Friday, November 18, 2011
notes
Input
Devices
An input
device is any hardware component that allows a user to enter data and
instructions into a computer. Six commonly used input devices are the keyboard,
mouse, microphone, scanner, digital camera, and PC camera.
A computer keyboard contains keys that allow
you to type letters of the alphabet, numbers, spaces, punctuation marks, and
other symbols. A computer keyboard also contains other keys that allow you to
enter data and instructions into the computer.
A
mouse is a small handheld device that contains at least one button. The mouse
controls the movement of a symbol on the screen called a pointer. For example,
as you move the mouse across a flat surface, the pointer on the screen also moves.
With the mouse, you can make choices, initiate a process, and select objects.
Output
Devices
An output device is any hardware
component that can convey information to a user.
Three
commonly used output devices are a printer, a monitor, and speakers.
A
printer produces text and graphics on a physical medium such as paper or
transparency film. A monitor, which looks like a television screen, displays
text, graphics, and video information. Speakers allow you to hear music, voice,
and other sounds generated by the computer.
System
Unit
The
system unit, sometimes called a chassis, is a box-like case made from metal or
plastic that protects the internal electronic components of the computer from
damage
The
circuitry in the system unit usually is part of or is connected to a circuit
board called the motherboard. Two main components on the motherboard are the
central processing unit and memory. The central processing unit (CPU), also
called a processor, is the electronic device that interprets and carries out
the basic instructions that operate the computer.
During
processing, the processor places instructions to be executed and data needed by
those instructions into memory. Memory is a temporary holding place for data
and instructions. Both the processor and memory consist of chips. A chip is an
electronic device that contains many microscopic pathways that carry electrical
current. Chips, which usually are no bigger than one-half inch square, are
packaged so they can be attached to a motherboard or other circuit board.
Some
computer components, such as the processor, memory, and most storage devices,
are internal and reside inside the system unit. Other components, such as the
key board, mouse, microphone, monitor, printer, scanner, digital camera, and PC
camera, usually are located outside the system unit. These devices are
considered external. A peripheral is any external device that attaches to the
system unit.
Storage
Devices
Storage
holds data, instructions, and information for future use. Storage differs from
memory, in that it can hold these items permanently. Memory, by contrast, holds
items only temporarily while the processor interprets and executes
instructions.
A
storage medium (media is the plural) is the physical material on which a
computer keeps data, instructions, and information. A storage device records
and retrieves items to and from a storage medium. Storage devices often
function as a source of input because they transfer items from storage into
memory.
Common
storage devices are a floppy disk drive, a Zip® drive, a hard disk drive, a
CD-ROM drive, a CD-RW drive, a DVD-ROM drive, and a DVD+RW drive. A drive is a
device that reads from and may write on a storage medium. This media includes
floppy disks, Zip® disks, hard disks, and compact discs. A floppy disk consists
of a thin, circular, flexible disk enclosed in rigid plastic. A floppy disk
stores data, instructions, and information using magnetic patterns. You insert
and remove a floppy disk into and from a floppy disk drive A Zip® disk is a
higher capacity disk that can store the equivalent of up to 170 standard floppy
disks. A hard disk provides much greater storage capacity than a floppy disk. A
hard disk usually consists of several circular platters that store items
electronically. These disks are enclosed in an airtight, sealed case, which
often is housed inside the system unit. Some hard disks are removable, which
enables you to insert and remove the hard disk from a hard disk drive, much
like a floppy disk . Removable disks are enclosed in plastic or metal
cartridges so you can remove them from the drive. The advantage of removable
media such as a floppy disk and removable hard disk is you can take the media
out of the computer and transport or secure it.
Communications
Devices
Communications
devices enable computer users to communicate and to exchange items such as
data, instructions, and information with another computer. A modem is a
communications device that enables computers to communicate usually via
telephone lines or cable. Modems are available as both external and internal
devices.
Communications
devices, such as modems, allow you to establish a connection between two
computers and transmit items over transmission media, such as cables, telephone
lines, or satellites.
Why Is A
Computer So Powerful?
A computer derives its power from its
capability of performing the information processing cycle operations (input,
process, output, and storage) with amazing speed, reliability, and accuracy;
storing huge amounts of data and information; and communicating with other
computers.
Speed
In the
system unit, operations occur through electronic circuits. When data,
instructions, and information flow along these circuits, they travel at close
to the speed of light. This allows billions of operations to be carried out in
a single second.
Reliability
The
electronic components in modern computers are dependable because they have a
low failure rate. The high reliability of the components enables the computer
to pro duce consistent results.
Accuracy
Computers
can process large amounts of data and generate error- free results, provided
the data is entered correctly and the program works properly. If data is
inaccurate, the resulting output will be incorrect. A computing phrase — known
as garbage in, garbage out (GIGO) points out that the accuracy of a computer’s
output depends on the accuracy of the input.
Storage
Many
computers can store enormous amounts of data and make this data available for
processing anytime it is needed. Using current storage devices, the computer can
transfer data quickly from storage to memory, process it, and then store it
again for future use.
Communications
Most
computers today have the capability of communicating with other computers.
Computers with this capability can share any of the four information processing
cycle operations — input, process, output, and storage — with another computer.
For example, two computers connected by a communications device such as a modem
can share stored data, instructions, and information.
When two
or more computers are connected together via communications media and devices,
they form a network. The most widely known net work is the Internet.
New
Trends in IT
Computer
became the no 1 reason for the concept of Globalization. With the computer
networks and the evolution of the technologies new aspects came to like
Internet and Multimedia. We know because of the Internet entire world has
become a Global Village. Well be discussing about the Internet and its
evolution in detail during this course. Now we’ll focus a little about
Multimedia.
Multimedia
introduces exciting new communication possibilities to computing in general and
mixed with the power of Internet provides imaginative new opportunities that
were unthinkable even a few years ago. Multimedia is commonly used for
information, education and entertainment. The industrial giants in
Telecommunications and Entertainment industries are committing huge investments
in the Multimedia Technology and its applications. ‘Multimedia’ became an
over-used word in the 1990s, and has evolved into many new forms.
Several
such applications will be briefed below.
Networked multimedia
Internet and Intranets
Virtual Reality
Networked
Multimedia
Networked Multimedia combines both
computing and telecommunications together with Multimedia features. It
highlights the following elements:
Features of Networked Multimedia
Multiple users
Multiple processes
Independent control
Conversational environment
Support of all media types
Fully integrated environments
Transparent interface
Near-instantaneous response
Real time storage and retrieval
Uses of
Networked Multimedia
·
Sending messages
·
e-Learning — on-line access to tutors
·
e-Business — viewing items on sale over
a network and the communication of the transaction
·
Processing — manipulating information
from one form e.g. moving video into another e.g. still pictures for storage
and transmission
·
Meetings — enabling participants at
different locations not only to see and hear each other but also view documents
in test, graphics and video formats.
·
Project Management — using
videoconferencing facilities it is possible for project teams to discuss issues,
share information and reach decisions.
