characters in their memory.
Models for
memory
As we design memory systems, we need to
model memory components and
systems in order to make design decisions. A simple model of memory
components for parallel memory design would include three major parameters
of a memory component of a given size.
1.
Area—the
physical size of the logical component. This is most important in chip design,
but it also relates to cost in board design.
2. Performance—the
access time of the component. There may be more than one parameter, with
variations for read and write times, page mode accesses, and so on.
2.
Energy—the
energy required per access. If performance is characterized by multiple modes,
energy consumption will exhibit similar modes.
Motherboard: A motherboard is the main printed circuit board (PCB) in
general-purpose computers and other expandable systems. It holds and allows
communication between many of the crucial electronic components of a system,
such as the central
processing unit (CPU) and memory, and provides connectors for other peripherals. Unlike a backplane, a motherboard
usually contains significant sub-systems, such as the central processor, the
chipset's input/output and memory controllers, interface connectors,
and other components integrated for general use.
THE COMPUTER GENERATIONS
A generation in computer talk is a step-in technology.
The history of computer development is often referred to in reference to the
different generations of computing devices. It provides a framework for the
growth of the computer industry. Originally, the term ‘generation’ was used to
distinguish between varying hardware technologies. But nowadays, it has been
extended to include both the hardware and the software, which together make up
an entire computer system.
There are totally five computer generations known till
today.
FIRST GENERATION (1942-1955)
We have already known about some of the early computers –
ENIAC, EDVAC, and EDSAC etc. These machines and other of their time were built
by using thousands of vacuum tubes. A vacuum tube was a fragile glass device
that used filaments as a source of electrons and could control and amplify
electronic signals. It was the only high-speed electronics-switching device
available in those days. These vacuum tube computers are referred to as first
generations computers. First generations computers based on binary-coded
language to perform operations and were able to solve only one problem at a
time. Each machine was fed with different binary codes and hence were difficult
to program. In addition, to run on different types of computers, instructions
must be rewritten or recompiled.
Advantages
1.
Vacuum tubes were the only electronic components
available during those days.
2.
Vacuum tube technology made possible the advent of
electronic digital computers.
3.
These computers were the fastest calculating devices
of their time. They could perform computations in milliseconds.
Disadvantages
1.
Too bulky in size.
2.
Unreliable
3.
Thousands of vacuum tubes that were used emitted
large amount of heat and burnt out frequently.
4.
Air Conditioning required.
5.
Prone to frequent hardware failures.
6.
Constant maintenance required.
7.
Nonportable.
8.
Manual assembly of individual components into
functioning unit required.
9.
Commercial production was difficult and costly.
10. Limited
commercial use.
SECOND GENERATION
(1955-1964)
The transistor, a smaller and more reliable successor to
the vacuum tube, was invented in 1947.The second generation emerged with
transistors being the brain of the computer. With both the first and the
second-generation computers, the basic components were a discrete or separate
entity.
Advantages
1.
Smaller in size as compared to first generations
computers.
2.
More reliable
3.
Less heat generated
4.
These computers were able to reduce computational
times from milliseconds to microseconds.
5.
Less prone to hardware failures
6.
Better portability
7.
Wider commercial use
Disadvantages
1.
Air- conditioning required
2.
Frequent maintenance required
3.
Manual assembly of individual components into a
functioning unit was required.
4.
Commercial production was difficult and costly.
THIRD GENERATION (1964-1975)
The third generation was based on IC technology and the
computers that were designed with the use of integrated circuits were called
third generation’s computers.
Advantages
1.
Smaller in size as compared to previous generation
computers.
2.
Even more reliable than second generations
computers.
3.
Even lower heat generated than second generation
computers.
4.
These computers were able to reduce computational
times from microseconds to nanoseconds.
5.
Maintenance cost is low because hardware failures
are rare.
6.
Easily portable.
7.
Totally general purpose. Widely used for various
commercial application all over the world.
8.
Less power requirement than previous generation
computers.
9.
Manual assembly of individual components into a
functioning unit not required. So human labour and cost involved at assembly
stage reduced drastically.
10. Commercial
production was easier and cheaper.
Disadvantages
1.
Air-conditioning required in many cases.
2.
Highly sophisticated technology required for the
manufacture of IC chips.
FOURTH GENERATION (1975 ONWARDS)
A fourth-generation computer, which is what we have now,
has LSI chips at brain. It is LSI technology, which has led to the development
of very small but extremely powerful computers. It was the start of a social
revolution. A whole computer circuit was soon available on a single chip, the
size of postage stamp.
Advantages
1.
Smallest in size because of high components density.
2.
Very reliable
3.
Heat generated is negligence.
4.
No air conditioning required in most cases
5.
Much faster in computations than previous
generations.
6.
Hardware failure is negligible and hence minimal
maintenance is required.
7.
Easily portable because of their small size
8.
Totally general purpose
9.
Minimal labour and cost involved at assembly stage
10. Cheapest among
all generations.
Disadvantages
1.
Highly sophisticated technology required for the
manufacture of LSI chips.
FIFTH GENERATIOS (YET TO
COME)
Scientist is now at work on the fifth generation’s
computers – a promise, but not yet a reality. They aim to bring us machines
with genuine I.Q., the ability to reason logically, and with real knowledge of
the world. Thus, unlike the last four generations, which naturally followed its
predecessor, the fifth generations will be totally different, totally novel,
and totally new.
The odds of coming out with a
fifth generation’s computer are heaviest for Japan. They have already started
work in this direction few years back. Japan has chosen the PROLOG (Programming in Logic) language as its
operating software and plans to have the final machine talk with human beings,
see and deliver pictures and hear the normal, natural language.