·
Consulting — in business and in
medicine clients can be interviewed remotely by experts saving the cost and
time and inconvenience of travel.
·
Monitoring — video images for security
and other purposes can be manipulated at a Central position.
Internet
and Intranets-Internet
The principle uses of the Internet are:
·
Remote log in (through telnet)
·
File Transfer (through ftp)
·
Electronic mail (or e-mail)
·
Network news (newsgroups or Bulletin
Boards — BBS)
·
Real-time communications (Internet Relay
Chat)
·
Information systems (principally the
World Wide Web — is a common interface for all above services)
Most
modern web sites use Multimedia to enhance services related to the above
Principles
Intranet
Intranet is an internal, corporate
network using the same basic architecture, protocols and applications as the
Internet.
As web
pages are non-platform specific, corporations can use them to disseminate
information throughout the world without worrying about which platform their
employees will use to visit the site. Specifically, intranets use the
applications that have developed around the WWW and offer seamless integration
between corporate networks and the Internet itself.
Virtual
Reality
Virtual Reality (VR) is an extension
of multimedia — it uses the basic multimedia elements of imagery, sound and
animation. YR can be regarded as the ultimate form of interactive multimedia.
·
The technology of VR provides a far more
dynamic interface and 3D environments which attempts to place the user ‘inside’
a life-like experience
·
VR requires considerable processing
power to provide realistic images, since computer generated worlds, objects and
their relationships are defined mathematically.
·
In VR, the ‘cyberspace’ is made up of
many thousands of geometric objects plotted in 3Dimensional space. These
objects are subject to constant recalculation and redisplay according to the
changes in view brought about by the actions of the user. Thousands of
computations must occur as fast as 30 times per second to provide smooth
movements
·
Levels of realism can be enhanced by
applying techniques of lighting, shading and texturing.
·
Using high-speed, dedicated computers,
multi-million-dollar flight simulators have led the way in commercial
application of YR. Pilots of F-16s, Boeing 777s, and space shuttles have made
many dry runs before doing the real thing.
History
of Computer Technology
Digital computer grounded in
ideas from 1700’s & 1800’s
Computer technology became
available in the 1940’s and 1950’s
Classification
of Computers
Classification of Computers we can
basically divide in to 3 sections. Based on generation, size and data
representation.
Generations:
1 to 5
Generation
& Time Period
First
Generation 1946 — 1956
Use of vacuum tubes, large,
limited memory, jobs execution coordinated manually, and speed up to 10,000
Second
Generation 1957— 1963
Use of transistors, more reliable,
less heat generation, less power requirement, speed 200,000 - 300,000
Third
Generation 1964 — 1979
·
Use of Integrated Circuits (ICS) by
printing hundreds and thousands of tiny transistors on to small silicon chips
·
Speed up to 5 Million Instructions Per
Second (MIPS)
·
Use of operating systems that automated
the running of programs & communications between CPU & peripheral
devices.
·
Availability of hardware independent
programming.
Fourth
Generation 1980s
Use of LS1 & VLSI circuits,
costs fallen to a very low level, large memory & storage, speed up to 200
MIPS
Fifth
Generation
5
generation computers will be Artificial Intelligence Systems called Al
computers, which simulate the human brain. Expert systems also fall into this
category.
Size: Micro, Mini, Mainframe, Super
Micro Computers
A microcomputer is a complete computer on a smaller scale and is
generally a synonym for the more common term, personal computer or PC, a computer designed for an individual. A microcomputer contains
a microprocessor (a central processing unit on a microchip), memory in the form
of read-only memory and random access memory, I/O ports and a bus or system of
interconnecting wires, housed in a unit that is usually called a motherboard.
In an ascending
hierarchy of general computer sizes, we find:
·
An embedded systems
programming computer, which is embedded in something and doesn't support direct
human interaction but nevertheless meets all the other criteria of a
microcomputer
·
Microcomputer, workstation,
as used to mean a more powerful personal computer for special applications
·
minicomputer, now restyled a
"mid-range server"
·
mainframe or mainframe
computer, which is now usually referred to by its manufacturers as a
"large server"
·
Supercomputer, formerly
almost a synonym for "Cray supercomputer" but now meaning a very
large server and sometimes including a system of computers using parallel
processing
·
A parallel processing system
is a system of interconnected computers that work on the same application
together, sharing tasks that can be performed concurrently
Mini Computers
A minicomputer, a term no longer much used, is a computer of a
size intermediate between a microcomputer and a mainframe. Typically,
minicomputers have been stand-alone computers (computer systems with attached
terminals and other devices) sold to small and mid-size businesses for general
business applications and to large enterprises for department-level operations.
In recent years, the minicomputer has evolved into the "mid range
server" and is part of a network. IBM's
AS/400e is a good
example.
Mainframes
Mainframe
is an industry term for a large computer, typically manufactured by a large
company such as IBM for the commercial applications of Fortune 1000 businesses
and other large-scale computing purposes. Historically, a mainframe is
associated with centralized rather than distributed computing. Today, IBM
refers to its larger processors as large servers and emphasizes that they can
be used to serve distributed users and smaller servers in a computing network.
Super
Computers
A
supercomputer is a computer that performs at or near the currently highest
operational rate for computers. A supercomputer is typically used for
scientific and engineering applications that must handle very large databases
or do a great amount of computation (or both). At any given time, there are
usually a few well-publicized supercomputers that operate at the very latest
and always incredible speeds. The term is also sometimes applied to far slower
(but still impressively fast) computers. Most supercomputers are really
multiple computers that perform parallel processing. In general, there are two
parallel processing approaches: symmetric multiprocessing (SMP) and massively
parallel processing (MPP).
Perhaps
the best-known builder of supercomputers has been Cray Research, now a part of
Silicon Graphics. Some supercomputers are at "supercomputer center,"
usually university research centers, some of which, in the United States, are
interconnected on an Internet backbone known as vBNS or NSFNet. This network is
the foundation for an evolving network infrastructure known as the National
Technology Grid. Internet2 is a university-led project that is part of this
initiative.
At the
high end of supercomputing are computers like IBM's "Blue Pacific,"
announced on October 29, 1998. Built in partnership with Lawrence Livermore
National Laboratory in California. Blue Pacific is reported to operated at 3.9
teraflop (trillion operations per second), 15,000 times faster than the average
personal computer. It consists of 5,800 processors containing a total of 2.6
trillion bytes of memory and interconnected with five miles of cable. It was
built to simulate the physics of a nuclear explosion. IBM is also building an
academic supercomputer for the San Diego Supercomputer Center that will operate
at 1 teraflop. It's based on IBM's RISC System/6000 and the AIX operating
system and will have 1,000 microprocessors with IBM's own POWER3 chip.
At the
lower end of supercomputing, a new trend, called clustering, suggests more of a
build-it-yourself approach to supercomputing. The Beowulf Project offers guidance
on how to "strap together" a number of off-the-shelf personal
computer processors, using Linux operating systems, and interconnecting the
processors with Fast Ethernet. Applications must be written to manage the
parallel processing.
Data
Representation: Analog, Digital, Hybrid.
Analog
Computer
An
analog computer operates in a completely opposite way to the digital computer.
For a start, all operations in an analog computer are performed in parallel.
Secondly, data are represented in an analog computer as voltages, a very
compact but not necessarily robust form of storage (prone to noise corruption).
A single capacitor (equivalent to the Digital’s computer use of a transistor)
in an analog computer can represent one continuous variable.
Educational Analog Computer,
introduced in 1960 by Heathkit.
The
Heath kit Educational Analog Computer is completely self-contained and contains
nine DC operational amplifiers with provision for balancing without removing
problem setup. It also features three initial condition power supplies, five
coefficient potentiometers, four sets of relay contacts, an electronically
regulated power supply and a built-in repetitive oscillator for automatic
operation. The complete EC-1 kit also contains an assortment of precision resistors,
capacitors, special silicon diodes and patch cords for setting up scores of
complex computer problems easily and accurately.
Hybrid
Computer
Purpose:
Special, General
Depending on the purpose we can
categorize the computers mainly on to two.
Special
Purpose Computers
Special
Purpose Computers are used for special purposes like scientific researches,
atomic weapon researches, designing sophisticated vehicles, weather forecasting
etc. Normally super computers comes under special purpose computers as we
discussed earlier.
General
Purpose Computers
General
Purpose Computers are the normal computers we are using.
Basic
Functions of a Computer
A
computer is collection of electronic parts (hardware) that gathers, processes
and relays information. Some of the parts can be separated in to two
categories: input and output devices.
Input
Devices
Input Devices, such as keyboard,
mouse, light pen, etc., collect information provided by the user and convert it
in to electronic signals for further processing.
Output
Devices
Output
Devices, such as the monitor, a speaker, or a printer, convert electronic
information in to some kind of format that the user understands. Outputs take
the form of images on the screen or paper, or sounds. Inputs are converted in
to electronic signals which are sent to the CPU or Central Processing Unit
(CPU) for processing. The CPU processes all communication along the various
components of the PC. Once the task is completed, them CPU relays information
to the user by means of an output device and waits for more input. This cycle
is repeated continuously.
Booting
the Computer
The PC
boot process starts when a small program within the BIOS chip looks for an
active partition and loads the operating system. There are two methods for
booting a computer: cold and warm booting.
Cold
Boot
Cold
booting occurs when the computer’s power switch is turned on. The effect of a
cold boot is that it brings the system to the very beginning of the boot
process, beginning with the POST. This may become necessary when the computer
stops responding to any commands, including warm boots.
Warm
Boot
A warm
boot bypasses the first two system tests and goes directly to the bootstrap
loader (the point where DOS loads). A warm boot is triggered by pressing the
Ctrl, Alt, and Del keys at the same time.
CTRL + ALT + DEL
Rebooting the computer should be
used only as a last resort. Any information in open application files could be
lost when rebooting takes place.
Basic
Concepts of Hardware, Software, and Live ware
System
Software
Computer
Software
Software,
also called a computer program or simply a program, is a series of instructions
that tells the hardware of a computer what to do. Some instructions allow you
to input data from the keyboard and direct the computer to store the data in memory.
Other instructions cause data in memory to be used in calculations such as
adding a series of numbers to obtain a total. Some instructions compare two
values in memory and direct the computer to perform alternative operations
based on the results of the comparison. Other instructions direct the computer
to print a report, display information on the monitor, draw a color picture on
the monitor, or store information on a disk.
A
computer carries out, or executes, the instructions in a pro gram by first
placing, or loading, the instructions into the memory of the computer. Usually,
the computer loads the instructions from storage into memory. For example, each
time a program executes, it might load from the hard disk into memory.
When you
purchase a program, such as one shown in Figure 1-11, you typically receive
media such as a - CD-ROM(s) or a DYD-ROM that contains the software. Some
programs can load into memory directly from the media. With other programs, you
must install a part or all of the soft ware on the computer’s hard disk before
you can use the program. Some programs also require you to insert the media,
such as a CD-ROM, into the drive while you use, or run, the program. Others do
not. Figure 1- 12 shows the steps a user may follow to run a computer program
that allows you to create a greeting card. This program requires a CD-ROM in
the CD-ROM drive.
When you
buy a computer, it usually has some software pre installed on its hard disk.
This enables you to use the computer as soon as you set it up.
Software
is the key to productive use of computers. With the proper software, a computer
can become a valuable tool. The two categories of software are system Software
and application software. The following pages describe these categories of
software.
System
Software
System
software consists of the programs that control the operations of the computer
and its devices. sys tem software serves as the interface between the user, the
application software, and the computer’s hard ware. Two types of system
software are the operating system and utility programs.
Operating
System
An
operating system (OS) is a set of programs containing instructions that
coordinate all the activities among computer hardware devices. The operating
system also contains instructions that allow you to run application software.
Many of today’s computers use Microsoft’s most recent operating system, called
Windows XP.
When you
start a computer, the operating system loads into memory from the computer’s
hard disk. It remains in memory while the computer is running and allows you to
communicate with the computer and other software.
Utility
Programs
A utility program is a type of
system soft ware that performs a specific task, usually related to managing a
computer, its devices, or its programs. An example of a utility program is an
uninstaller, which removes a program that has been installed on a computer.
Most operating systems include several utility programs for managing disk
drives, printers, and other devices. You also can buy stand-alone utility
programs, which allow you to perform additional computer management functions.
Language
Translators
The
programs written by using a computer programming language should be converted
to the machine code in order to run the program correctly and to get the wanted
output. Computer programming languages are in human understandable for mat and
machine code cannot be understood by human.
Converting Human readable format computer
programs in to machine readable format is done by Language Translators.
Translators
we can basically divide in to two parts called Compilers and Interpreters. The
main difference between these two is the way that they convert the Computer
programming language to machine language.
Compiler
A
compiler is a special program that processes statements written in a particular
programming language and turns them into machine language or "code"
that a computer's processor uses. Typically, a programmer writes language
statements in a language such as Pascal or C one line at a time using an editor.
The file that is created contains what are called the source statements.
The programmer then runs the appropriate language compiler, specifying the name
of the file that contains the source statements.
When
executing (running), the compiler first parses (or analyzes) all of the
language statements syntactically one after the other and then, in one or more
successive stages or "passes", builds the output code, making sure
that statements that refer to other statements are referred to correctly in the
final code. Traditionally, the output of the compilation has been called object
code or sometimes an object module. (Note that the term
"object" here is not related to object-oriented programming.) The
object code is machine code that the processor can process or
"execute" one instruction at a time.
More
recently, the Java programming language, a language used in object-oriented
programming, has introduced the possibility of compiling output (called byte
code) that can run on any computer system platform for which a Java virtual
machine or byte code interpreter is provided to convert the bytecode into
instructions that can be executed by the actual hardware processor. Using this
virtual machine, the bytecode can optionally be recompiled at the execution
platform by a just-in-time compiler.