Configuration of Computer System: In the
Microcomputer Category, the latest appearance is that of low-cost
Personal Computer which can perform many functions. They are called so because
they are designed for personal use of individuals or small business units,
office automations units or professionals. PC can be used for a variety of
applications like Computer literacy BASIC programming, Fun and games, business
and Professional applications, telecommunications, Data Base Management System,
Accounting, and Word Processing.
IBM introduced a computer called Personal Computer
(IBM-PC) on 12.2.81. An IBM PC compatible system is at present cost Rs. 20000
to 1,00,000. Over 150 different brands, which are said to be IBM PC
compatibles, have been introduced in India.
A PC is a general-purpose
computer that can easily fit on a normal size office table. As the name
implies, PCs were mainly designed to meet the personal computing needs of individuals
either in their working places or at their homes.
The configuration of PCs varies from one PC to another
depending on their usage. However, the most commonly used configuration
consists of a system unit, a monitor (display screen), a keyboard and a mouse.
The System Unit consists of the main circuit board (consisting of CPU, Memory,
etc), the Hard Disk, the floppy disk drive, the CD-ROM drive, any special add-on
cards (such as network interface card, sound card), and ports for connecting
peripherals devices (such as printer).
The two most commonly used models of PCs are the desktop
model and the tower model. Although, the desktop model was more popular few
years ago, the tower model is gaining popularity now. A PC generally employs
several chips (CPU chip, RAM chips, I/O handling chips etc.) on a main circuit
board called a system board or motherboard. Often PCs are distinguished by the
main component of the motherboard, that is the microprocessor chip, which is
used as their CPU.
Types of PC: -
Desktop- Desktop computer
is the most common microcomputer. It is principally intended for stand-alone
use by an individual. These microcomputers typically consist of a system unit,
a display monitor, a keyboard, internal hard disk storage and other peripheral
devices. Desktop computer are not very expensive to purchase by the individuals
or the small businesses. Some of the major personal computer manufactures are
APPLE, IBM and Hewlett-Packard.
Laptop- A laptop is a
portable computer, that is, a user can carry it around. Since the laptop
computer resembles a notebook, they are also known as notebooks. Laptops are
small computers enclosing all the basic features of a normal desktop computer.
The biggest advantage of this computer is that one can use this computer
anywhere and at any time, especially when one is traveling and does not have a
place to keep it. Moreover, these computers do not need any external power supply
as a rechargeable battery is completely self-contained by them. These computers
are expensive as compared to desktop computers.
Notebook- Notebook computers are portable computers
which are mainly meant for use by people who need computing power wherever they
go. As their name implies, notebook computers are approximately of the size of
an 81/2 *11-inch notebook and can easily fit inside a briefcase.
Since they have to be carried along, they are also light in weight, weighing
around 2 Kg. They are also known as laptop PCs because they are as powerful as
a PC and their size and weight allows them to be used by comfortably placing
them on one’s lap.
A notebook computer uses an almost full-size keyboard, a
small flat-screen liquid crystal color display, and a trackball instead of a
mouse. They also have a hard disk, a floppy disk drive, and sometimes a CD-ROM
drive also. Many models of notebook computers can be connected to a network so
that they can download data from other computers on the network as and when
such a need arises, or to access to the Internet. Notebook computers are
designed to be used even at places where there is no power point available to
connect them with a power source. Hence, they are designed to operate with
chargeable batteries. With a fully charged battery, a notebook computer can be
used for a few hours.
Notebook computers
normally run MS-DOS or WINDOWS operating system. They are mostly used for word
processing, spreadsheet computing. Data entry and preparing presentation materials
while a person is traveling. The processing capability of a notebook computer
is normally as good as an ordinary PC because both use the same type of a
processor such as an Intel Pentium Processor. Notebook computers are typically
more expensive than a normal PC.
Palmtop- A small
computer that literally fits in your palm is called palmtop. Compared to full
size computers, palmtops are severely limited, but they are practical for
certain functions such as phone books and calendars. Palmtops that use a pen
rather than a keyboard for input are often called hand-held computers or PDAs.
Because of their small size, most palmtop computers do not include disk drives.
however, many contain PCMCIA slots in which you can insert disk drives, modems,
memory and other device
PDA- Personal Digital Assistants (PDAs)
are small networked computers which can fit in the palm of your hand. PDAs have
evolved over the years, beginning with first-generation devices such as the
Apple Newton (Apple coined the term “PDA”) and Palm Pilot. They offered
features such as calendar and note-taking capability. PDA operating systems
include Apple iPhone OS, Symbian OS, Palm, Windows CE, Windows Mobile,
Blackberry, and Google's Android, among others.
TYPES
OF COMPUTERS: -The
computers have been classified into three categories
1) Digital Computer
2) Analog Computer
3) Hybrid Computer
DIGITAL COMPUTERS: - The digital computers work upon discontinuous
data. They convert the data into digits (binary digits 0 and 1) and all
operations are carried out on these digits at extremely fast rates. A digital
computer basically knows how to count the digits and add the digits. Digital
Computers are much faster than analog computers and far more accurate.
Computers used for business and scientific applications are digital computers.
Digital Computers can be classified in two ways:
(1)
Purpose- wise
(2)
Size and performance wise
Purpose Wise digital
computers are classified into two types:
Special-Purpose Computer is the one that
is designed to perform a specific task. The instruction (programs) to carry out
the task is permanently stored in the machine. For the specific tasks, this
type of computer works efficiently but such computers are not versatile.
General- Purpose Computer is the one that can work on different types of programs input to it
and thus be used in countless applications. The programs are not permanently
stored but are input at the time of execution. These computers are very
versatile.