Traditionally
in some operating systems, an additional step was required after compilation -
that of resolving the relative location of instructions and data when more than
one object module was to be run at the same time and they cross-referred to
each other's instruction sequences or data. This process was sometimes called linkage
editing and the output known as a load module.
A compiler works with what are
sometimes called 3GL and higher-level languages.
An
assembler works on programs written using a processor's assembler language.
Interpreter
An
interpreter is a computer program that executes other programs. This is in
contrast to a compiler which does not execute its input program (the source
code) but translates it into executable machine code (also called object code)
which is output to a file for later execution. It may be possible to execute
the same source code either directly by an interpreter or by compiling it and
then executing the machine code produced.
It takes
longer to run a program under an interpreter than to run the compiled code but
it can take less time to interpret it than the total required to compile and
run it. This is especially important when prototyping and testing code when an
editinterpret- debug cycle can often be much shorter than an
edit-compile-run-debug cycle.
Interpreting
code is slower than running the compiled code because the interpreter must
analyze each statement in the program each time it is executed and then perform
the desired action whereas the compiled code just performs the action. This
run-time analysis is known as "interpretive overhead". Access to
variables is also slower in an interpreter because the mapping of identifiers
to storage locations must be done repeatedly at run-time rather than at compile
time.
DBMS
Software
A
database is a collection of data organized in a manner that allows access,
retrieval, and use of that data. In a manual database, you might record data on
paper and store it in a filing cabinet. With a computerized database, the
computer stores the data in an electronic format on a storage medium such as a
floppy disk or hard disk. Database software, also called a database management
system (DBMS), is software that allows you to create, access, and manage a
database. Using database software, you can add, change, and delete data in the
database; sort and retrieve data from the database; and create forms and
reports using the data in the database.
Application
Software
Application
software consists of programs that perform specific tasks for users. Popular
application software includes word processing software, spreadsheet software,
data base software, and presentation graphics software. Word processing
software allows you to create documents such as letters, memorandums, and
brochures. Spreadsheet software allows you to calculate numbers arranged in
rows and columns. Users perform financial tasks such as budgeting and
forecasting with spreadsheet software. Database soft ware allows you to store
data in an organized fashion, as well as retrieve, manipulate, and display that
data in a variety of formats. With presentation graphics software, you create
documents called slides that add visual appeal to presentations. Software
vendors often bundle and sell these four applications together as a single
unit. This bundle, called a suite, costs much less than if you purchased the
applications individually. Microsoft’s Office XP is a very popular suite.
Many
other types of application software exist that enable users to perform a
variety of tasks. Some widely used applications include the following:
reference, education, and entertainment; desktop publishing; photo and video
editing; multimedia authoring; network, communications, electronic mail (e-mail),
and Web browsers; accounting; project management; and personal information
management. Chapter 2 discusses Web browsers and e-mail, and Chapter 3
discusses the other applications.
Application software is available
in a variety of forms: packaged, custom, freeware, public domain, shareware,
and from application service providers.
Word Processing
Microsoft Word
Corel WordPerfect
Lotus Word Pro
Microsoft Pocket Word
Spread Sheet
Microsoft Excel
Corel Quattro Pro
Lotus 1
Microsoft
Pocket Excel
Database
Microsoft Access
Corel Paradox
Lotus Approach
Microsoft Visual FoxPro
Oracle
Presentation Graphics
Microsoft PowerPoint
Corel Presentations
Lotus Freelance Graphics
Personal Information Manager
Microsoft Outlook
CorelCENTRAL
Lotus Organizer
Palm MultiMail
Software Suite
Microsoft Office
Corel WordPerfect Office
Lotus SmartSuite
Project Management
Microsoft Project
Primavera SureTrak Project
Manager
Accounting
Intuit QuickBooks
Peachtree
Col11plete Accounting
Application
Packages
Copyrighted
application or system software that meets the needs of a wide variety of users,
not just a single user or company, is called packaged software. You can
purchase packaged software from stores that sell computer products. You also
can purchase packaged software from companies on the Internet.
Custom-Made
Software
Sometimes a user
or company with unique soft ware requirements cannot find packaged software
that meets all its needs. In this case, the person or company can opt for
custom software. Custom software, written by a programmer, is a tailor-made
application or system program developed at a user’s request to perform specific
functions.
Freeware
in Public-Domain
Software
and Shareware
Freeware
is application or system software provided at no cost to a user by an
individual or a company. Freeware is copyrighted. You cannot resell it as your
own. Public- domain software also is free software, but it has been donated for
public use and has no copyright restrictions.
Shareware
is copyrighted software that is distributed free for a trial period. If you
want to use a shareware program beyond that period, you send a payment to the
person or company that developed the program. Companies that develop shareware
rely on the honor system. The company trusts you to send payment if you
continue to use the software beyond the stated trial period. Upon sending this
small fee, the developer registers you to receive service assistance and
updates.
Examples
of shareware, freeware, and public-domain software include utility programs,
graphics programs, and games. Thousands of these pro grams are available on the
Internet to download, or copy to your computer. You also can obtain copies of
these programs from the developer, a coworker, or a friend.
Application
Service Provider
Storing and maintaining programs can be a
costly investment for individuals and businesses. Some opt to use an
application service provider for their software needs. An application service
provider (ASP) is a third-party company that manages and distributes software
and services on the Internet. That is, instead of installing the software on
your computer, you run the programs from the Internet. Some vendors pro vide
access to the software at no cost. Others charge for use of the program.
Programming
Languages
Hundreds
of programming languages exist. Only a few, however, are used widely enough
today for the industry to recognize them as standards. Most of these are
high-level languages that work on a variety of computers. This section
discusses these programming languages, their origins, and their primary
purpose. Although the Java programming language is used in many business
applications today, it originally was used primarily for Web development. Thus,
Java is discussed in the Web page development section of this chapter.
To
illustrate the similarities and differences among programming languages,
figures on the following pages show program code in several programming
languages. The code solves a simple payroll problem — computing the gross pay
for an employee. The steps to compute gross pay can vary from one system to
another. The examples on the following pages use a simple algorithm to help you
easily compare one programming language with another.
To
compute the gross pay, first multiply the regular time hours worked by the
hourly rate of pay to obtain the regular time pay. If the employee has overtime
hours, the employee’s overtime pay is 1.5 times the hourly rate of pay
multiplied by overtime hours. Then, add the regular time pay and overtime pay
together.
BASIC
John
Kemeny and Thomas Kurtz developed a programming language called Beginner’s
All-purpose Symbolic Instruction Code, or BASIC, in the mid-1960s at Dartmouth
College. Kemeny and Kurtz designed BASIC for use as a simple, interactive
problem-solving language. BASIC originally was intended as the language used in
a student’s first programming course because it is so easy to learn and use.