Size and Performance wise digital
computers can be classified into following four types:
Microcomputers- a microcomputer
is a computer who’s CPU is a microprocessor. A microprocessor is a processor
whose all components are on a single integrated circuit chip. Those are
normally single-microprocessors, single-user systems designed for performing
basic operations like educational, training, small business applications,
playing games etc. These are mainly used in offices, homes, schools, shops,
stores etc. IBM PCs, Apple Mac, IBM PS/2 are some popular computers of this range.
Compared to mainframes and minis the microcomputer may be
comparatively slow and their capabilities limited, but they do provide good
value for money. The market of cheap microcomputers has tremendously
(extremely) expanded in recent years. In India, several companies have been
licensed to manufacture microprocessors.
Minicomputers- Minicomputers are
more powerful computers than microcomputers in terms of processing power and
capabilities. Minicomputers are mainly multi-user systems where many users
simultaneously work on the systems. Mini computers posses’ greater storage
capacity and larger memories as compared to microcomputers. These are even
capable of handling more input-output devices. The most important advantage of
a minicomputer over the mainframe is that it is cheaper in cost, small in size
and very reliable. It does not require air conditioning and can be operated at
room temperature. Examples are: PDP-11,
VAX, and 7500 MAGNUM etc.
Mini computer can accept and transfer data from I/O devices
at the maximum speed of 4 million bytes per second. They usually employ
microprocessors (chips) in the CPU, both for data storage as well as data
manipulations.
Mainframe Computers- Mainframe
computers are designed to handle huge volumes of data and information. These
very large and expensive computers have great processing speed and very large
storage capacity and memory as compared to minicomputers. These computers even possess
and work with more than one processor at the same time. Thus one can say these
are multi-user, multiprocessor systems. For mainframe computers very sophisticated
operating systems are needed to control and supervise their operation.
Mainframe allows its user to maintain large information storage at a
centralized location and be able to access and process this data from different
computers located at different locations. They are typically used by large
businesses and for scientific purposes. Examples are: ICL 39, CDC 6600, VAX
8842, IBM 3090/600, IBM 4381.
Super Computers –Super computers
are the most powerful computers among digital computers. These consist of several
processors running together thereby making them immensely (hugely) faster and
powerful. These computers are capable of handling huge amounts of calculations
that are beyond human capabilities. Super computers can perform billions of instructions
per second. Some of today’s super computers have the computing capability equal
to that of 40,000 microcomputers. A Japanese super-computer has calculated the
value of Pi to 16 million decimal places. These computers cost in 15–20-million-dollar
range. A super computer can process a great deal of information and make
extensive calculations very, very quickly. They can resolve complex
mathematical equations in a few hours, which would have taken a scientist with
paper and pencil a lifetime, or, years, using a hand calculator. Typically,
super computers are used to solve multi-variant mathematical problems of
existent physical processes, such as aerodynamics, metrology and plasma
physics. These are also required by the military strategists to simulate
defense scenario. These are mainly used in application used in applications
like weather forecasting, nuclear science research, seismology etc. Examples
are: CRAY X-MP/14, CDC –205, ETA, PARAM, and ANURAG.
PARAM and ANURAG are super computers produced by India
and are exported to many European countries.
ANALOG COMPUTERS: - In analog computers, continues quantities are
used. Computations are carried out with physical quantities such as voltage,
length, current, temperature etc. The devices that measure such quantities are
analog devices e.g. voltmeter ammeter. Analog computers operate by measuring
rather than counting. Such computers do not directly deal with the numbers.
They measure continuous physical magnitudes (such as temperature, pressure and
voltage), which are analogous to the numbers under considerations. For example,
the petrol pump may have an analog computer that converts the flow of pumped
petrol into two measurements- the quantity of petrol and the price for that
quantity. The main advantage of analog
computers is that all calculations take place in parallel and hence these are
faster. It is very easy to get graphical results directly using analog
computer. But their accuracy is poor as compared to digital counterparts.
Analog computers are mostly used in engineering and scientific applications. An
electronic weighing scale is an example of analog computer.
Difference between Analog
and Digital Computer: -
|
Analog |
Digital |
|
Operates on physical
quantities like temperature, resistance, length, speed etc. |
Operates on numbers like
amount of cheque, number of miles etc. |
|
Measures |
Counts |
|
Continuous |
Discontinuous |
|
Storage capacity
relatively high |
Storage capacity
relatively low |
|
Mainly Process Control |
Scientific and Commercial
use |
HYBRID COMPUTER: - Hybrid computers utilize the best qualities
of both the digital and analog computers. In these computers some calculations
take place in analog manner and rest of them take place in digital manner.
Hybrid computers are best used in hospital where analog part is responsible for
measurement of patient’s heart beat, blood pressure, temperature and other
vital signs and then the operation is carried out in digital fashion to monitor
patient’s vital signs. Hybrid computers are also used in weather forecasting,
scientific applications, and various fields of engineering and in industrial
control processes.
Analog and hybrid computers perform specialized task but
the digital computers are used almost everywhere in business and scientific
applications.
Memory:- The memory of
a computer is more like a predefined working place, where it temporarily keeps
information and data to facilitate its performance. When the task is performed,
it clears its memory and memory space is then available for the next task to be
performed. When the power is switched off, everything stored in the memory gets
erased and cannot be recalled.
The memory of
computer is often called main memory or primary memory. The memory of a computer can be thought of as
cells. Each of these cells is further broken down into smaller parts known as
bits. A bit means a binary digit i.e. either 0 or 1. A bit is an elementary
unit of the memory. A group of 8 bits is called a byte and a group of 4 bits is
called a nibble.
One byte is the smallest unit, which can represent a data
item or a character. Other units of memory are KB, MB, and GB.
1KB = 1024 BYTES = 210
1 MB = 1024 KB = 1024*1024 BYTES
1 GB = 1024 MB = 1024*1024*1024 BYTES
Since computer’s main memory
(primary memory) is temporary, secondary memory space is needed to store data
and information permanently for later use. The two most common secondary
storage media are the floppy diskette and the hard disk.
There are two types of memories: Primary and Secondary.
The primary memory or the main memory is part of the main computer system. The
processor directly stores and retrieves information from it. The primary memory
itself is implemented by two types of memory technologies. The first is called
random access memory(RAM) and other is read only memory(ROM).
Random Access
Memory(RAM):- Random access memory is a volatile memory. It means the information
stored in it remains as long as the power is switched ON. As soon as the power
is switched OFF, the information contained in it vanishes. RAM can be defined
as a block of sequential memory locations, each of which has a unique address
determining the location and those location contain a data element.
Physically, this
memory consists of some integrated circuit chips either on the motherboard or
on a small circuit board attached to the motherboard. A computer’s motherboard
is designed so that its memory capacity can be easily enhanced by adding more
memory chips. The additional RAM chips, which plug into special sockets on the
motherboard are also known as single in-line memory modules (SIMMs).
There are two types of RAM.
Dynamic Random
Access Memory (DRAM):- This type of RAM holds the data in dynamic manner with the help of a refresh circuitry.
Each second or even less than that the content of each memory cell is read and
the reading action refreshes the contents of the memory. Due to this refreshing
action, the memory is called dynamic RAM. DRAMs are made from transistor and
capacitor. The capacitor holds the electrical charge if the bit contains 1 and
no charge if the bit is 0. the transistors reads the contents of the capacitor.
The charge is held for a short period and than it fades away, that is, when
refresh circuitry comes in.
Static Random
Access Memory(SRAM):- SRAM along with DRAM is essential for a system to
run optimally, because it is very fast as compared to DRAM. It is effective
because most programs access the same data repeatedly and keeping all this
information in the fast SRAM allows the computer to avoid accessing the slower
DRAM. Data is first written to SRAM assuming that it will be used again soon.
SRAM is generally included in a computer system by the name of cache. A static
RAM is faster, costlier, and consumes more power than dynamic RAM. Due to these
reason, large memories use dynamic RAM,
and static RAM is used mainly for specialized applications. The main memory of
most computers uses dynamic RAM.
Read Only Memory (ROM):- A special type
of RAM, called read only memory(ROM) is a non-volatile memory chip in which
data is stored permanently and can not be altered by the programmer. As the
name suggests, read only memory can only be read, not written. The contents of
ROM are not lost even in case of sudden power failure, making it non-volatile
in a nature. Read only memory is also
random access in nature, which means the CPU can randomly access any location
within ROM.
In fact, storing data permanently
into this kind of memory is called “burning in the data”, because data in such
memory is stored by fuse-links. Once a fuse-link is burnt it is permanent. ROMs
are mainly used to store programs and data which do not change and are
frequently used. For example, the most basic computer operations are carried out by wired electronic circuits. However,
there are several higher level operations that are very frequently used but
will require very complicated electronics circuits for their implementations.
Hence, instead of building electronics for these operations, special programs
are written to perform these operations. These programs are called microprograms
because they deal with low-level machine functions and are essentially
substitutes for additional hardware.
Microprograms are written to aid the control unit in directing all the
operations of the computer systems. ROMs are mainly used by computer
manufacturers for storing these microprograms so that they can not be modified
by the users. A good example of a microprogram that is stored in a ROM chip of a
computer is the set of instructions that is needed to make the computer system
ready for use when its power is switched on. This microprograms called system
boot programs, contains a set of start up instructions to check if the system
hardware like memory, I/O devices etc are functioning properly and looks for an
operating system and loads its core part in the volatile RAM of the system to
produce the initial display screen prompt. Note that this microprogram is used
every time the computer is switched on and needs to be retained when the
computer is switched off.
There are various
types of ROM which are given below:
- Programmable Read Only Memory (PROM)- PROM is a
ROM that can be programmed to record information using a facility known as
PROM programmer. Once the chip has been programmed, the recorded
information can not be changed i. e. PROM becomes same as ROM.
- Erasable Programmable Read Only Memory- EPROM
is another type of ROM that can not erased and the chip can be programmed
to record different information using a special PROM program facility.
Erasure to achieved by exposing the chip to ultraviolet light. When an
EPROM is in use, information can only be read and the information remains
on the chip until it is erased. EPROM are of two kinds-UVPROM(Ultraviolet
PROM) and EAPROM(Electrically Alterable PROM).
- Electrically Erasable PROM (EEPROM)- This type
of ROM can be programmed and erased by electrical signals. It does not
require exposure to ultraviolet light to erase its contents as EPROM
memory does and provides an easy means to load and store temporary or
permanent information in a form of ROM memory. Information loaded in this
memory can be retained for many years without any power supplied, one of
its new applications is as a backup to RAM memory whose contents are lost in a power failure. When power
is returned, the EEPROM memory can be used to replace the lost contents of
the RAM memory and the microcomputer can continue working just as if
nothing had happened. Even newer devices are combining RAM and EEPROM
memory in a single integrated circuit.
Due to the ease with which stored programs can be altered, EEPROM
is also known as flash memory. Flash memory is used in many new I/O and
storage devices. The intelligence of these devices can be upgraded by
simply down-loading new software from a
vendor-supplied
disk to flash memory.