Today, BASIC is used on both personal computers and mid-range servers to
develop some business applications. Many versions of BASIC exist, including
QBasic, QuickBASIC, and MS-BASIC.
Visual
Basic
Developed
by Microsoft Corporation in the early 1990s, Visual Basic is a Windows based
application that assists programmers in developing other event-driven
Windows-based applications. The first step in building a Visual Basic
application is to design the graphical user interface using Visual Basic
objects. Visual Basic objects, or controls, include items such as command
buttons, text boxes, and labels. Next, you write any code needed to define
program events. An event in Visual Basic might be the result of an action
initiated by a user. When a user clicks an object in a Visual Basic
application, the application executes the Click event. You define Visual Basic
events using code statements written in Visual Basic’s built-in programming
language.
COBOL
COBOL
(Common Business-Oriented Language) developed out of a joint effort between the
United States government, businesses, and major universities in the early
1960s. Naval officer Grace Hopper, a pioneer in computer programming, was a
prime developer of the COBOL language.
COBOL is
a procedural programming language designed for business applications. Although
COBOL pro grams often are lengthy, their English- like statements make the code
easy to read, write, and maintain (Figure 15-26). COBOL is especially useful
for processing transactions on main frames. COBOL programs also run on other
types of computers. The most popular personal computer COBOL program is Micro
Focus Net Express®, which allows you to create procedural and object-oriented
COBOL programs and migrate them to the Web.
C
The C
programming language, developed in the early 1970s by Dennis Ritchie at Bell
Laboratories, originally was designed for writing system software. Today, a
variety of software programs are written in C. This includes operating systems
and application software such as word processing and spreadsheet programs.
C is a
powerful language that requires professional programming skills. Many
programmers use C for business and scientific problems. C runs on almost any
type of computer with any operating system, but it most often is used with the
UNIX operating system. In fact, most of the UNIX operating system is written in
C.
C++
Developed in the 1980s by Bjame
Sroustrup at Bell Laboratories, C++ (pronounced SEE-plus-plus) is an
object-oriented programming language. C++ is an extension of the C programming
language. It includes all the elements of the C language plus has additional
features for working with objects, classes, events, and other object-oriented
concepts. Programmers commonly use C++ to develop application software, such as
word processing and spreadsheet programs, as well as database and Web
applications. Although C++ is an outgrowth of the C programming language, you
do not need C programming experience to be a successful C++ programmer. Some
programmers use a newer programming language called C# (pronounced SEE-sharp).
C# com bines features of C and C++ and is best suited for development of Web
applications.
JAVA
Developed
by Sun Microsystems, Java is a compiled object-oriented programming language
used to write stand-alone applications, as well as applets and servlets. Java
applet examples might include input forms, rotating images, fireworks,
interactive animations, or a game. Figure 15-37 shows a sample Java program and
its resulting screen.
The Java
language is very similar to C++. One difference is that Java source code is
compiled into bytecode, instead of object code. The operating system cannot
execute bytecode. A Java interpreter executes the bytecode. Java-enabled Web
browsers contain Java bytecode interpreters.
Code
segments used to create a Java application are called JavaBeans, or Beans. A
JavaBeans is platform independent. This enables the code to run on any computer
or operating system. Many programmers believe that Java will be the programming
language of the future because of its simplicity, robustness, and portability.
Input
Devices
Keyboard
devices
Offline
Data Preparation Methods
Table 1
below illustrates the Data Preparation Devices with corresponding input devices
and media. These methods need special data preparation devices. The term “Off
linen is used because this activity is done outside the computer and before
input.
In Sri
Lanka the punch card system was popular with the computers used from 1969 to
the late 1970s. Later, it was replaced by Key-to-Diskette systems, which are
still in use in Sri Lanka at a few places having high volume batch processing
and less time critical applications such as: Data Processing of Public
Examinations, Provident Fund applications, Census applications, Archival of
high volume business documents etc.
Key
Board Devices - On-line Terminals
The more common types of terminals
are:
Visual
Display Unit (VDU) or Video Terminal
VDU has ‘a television’ type screen
and a keyboard. Here, input is by a keyboard and output through display on
Cathode Ray Tube (CRT). This is the most common method of input.
General Features of a Video
Terminal
I.
It is an input/output (dual purpose)
device.
II.
The keyboard generally resembles a
typewriter (QWERTY) keyboard, butthere is a wide range of variations.
III.
Display clarity (Resolution) depends on
the pixel density. Pixel is the smallest display element to represent single
color. Each character is formed by combination of pixels.
IV.
Standard screen display includes 24 rows
by 80 column character matrixtotaling 1920 characters.
V.
Screen movement is generally; Line by line -
Scrolling or page by page -Paging.
VI.
Most of Video Terminals are ‘dumb’
terminals. New models appear now in the market includes microprocessor with
various levels of facilities.
Terminals
A
terminal is a form of input (and output) device that connects you to a
mainframe or other type of computer called a host computer or server. There are
four types of terminals:
A dumb
terminal can be used to input and receive data, but it cannot process data
independently. It is used only to gain access to information from a computer.
Such a terminal may be used by an airline reservations clerk to access a
mainframe computer for flight information
An
intelligent terminal includes a processing unit, memory, and secondary storage
such as a magnetic disk. Essentially, an intelligent terminal is a micro
computer with communications software and a telephone hookup (modem) or other
communications link. These connect the terminal to the larger computer or to
the Internet. Microcomputers operating as intelligent terminals are widely used
in organizations.
A
network terminal is also known as a thin client or network computer, is a
low-cost alternative to an intelligent terminal. Most network terminals do not
have a hard-disk drive and must rely on the host computer or server for
application and system software. These devices are becoming increasingly
popular in many organizations. An Internet terminal, also known as a Web
terminal, provides access to the Internet and displays Web pages on a standard
television set, these special purpose terminals have just recently been
introduced to offer Internet access to people without microcomputers. Unlike
the other types of terminals, Internet terminals are used almost exclusively in
the home.
A
Point-of-Sale (POS) Terminal
A Point-of-Sale
(POS) Terminal is a smart terminal used very much like a cash register, but it
also captures sales and inventory’s data at the point of sale and sends it to
the central computer for processing. Many supermarkets have POS terminals that
are connected directly to a central computer so that the sales data can be
immediately recorded. This type of terminal usually displays the price, the
product number, and possibly the product description. In addition, this type of
terminal is equipped with a cash-register-type keyboard, a cash drawer, and a
printer to print the receipt. It can operate on standalone basis and data
stored can be transferred to the main computer by using a computer medium such
as a diskette.
Banking
/ Financial Transaction Terminal
The most common terminals are:
I.
Automatic Teller Machine (ATM)
A special intelligent terminal located outside the
bank for a customer to operate on his own to perform a limited number of
banking transactions. e.g.: cash withdrawal, cash transfers, utility bill
payment etc.
II.
Teller Operator terminal
This is a specialized dumb/smart terminal for the
bank teller operators who work in the front office of a bank to handle customer
transactions.