Primary vs
Secondary Memory :
Now, Let’s see the
difference between Primary memory and Secondary memory
|
SR.NO. |
PRIMARY
MEMORY |
SECONDARY
MEMORY |
|
1. |
Primary memory is temporary. |
Secondary memory is permanent. |
|
2 . |
Primary memory is directly accessible by Processor/CPU. |
Secondary memory is not directly accessible by the CPU. |
|
3 . |
Nature of Parts of Primary memory varies, RAM- volatile in nature. ROM-
Non-volatile. |
It’s always Non-volatile in nature. |
|
4. |
Primary memory devices are more expensive than secondary storage devices. |
Secondary memory devices are less expensive when compared to primary
memory devices. |
|
5. |
The memory devices used for primary memory are semiconductor memories. |
The secondary memory devices are magnetic and optical memories. |
|
6. |
Primary memory is also known as Main memory or Internal memory. |
Secondary memory is also known as External memory or Auxiliary. |
Various Storage Devices: - Storage devices
are classified into two parts according to their working and the method
provided to access the information stored in it; serial access storage devices
and direct access storage devices.
Serial Access Storage Devices:- These devices provide
only serial access to the information stored. This means that all the previous
records need to be read to access the next record – one cannot directly access
a particular record. In a serial access storage device simultaneous reading and
writing cannot be done in the file. Magnetic tape is an example of serial access
storage device.
Direct Access Storage Devices (DASD):- In a direct access
storage device there is no need to read all the previous records to access a
particular record. The record can be accessed directly. In DASD the data stored
can also be accessed sequentially, if need be. Reading and writing can be done
simultaneously in the file in a DASD floppy disk and magnetic disks. CD ROMs
are also direct access storage devices.
Magnetic Tape: - Magnetic tapes are used as external
storage device to keep back-up copies of precious software and data. It is a
serial access storage device and provides sequential access only. Tape is a
plastic ribbon that is coated on one side with a magnetic material
(iron-oxide). Information is stored using binary code in the form of magnetized
or non-magnetized spots. An electro-magnetic head arranges these magnetic
particles to store data. These particles are interpreted when we read from the
tape and are then converted back to information. They come in the range of 12.5
mm to 25 mm width and 500 meters to 1200 meters length.
Recording Data on Magnetic Tapes: - Magnetic tapes
have their own coding system. Information is recorded on the tape in the form
of tiny invisible magnetized and non-magnetized spots (representing 1's and 0's).
Tape is divided into vertical columns called frames and horizontal rows called
tracks. Only one character is recorded per frame. Data is recorded in two
coding formats – BCD (Binary Coded Decimal) which is 7 bit format (6 bit BCD
and 1 bit for parity checking), EBCDIC (Extended Binary Coded Decimal
Interchange Code) which is 9 bit format (8 bit EBCDIC code and 1 bit for parity
checking). A parity or check bit is used to detect errors that may occur due to
loss of a bit from a string of 6 bit BCD or 8 bit EBCDIC format during input or
output operation.
Fixed and Variable Length Records: - The data is
normally stored on a tape in blocks. On some tapes, the block is of fixed
length. It may vary in length for others. In fixed length record block, the data
size cannot exceed a predetermined maximum number of characters. In variable
length record block, there is no such limitation on number of characters and
the record may contain any number of characters.
Blocking of Records: - A magnetic
tape alternates sections of data called blocks and regions of blank tapes
(about 3/4 inch in length) called Inter Record Gap (irg). The gap has to be provided to allow for stopping and starting
of the tape. Data transfer takes place only when the tape is moving at constant
speed. Also it is transferred block wise in magnetic tapes. A block may contain
one record or more than one record. Number of records in one block is known as
Blocking Factor of that tape. The data between two IRG forms one
physical record while it may contain number of logical records within it.
Multiple Record Block:-
Header and Trailer Labels
In addition to
the data records, each tape file contains at least two additional records.
u Header Label It is the first record on the file. Header label contains control
information for verifying the accuracy, completeness and authenticity of file
name, a password, a file creation date and access control information about the
file.
u Trailer Label It
is the last record on the file. A trailer label contains the number of physical
and logical records contained in the file.
Magnetic Tape Drive:- Magnetic tape
drive are the devices that can either read data from a tape into the CPU or can
write information being produced by the CPU onto the tape. Data is read from
the tape with the help of a read-write assembly. There is one read-write head
for each track.
Data Transfer Rate (DTR):- It is an
important feature of secondary storage devices. DTR is the product of tape
density and tape speed. Data Transfer Rate (bytes/second) = Packing Density
(bytes/inch) x Tape Speed (inches/second). Typical data transfer rate is 100000
bytes/second or above.
Tape Density:- The density of tape means number
of frames recorded in 1 inch of tape. Typical tape densities are 556 BPI, 800
BPI, 1000 BPI, 3250 BPI or 6250 BPI. (Bytes per Inch).
Tape Speed: - The speed of tape is measured in inches per second.
It normally lies between 50-200 inches/second.
Advantages of Magnetic
Tapes
u Magnetic tapes provide
virtually unlimited storage. Number of tapes can be used as per requirement for
storing of data.
u A magnetic tape provides high data density. A typical tape can store
6250 characters per inch. So a tape of 28,800 inches can store 180 million
characters.
u Magnetic tapes are economical to use – their cost is very low.
u Rapid data transfer rate is about one million bytes per second.
u Magnetic tapes and cartridges are very easy to handle and use.
u Tape is a convenient way
of carrying large volumes of information from one place to another.
u It can be erased and
reused many times.
Limitations of
Magnetic Tapes
One of the main limitation with magnetic
tapes is that they lack direct access to records and are susceptible to environmental
disturbances. Also the require human intervention for operation. The data
transmission in magnetic tapes is slow as compared to disks. There is
variability or tape drives, i.e. different types of tape drives are available
and there is no standardization, which makes it difficult to recover from
parity errors.
Magnetic Disks:- Magnetic disks are used as an input,
output or external storage device. They are a popular medium for Direct Access
Storage Devices (DASD). It is a thin, circular metal plate/platter coated on
both sides with a magnetic material. It usually comes in the form of a disk
pack, also known as hard disk. All the disks in the disk pack rotate at a very
high speed of 700 or 3600 rpm. They come in two varieties according to the operations
of the read-write mechanism – fixed head and moving head. Disk packs also come
in two configurations – fixed disk pack, removable disk pack.
Storage of Information:-
Information is stored on both the surfaces of each disk platter
except the upper surface of the top platter and lower surface of the bottom
platter. Each disk platter is divided into concentric circles known as tracks.
A set of corresponding tracks in all the surfaces of a disk pack is called a
cylinder. Data is stored in one cylinder first and then the head moves to the
next cylinder, this saves time wasted in moving the head track by track for a
particular disk. Information is recorded as a series of magnetized (signifying
a 1-bit) or non-magnetized (signifying a 0-bit) spots. Each track contains
equal number of characters. Information can be erased from anywhere on the disk
and new data can be recorded on it. Normally there are 200 tracks on a disk
surface numbered 0 to 199. Each track is divided into 8 or 12 equal sectors.
Sectors are used to store information – about 512 bytes can be stored per
sector.
Addressing of Records:-
The heads are attached to access arms which are moved in and out
over the spinning disk. The heads can thus be quickly located over any track to
read or write data. These tracks begin at the outer edge and continue towards
the centre. Each track has a unique number (000-199).
Accessing of Data:-
Data is recorded on the tracks of the spinning disk surface and
read from this surface by one or more read/write heads. There are two basic
types of disk systems – the moving head system and the fixed head system.
Accessing of data is different for both.
Moving Head System
Moving head system consists of one read-write head for each disk
surface mounted on an access arm which can be moved in and out. Each read-write
head moves horizontally across the surface of the disk. Every surface of disk
pack has its own head and all heads move together. One cylinder is accessed
simultaneously by the set of read-write head. Then the head moves for the next
cylinder.
Fixed Head system:-
In a fixed head system the access arm is non-movable and a number
of heads are attached on this arm. These read-write heads are distributed over
the disk surface, one head for each track. As a result no head movement is
required and information is accessed more quickly. Because of the space
required for the additional read-write heads, fixed head disks have less
capacity and cost more per byte of data stored.
Access Time:-
Information is accessed from the disk by referencing the disk
address. Disk address is specified in terms of surface number, track number and
sector number. Access time in any disk system is made up of three components –
Seek Time, Latency Time and Data Transfer Time.
u Seek Time Time
required to position the head over the proper track is called the seek time,
normally measured in milliseconds. For fixed head system, seek time is always
zero.
u Latency Time Time
required to rotate the disk pack to bring the correct sector under the
read-write head is known as latency time. Average latency time is of the order
of 8-10 ms.
u Data Transfer Time Time
required to read or write the actual data on the disk is called data transfer
time.
Storage Capacity:-
The storage capacity of a magnetic disk largely depends upon the
number of disks in the disk pack and the number of tracks per inch and bits per
inch of track. Total number of bytes that can be stored in a disk pack =
(Number of cylinders × Tracks per cylinder × sectors per track × Bytes per
sector). Storage capacity of a disk pack is in the range of 200 to 10000
megabytes (Mbytes).
Advantages of Magnetic Disks:-
Magnetic disks are DASD devices, i.e. time taken to locate a
particular record is independent of the position of that record. Disk storage
is more durable than that of tape storage. Magnetic disks support on-line
processing because of its direct accessing property. In a nut-shell the
advantages of magnetic disks are: easy accessibility, durability, reusability,
compactness and providing sequential as well as direct access.
Disadvantages: -
The disadvantage of magnetic disk packs is that it is costlier than
tape. Tapes are more economical to use. The disk packs are not easily portable,
i.e. there are difficulties in
removing a diskpack. Also they are less secure because they have direct access
and data can get overwritten by mistake any where on the disk or it can get
corrupted anywhere along the file. In the case of tapes, since all of it is not
exposed for accessing it is more secure and safe.
Types of Disks
All magnetic disks are platters coated with a magnetic material.
They come in different sizes. They are portable or permanently mounted on the
computer system. The complete unit to read and write on these disks is called
Disk Drive Unit. Disks can also be differentiated on the basis of their size,
sectors, number of sides, storage capacity and density.
Winchester Disk/Hard Disk
It is a group of large metal or plastic disks permanently sealed in
a container. Read-write heads and access mechanisms are also inbuilt within the
container. These sealed containers are not generally removed from their disk
drives. Smaller size of Winchester disks are used in mini computers and PCs.
Winchester disks minimize contamination by prohibiting the circulation of
outside air between the disk. In operation, the disk is rotated at a high speed
on the spindle. There is one read-write head for each surface. The heads can
move to and fro to select desired track position. During operation, heads don't
touch the magnetic surface, a thin cushion of air is maintained between a
rotating disk and read-write head. To store or retrieve data, the system finds
the disk address used to contain the data by moving the read/write head to the
appropriate track where it waits until the desired sector passes by. The
capacity of hard disks is very high compared to other disks. Nowdays hard disks
of capacity 4.3 GB and 8 GB are available. Although hardisks are fixed in PC,
porting of disks from one place to another is more secure than before because
all the disks are safely packed inside a protective covering.