III.
Normal Video Terminals
Portable
Terminal
It is
consists of a Key Board, flat screen monitor and built in communication
software/hardware which can connect from a remote location through a
communication link to a computer. The portability of the terminal is due to its
light weight. There are a wide range of products available. (e.g.: Terminal for
traveling salesman.)
Non
Keyboard Devices
Mouse
The
mouse, is one of the devices connected to the computer by a small cable. As the
mouse is rolled across the desktop, the cursor moves across the screen. When
the cursor reaches the desired location, the user usually pushes button on the
mouse once or twice to signal a menu selection or a command to the computer.
Mouse technology is often used with graphics-oriented personal computers. With
special software for graphics, the mouse can be used like a pen or a Paint
Brush to create figures and patterns directly on the video display screen.
e.g.: use with Corel Draw or Harward Graphics, Desk Top Publishing Applications
etc.
Joystick
It is a device which contains
lever which can be used to move objects on the screen.
e.g.: It is normally used to play
computer games.
Light
Pen
The
light pen is a special attachment to a graphics terminal. The pen is touched on
the video display screen at the desired location and switched on, then you can
draw the image on the screen. Light pens are frequently used by graphic
designers, illustrators, and drafting engineers. Of course, data may also be
entered using keyboard e.g. Computer Aided Design (CAD), Architectural
applications etc.
Touch
Screen
Limited
amounts of data can be entered via a terminal or a microcomputer that has a
touch screen. The user simply touches the screen at the desired locations,
marked by labeled boxes, to point out” choices to the computer. e.g.: Inquiry
Terminals for general public such as Terminals located in the lobby areas of a
large building complex. This will enable users to operate without much
technical knowledge & skill.
Scanning
Devices
Scanners
A
Digital Scanner translates images such as pictures or documents into digital
form. This is one area where substantial research & development has been
taking place in the recent past. A wide variety of products are now available
in the marketplace. The most common products are:
Hand
Held Scanner
Used
mostly with small scale publishing (Desk Top Publishing) systems to input
pictures, logos etc.
Flat Bed
Scanner
Used to
input text based documents with or without images. Special software packages
are now available to process these text based images such as editing, merging
etc.
Character
Scanners
These
are hand held devices which can read data printed in special type fonts (e.g.:
OCR) or hand written data based on recommended guidelines. In addition, there
are Optical Character Readers which operate at high speed to handle high volume
batch oriented input like Electricity billing meter readings.
Optical
Character Recognition (OCR)
Optical character recognition is a
device that is used to scan the text, which is found on a document, and then
the data is converted into electrical signals for the computer. This device can
be used to read a special type of font. However, the more expensive OCR devices
can read a variety of fonts; some can even read hand written documents.
Optical
Mark Recognition (OMR)
The
input device is Optical Mark Reader. In some of the foreign examinations like
SAT or TOEFL the candidate application forms as well as the answer sheets are
specially designed (Mark Sense Cards) to record data by using pencil marks.
Open University of Sri Lanka and University of Colombo BIT program also use a
similar method. Data recorded in this form is converted into computer-usable
form by an Optical Mark Reader (OMR). The OMR device has a high-intensity light
inside which is directed in the form of a beam at the sheet of paper being fed
through it. The beam scans the marked forms and detects the number and location
of the pencil marks. The data is then converted into electrical signals for the
computer. OMRs come in a variety of sizes and shapes that depend on the size of
the forms to be read and the required loading and processing capacity of the
reader.
Magnetic-Ink
Character Recognition (MICR)
This is
specially designed for the input of high volume cheques in the banking
industry.
The
advantages of the MICR system are that Human involvement is minimum, thus the
potential for errors is small, The codes can be read by both people and
machines, It is fast, automatic, and reliable (2400 checks/mm).
The
cheque leaf taken into the machine, and magnetize the ferric particles present
in the data printed at the bottom edge of it. At the first read station it
recognizes data based on magnetic induction & converts into electrical
signals. Generally at the second read station it reads again & compare with
the first reading for accuracy. In addition to reading, it sort the cheque
leaves in a bundle according to the bank code & output to respective pigeon
holes. e.g.; Cheque Clearing House of Sri Lanka.
Bar Code
Reader
Bar Code is a collection of thick
and thin lines and spaces that represent data in binary.
E.g.:
Parcel tracking system in
courier companies
File tracking system in an
office
Issue of items in a supermarket.
Record the movement of books in
a library
In
courier application the parcel reference number is bar coded & recorded in
all relevant documents & the parcel itself. This reference number is input
to the computer system by using a portable barcode scanner or wand reader. The
wand has a scanning device that analyzes light & dark bars for width &
spacing. This wand reader is a special attachment to a video terminal.
Card
Reading Devices
Time/Punch
Card
IBM made
the punch-card technology into the business standard of the 1950's and 1960's.
There are manual punch card readers available. But now a days this technology
is not using widely.
Voice and
Image Input Devices
Microphone
A
microphone wants to take varying pressure waves in the air and convert them
into varying electrical signals. There are five different technologies commonly
used to accomplish this conversion:
Carbon
microphones -
The oldest and simplest microphone uses carbon dust. This is the technology
used in the first telephones and is still used in some telephones today. The
carbon dust has a thin metal or plastic diaphragm on one side. As sound waves
hit the diaphragm, they compress the carbon dust, which changes its resistance.
By running a current through the carbon, the changing resistance changes the
amount of current that flows.
Dynamic
microphones -
A dynamic microphone takes advantage of electromagnet effects. When a magnet
moves past a wire (or coil of wire), the magnet induces current to flow in the
wire. In a dynamic microphone, the diaphragm moves either a magnet or a coil
when sound waves hit the diaphragm, and the movement creates a small current.
Ribbon microphones - In a ribbon microphone, a thin ribbon is suspended in a
magnetic field. Sound waves move the ribbon which changes the current flowing
through it.
Condenser
microphones -
A condenser microphone is essentially a capacitor, with one plate of the
capacitor moving in response to sound waves. The movement changes the
capacitance of the capacitor, and these changes are amplified to create a
measurable signal. Condenser microphones usually need a small battery to
provide a voltage across the capacitor.
Crystal
microphones -
Certain crystals change their electrical properties as they change. By
attaching a diaphragm to a crystal, the crystal will create a signal when sound
waves hit the diaphragm.
Web
Camera
A cam,
homecam, or Webcam is a video camera, usually attached directly to a computer,
whose current or latest image is requestable from a Web site. A live cam is one
that is continually providing new images that are transmitted in rapid
succession or, in some cases, in streaming video. Sites with live cams sometimes
embed them as Java applets in Web pages. Cams have caught on; there are now (we
estimate) several thousand sites with cams. The first cams were positioned
mainly on fish tanks and coffee machines. Many of today's live cams are on
sexoriented sites. For travel promotion, traffic information, and the remote
visualization of any ongoing event that's interesting, webcams seem like an
exciting possibility that will become more common as users get access to more
bandwidth.