Floppy Diskette
Hard disks are costly and unfit to transfer data from one place to
another so floppy diskettes of low capacity are used. These diskettes are
economical and very flexible to use and are fixed inside the computer system.
The diskette can be frequently taken out and inserted in the disk drive unit
While in operation, floppy disk drive heads actually touch the surface of the
magnetic disk. This results in quick wear and tear of the disks and the
read-write heads. These diskettes are made up of plastic and have a magnetic
coating. The disk is covered in a protective sheath with opening for editing
and writing.
Nowadays two
sizes of disks are commonly available – 5.25 inches and 3.5 inches. The 3.5
inch diskette comes in a rigid packet for protection so they are not really
‘floppy’. These diskettes store more data due to certain advantages. They have
over 5.25 inch diskettes. The 3.5 inch diskette has actually replaced 5.25 inch
diskettes. Floppy diskettes hold only one disk at a time and so only one
diskette can work with the disk drive unit at a time.
The capacity of
floppy disks, mini disks, is very low compared to hard disks. A 5.25 inch disk has
360 KB and 1.2 MB capacity while that of 3.5 inch disk can store about 1.44 MB
of data. The 3.5 inch disk is also referred to as micro floppy disk. The cover
of these disks has a sliding opening for editing and writing of heads which is
automatically closed when not in use to protect the disk from dust and
moisture.
The index role on the recording media is used by the disk drive
unit to locate the starting of the first sector. In a 5.25 inch floppy disk
write protect opening is occurred by a write protect notch and writing to the
disk is permitted only when the notch is open and in 3.5 inch diskette a write
protect tab is used to make the diskettes read only by covering the notch and
opening the tab respectively. Another hole in a 3.5 inch floppy disk is a high
density detection hole. It distinguishes the 1.44 MB diskette from 720 KB low
density diskette, which is of the same size and shape.
Zip Drive: - These are high-capacity floppy disk drives developed by the Iomega
Corporation. Zip disks are slightly larger than the conventional floppy disks,
and are about twice as thick. They can hold 100MB of data. Because they are
relatively inexpensive and durable, they have become a popular media for
backing up hard disks and for transporting large files.
Optical Disks: - Storage
techniques can be based both on the principles of magnetism as well as on the
use of light. Optical technology involves the use of laser beams-highly
concentrated beams of light. Most optical disks are read-only storage devices:
however, some of the more expensive optical disks can be written and erased. A common
version of the optical disk is the CD-ROM.
CD:
- CD-ROM
stands for Compact Disk-Read Only Memory. It is a small optical disk in which a
laser beam is used to store and read information. It is a read only storage
device, i. e. once CD-ROM is produced the information stored on it cannot be
changed by using common CD-ROM drives. It is mainly used to distribute software
data, multimedia presentations, etc. It looks like a shiny aluminum foil
encased in a plastic container. It is a circular disk with a 4.75-inch
diameter. Another version of CD-ROM, the 3.5 inch, is not as popular as the 4.75-inch
disks.
Data Storage and Retrieval: -
The data is stored on a CD-ROM disk in a completely different way
as compared to the data storage of a magnetic disk. On a CD-ROM, data is stored
in a continuous 'spiral' format, like the process of recording songs on the
audio tracks. On a magnetic disk, as you move towards the center, the sector
size decreases, whereas on a CD-ROM disk each sector is of the same size. As
each sector of the CD-ROM is of the same size, the CD-ROM drive is rotated
using a constant linear velocity. This method increases the disk storage
capacity but maintaining a constant linear velocity requires complicated logic
circuits and the disk access operation also slows down as compared to the hard
disk drive.
The data on the
CD-ROM is stored on a recording surface which is made up of aluminum or gold
layer; this layer is protected by a transparent plastic coating. On this
recording surface, data is recorded as ‘pits’ and ‘lands’ as shown in the
following figure. The lands are flat surface on the disk surface and pits are
small depressions on the disk surface. These pits are created by focusing a
sharp laser beam on the disk surface.
The laser beam
actually burns the surface so a pit is created at that place. These pits and
lands are recognized as 1 and 0. For reading, the laser beam is focused on the
disk surface. The beam gets reflected at land and no reflection is there at
pits. Sequence of reflected light is interpreted as 0s and 1s by the drive unit
and sent to the computer.
Merits and Demerits of CD-ROM:-
The main advantage of these disks is that unlike the magnetic
storage media, if these disks are kept properly, the stored information on
these disks will last forever. These CD-ROMs are not susceptible to electrical
and magnetic field disturbances because whatever is stored is permanent and it
is not in the form of electrical charge or magnetic polarity of particles.
Another big advantage of the CD-ROM disk is its high storage capacity. One
CD-ROM can store 600 MB of data. The only demerit associated with it is that it
is read only storage device and data cannot be changed by using a normal CD-ROM
drive.
One of the most
common applications of the CD-ROM is entertainment software such as video
games, etc. Many types of educational software encyclopedias requiring lot of
space are now a days available on CD-ROMs. They are also being used extensively
for multimedia application.
VCD: - The format is a standard digital data format for storing video on
a compact disc. VCDs are playable in dedicated VCD players and widely playable
in most DVD players, personal computers and
some video game consoles. However, they are less playable in some Blu-ray Disc players, vehicle audio with
DVD/Blu-ray support and video game consoles such as the Sony PlayStation and Xbox due to lack of support backward compatibility for the older MPEG-1 format
or inability to read MPEG-1 in .dat files
alongside MPEG-1 in standard MPEG-1 files.
CD-R:
- CD-R
drive, which is short for Compact Disk-Recordable drive, is a type of disk
drive that can create CD-ROMs and audio CDs. This allows the users to master a
CD-ROM or audio CD for publishing. Until recently, CD-R drives were quite
expensive, but process have dropped dramatically. A feature of many CD-R
drives, called multisession recording, enables you to keep adding data to a
CD-ROM over time. This is extremely important if you want to use the CD-R drive
to create backup CD-ROMs. To create CD-ROMs and audio CDs, you will need not
only a Cd-R drive, but also a CD-R software package. Often, it is the software
package, not the drive itself that determines how easy or difficult it is to
create CD-ROMs. Cd-R drives can also read CD-ROMs and play audio CDs.
CD-RW: - CD-RW disk is short for CD-Rewritable disk and this is a new type of CD
disk that enables you to write onto it in multiple sessions. One of the
problems with CD-R disks is that you can only write to them once. With CD-RW
drives and disks, you can treat the optical disk just like a floppy or hard
disk, writing data onto it multiple times.
The first CD-RW drives became available in mid-1977. they
can read CD-ROMs and can write onto today’s CD-R disks, but they cannot write
on CD-ROMs. Many experts believe that they will be a popular storage medium.
Digital Versatile Disk (DVD)-> DVD is a standard format for the distribution and interchange of
digital content in both computer based and consumer electronic products. The
format is designed to store various types of large multimedia data on a new and
improved version of the CD (Compact Disk), which works on optical disk
technology. That is, both DVD-ROM and CD-ROM store data using pits embossed in
the disk surface. However, these pits are about 4 ½ times as dense on the
DVD-ROM as on the CD-ROM and can store about seven times more data per side.
(The greater density is due to a more efficient data modulation scheme and error
correction method that packs more data bits into every bit). Physically, both
DVD and CD are identical in both diameter and total thickness. Currently
available DVDs have 4.7 GB capacity per side. They provide about 135 minutes of
video playback at an average data rate of little less than 5 megabits/second.
DVD-RW: DVD+RW is the name of a standard for optical
discs: one of several types of DVD, which hold up to about 4.7 GB per disc
(interpreted as approximately 4.7 × 109 bytes; actually 2295104
sectors of 2048 bytes
each which comes to 4700372992 bytes, 4590208 kilobytes, 4482.625 megabytes, or
4.377563476 gigabytes) and are used for storing films, music or other data.
DVD+RW supports random
write access, which means that data can be added and removed without erasing
the whole disc and starting over (up to about 1000 times). With suitable
support from the operating system, DVD+RW media can thus be treated
like a large floppy disk.
DVD+RW was primarily developed for holding discrete
data sets (which change with time) or as recyclable discs for backing up
collections of files. However, they (and DVD-RW) are less
popular for computer use than DVD-R or DVD+R discs, because they are not suitable for permanent
backup files (because non-rewritable media is significantly cheaper). For
similar reasons, rewritable discs are not as widely used for permanent storage
of home DVD video recorders as DVD-R and DVD+R.
On the other hand, DVD+RW or DVD-RW make an
inexpensive medium for multiple temporary recordings: they can be used for the
daily discs of a backup cycle (which are overwritten after a number of days or
weeks), and became very popular for their convenience and cheapness as a medium
for time-shifting TV (recording programs for a single later viewing and
erasing), DVD+RW discs are now playable in three quarters of today's DVD
players, many of them Hybrid (DVD±RW) drives
Blue Ray Disc: Blu-Ray Disc (BRD), Often Known Simply
as Blu-Ray, Is A Digital Optical Disc Storage Format.
It Is Designed to Supersede the DVD Format, Capable of Storing
Several Hours of Video in High-Definition (HDTV 720p and 1080p). The
Main Application of Blu-Ray Is as A Medium for Video Material Such as Feature
Films and For the Physical Distribution of Video Games for The PlayStation
1,2,3, 4, 5, Xbox One and Xbox Series X. The Name
"Blu-Ray" Refers to The Blue Laser (Which Is Actually a Violet Laser)
Used to Read the Disc, Which Allows Information to Be Stored at A Greater
Density Than Is Possible with The Longer-Wavelength Red Laser Used for DVDs.
USB flash drive/Pen Drive: - A USB flash
drive is essentially the flash memory integrated with a USB 1.1 or 2.0
interface used as a small, lightweight, removable data storage device. USB
flash drives are also known as Pen Drives, Thumb Drives, Flash Drives and a
wide variety of other names. They are also sometimes erroneously called memory
sticks, which is a Sony trademark describing their proprietary memory card
system.
A flash drive consists of a small printed circuit board
encased in a robust (healthy) plastic casing, making the drive sturdy enough to
be carried about in a pocket. Only the USB connector protrudes (extend beyond)
from this plastic protection, and is often covered by a removable plastic cap.
SD/MMC Memory Cards: Secure Digital, officially abbreviated as SD, is
a proprietary non-volatile memory card format
developed by the SD Association (SDA) for use in portable
devices. The companies also formed the SD Association (SDA), a non-profit
organization, in January 2000 to promote and create SD Card standards.[3] SDA
today has about 1,000 member companies.
Solid State
Drive- SSD is a solid-state storage device
that uses integrated circuit assemblies to store data persistently, typically using flash memory, and
functioning as secondary storage in the hierarchy
of computer storage. It is also sometimes called a solid-state
device or a solid-state disk,[1] even
though SSDs lack the physical spinning disks and movable read–write heads used
in hard disk drives (HDDs) and floppy disks.[2
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