Video
Camera
A typical analog camcorder
contains two basic parts:
A camera section, consisting of
a CCD, lens and motors to handle the zoom, focus and aperture
A VCR section, in which a
typical TV VCR is shrunk down to fit in a much smaller space.
The camera component's function is
to receive visual information and interpret it as an electronic video signal.
The VCR component is exactly like the VCR connected to your television: It
receives an electronic video signal and records it on video tape as magnetic
patterns
Virtual Reality
Virtual reality is the simulation
of a real or imagined environment that can be experienced visually in the three
dimensions of width, height, and depth and that may additionally provide an
interactive experience visually in full real-time motion with sound and
possibly with tactile and other forms of feedback. The simplest form of virtual
reality is a 3-D image that can be explored interactively at a personal
computer, usually by manipulating keys or the mouse so that the content of the
image moves in some direction or zooms in or out. Most of these images require
installing a plug-in for your browser. As the images become larger and
interactive controls more complex, the perception of "reality"
increases. More sophisticated efforts involve such approaches as wrap-around
display screens, actual rooms augmented with wearable computers, and joystick
devices that let you feel the display images.
Virtual reality can be
divided into:
The simulation of
real environments such as the interior of a building or a spaceship often with
the purpose of training or education
The development of
an imagined
environment, typically for a game or educational adventure
Gloves
Gloves
designed with flexible sensors which can accurately and reliably measure the
position and movement of the fingers and wrist. Additional features of the
glove would include the ability to simulate resistance at object borders during
grasp of a simulated object in the VR environment. The glove will also be used
to determine response times, posture, and movement dynamics of the fingers and
hand. Touch sensitive response boards will also be necessary so that a simple
touch of a finger anywhere on the board will record the contact to enable
movement or response times to be recorded complementing the basic functions of
the glove in space.
Headphone
Headgear
that can display high resolution static or dynamic images. This device (goggles
or head-mounted) would be used to display experimental stimuli with the option
of aural accompaniment. Headphone is a lightweight device which you can use to
hear quality audio.
E.g.:
The
HP-60 Super Bass Headphone Set is CD digital quality, 16hz - 22,000hz, 32 Ohm,
light weight, easy clean vinyl padded ear pieces, and works with all portable
audio devices.
Motion
Capture
Motion
capture is defined as "The creation of a 3D representation of a live
performance." in the book Understanding Motion Capture for Computer
Animation and Video Games by Alberto Menache. This is in contrast to animation
that is created 'by hand' through a process known as keyframing
Monitors
Mono(Monochrome) – Display a
single Color.
CGA - Color Graphic Adapter (4
Colors)
EGA – Enhance Graphic Adapter (16
Colors)
VGA – Video Gr. Ad. (16 colors
& 256 shading)
SVGA – Super VGA
Printers
Impact Printers
The Print mechanism strikes
against the paper.
Non
Impact Printers
The Print mechanism does not
strike against the ribbon or paper.
Character
Printers
Speed in Characters Per Second
(CPS)
Used for low to medium volume
applications
E.g.: Matrix, Ink Jet.
Line
Printers
Speed in Lines Per Minute (LPM)
Used for high volume medium
quality applications.
E.g.:
printing of public examination results.
Page
Printers
Speed in Pages per Minute (PPM)
Used for medium to high volume,
high quality applications.
E.g.: Print a master copy of a
magazine by using a Laser Printer.
Color
Printers
Color
printers use a Color Cartridge. Time to time you have to place the cartridge
when the cartridge is over.
Black
and White Printers
Black
and White Printers use a Black and White Cartridge. Time to time you have to
place the cartridge when the cartridge is over. There are printers which
supports both Color and Black and White cartridges.
Text
Printers
To print text there are specially
text printers. The quality is very low in these printers.
Graphic
Printers
These
are very high quality printers. These printers are expensive than normal
printers. You can get very high quality Images printouts by using a graphic
printer. Graphic designers and the people in studios use Graphic printers.
Dot
Matrix Printers
The most
widely used printer which prints one character at a time. The technology has
improved during the recent past to include very high quality & reasonably
fast printers.
Daisy
Wheel Printers
A kind of impact printer where the
characters are arranged on the ends of the spokes of a wheel (resembling the
petals on a daisy).
The
wheel (usually made of plastic) is rotated to select the character to print and
then an electrically operated hammer mechanism bends the selected spoke forward
slightly, sandwiching an ink ribbon between the character and the paper, as in
a typewriter.
One advantage of this arrangement
over that of a typewriter is that different wheels may be inserted to produce
different typefaces.
InkJet
Printers
Ink printers work in much the same
fashion as dot-matrix printers in that they form images or characters with
little dots. However, the dots are formed, not by hammer like pins, but by tiny
droplets of ink, and the text these printers produce is of letter quality.
These printers can almost match the speed of dot- matrix printers - up to about
270 eps-and they produce less noise.
Thermal
Printers
Thermal printers use heat to
produce an image on special paper. The print mechanism - rather like a dot - matrix
print head - is designed to heat the surface of chemically treated paper so
that a dot is produced based on the reaction of the chemical to the heat. No
ribbon or ink is involved. It can print multiple colors.
Laser
Printers
Laser printer technology is much
less mechanical than impact printers resulting in much higher speeds and
quieter operation. The process resembles the operation of a photocopy machine.
A laser beam is directed across the surface of a light-sensitive drum and fixed
as needed to record an image in the form of a pattern of tiny dots. The image
is then transferred to the paper. This printer prints a page at a time - in the
same fashion as a copying machine, using a special toner. When high-speed laser
printers (also called page printers) were introduced, they were very expensive.
However, recent laser printer technology has made desktop versions available at
very reasonable prices.
Plotters
A plotter is a specialized output
device designed to produce high-quality graphics in a variety of colors. Drum
plotters and flat bed plotters both use pens and electrostatic plotters do not.
Drum
Plotter
The paper is mounted on the
surface of a drum. The drum revolves and the plotter pens (which are similar to
felt-tip pens) are horizontally positioned over the target area. When the paper
has rotated to the correct point, the pens are dropped to the surface moved
left and right under program control across the paper as the drum revolves.
When the image is complete, the pens are raised from the surface.
Flat Bed
Plotter
These are designed so that the
paper is placed flat and one or more pens move horizontally and vertically
across the paper.
Electrostatic
Plotters
Use electrostatic charges to
create images out of very small dots on specially treated paper.
Electrostatic plotters are faster than pen plotters and can produce images of
very high resolution, e.g.: Used by Architects, Surveyors, and Engineers.
Voice
Output Devices
Voice
output should be a more useful medium. This technology has had to overcome many
hurdles. The most difficult has been that every individual perceives speech
differently; that is, the voice patterns, pitches, and reflections we can hear
and understand are different for all of us. It is not always easy to understand
an unfamiliar voice pattern. At this point, two different approaches to voice
output have evolved:
Speech
coding
This
relies on human speech as a reservoir of sounds to draw from in building the
words and phrases to be output. Sounds are codified and stored on disk to be
retrieved and translated back as sounds. Speech coding has been used in
applications such as automobiles, toys, and games.
Speech
synthesis
In this
method voice is produced electronically without the use of a human voice. The
largest application to date for the speech synthesis approach to voice output
converting text into “spoken” words has many potential uses, including
providing reading machines for the blind. And, of course, sound output does not
have to be in voice form, it can be music or special-effects sounds, such as
the sound accompaniment for computer animation, in cartoons etc.
COM
Devices
Serial
port or "com port". A connector on a computer to which you can attach
a serial line connected to peripherals which communicate using a serial
(bit-stream) protocol. The most common type of serial port is a 25-pin D-type
connector carrying EIA-232 signals. Smaller connectors (e.g. 9-pin D-type)
carrying a subset of EIA-232 are often used on personal computers. The serial
port is usually connected to an integrated circuit called a UART which handles
the conversion between serial and parallel data. In the days before bit-mapped
displays, and today on multi-user systems, the serial port was used to connect
one or more terminals (teletypewriters or VDUs), printers, modems and
other serial peripherals. Two computers connected together via their serial
ports, possibly via modems, can communicate using a protocol such as UUCP or CU
or SLIP
Processor
A
processor is the logic circuitry that responds to and processes the basic
instructions that drive a computer. The term processor has generally replaced
the term central processing unit (CPU). The processor in a personal computer or
embedded in small devices is often called a microprocessor.
Arithmetic
and Logic Unit
An
arithmetic-logic unit (ALU) is the part of a computer processor (CPU) that
carries out arithmetic and logic operations on the operands in computer
instruction words. In some processors, the ALU is divided into two units, an
arithmetic unit (AU) and a logic unit (LU). Some processors contain more than
one AU - for example, one for fixed-point operations and another for
floating-point operations. (In personal computers floating point operations are
sometimes done by a floating point unit on a separate chip called a numeric
coprocessor.)
Typically, the ALU has direct input
and output access to the processor controller, main memory (random access
memory or RAM in a personal computer), and input/output devices. Inputs and
outputs flow along an electronic path that is called a bus. The input consists
of an instruction word (sometimes called a machine instruction word) that
contains an operation code (sometimes called an "op code"), one or
more operands, and sometimes a format code. The operation code tells the ALU
what operation to perform and the operands are used in the operation. (For
example, two operands might be added together or compared logically.) The
format may be combined with the op code and tells, for example, whether this is
a fixedpoint or a floating-point instruction. The output consists of a result
that is placed in a storage register and settings that indicate whether the
operation was performed successfully. (If it isn't, some sort of status will be
stored in a permanent place that is sometimes called the machine status word.)
In general, the ALU includes storage
places for input operands, operands that are being added, the accumulated
result (stored in an accumulator), and shifted results. The flow of bits and the
operations performed on them in the subunits of the ALU is controlled by gated
circuits. The gates in these circuits are controlled by a sequence logic unit
that uses a particular algorithm or sequence for each operation code. In the
arithmetic unit, multiplication and division are done by a series of adding or
subtracting and shifting operations. There are several ways to represent
negative numbers. In the logic unit, one of 16 possible logic operations can be
performed - such as comparing two operands and identifying where bits don't
match. The design of the ALU is obviously a critical part of the processor and
new approaches to speeding up instruction handling are continually being
developed.
Control
Unit
Control
Unit controls every single hardware part attached to the computer. Its main
task is controlling the hardware devices which are attached to the machine
according to the signals of CPU.
Memory
Organization
Memory
is the electronic holding place for instructions and data that your computer's
microprocessor can reach quickly. When your computer is in normal operation,
its memory usually contains the main parts of the operating system and some or
all of the application programs and related data that are being used. Memory is
often used as a shorter synonym for random access memory (RAM). This kind of
memory is located on one or more microchips that are physically close to the
microprocessor in your computer. Most desktop and notebook computers sold today
include at least 16 megabytes of RAM, and are upgradeable to include more. The
more RAM you have, the less frequently the computer has to access instructions
and data from the more slowly accessed hard disk form of storage.
Memory
is sometimes distinguished from storage, or the physical medium that holds the
much larger amounts of data that won't fit into RAM and may not be immediately
needed there. Storage devices include hard disks, floppy disks, CD-ROM, and
tape backup systems. The terms auxiliary storage, auxiliary memory, and
secondary memory have also been used for this kind of data repository.
Additional
kinds of integrated and quickly accessible memory are read-only memory (ROM),
programmable ROM (PROM), and erasable programmable ROM (EPROM). These are used
to keep special programs and data, such as the basic input/output system, that
need to be in your computer all the time.
Primary
Storage
RAM
RAM
(random access memory) is the place in a computer where the operating system,
application programs, and data in current use are kept so that they can be
quickly reached by the computer's processor. RAM is much faster to read from
and write to than the other kinds of storage in a computer, the hard disk,
floppy disk, and CD-ROM. However, the data in RAM stays there only as long as
your computer is running. When you turn the computer off, RAM loses its data.
When you turn your computer on again, your operating system and other files are
once again loaded into RAM, usually from your hard disk.
RAM can be compared to a person's
short-term memory and the hard disk to the long-term memory. The short-term
memory focuses on work at hand, but can only keep so many facts in view at one
time. If short-term memory fills up, your brain sometimes is able to refresh it
from facts stored in long-term memory. A computer also works this way. If RAM
fills up, the processor needs to continually go to the hard disk to overlay old
data in RAM with new, slowing down the computer's operation. Unlike the hard
disk which can become completely full of data so that it won't accept any more,
RAM never runs out of memory. It keeps operating, but much more slowly than you
may want it to.
How Big
is RAM?
RAM is
small, both in physical size (it's stored in microchips) and in the amount of
data it can hold. It's much smaller than your hard disk. A typical computer may
come with 256 million bytes of RAM and a hard disk that can hold 40 billion
bytes. RAM comes in the form of "discrete" (meaning separate)
microchips and also in the form of modules that plug into holes in the computer's
motherboard. These holes connect through a bus or set of electrical paths to
the processor. The hard drive, on the other hand, stores data on a magnetized
surface that looks like a phonograph record. Most personal computers are
designed to allow you to add additional RAM modules up to a certain limit.
Having more RAM in your computer reduces the number of times that the computer
processor has to read data in from your hard disk, an operation that takes much
longer than reading data from RAM. (RAM access time is in nanoseconds; hard
disk access time is in milliseconds.)
Why
Random Access?
RAM is
called "random access" because any storage location can be accessed
directly. Originally, the term distinguished regular core memory from offline
memory, usually on magnetic tape in which an item of data could only be
accessed by starting from the beginning of the tape and finding an address
sequentially. Perhaps it should have been called "nonsequential
memory" because RAM access is hardly random.
RAM is organized and controlled in a way
that enables data to be stored and retrieved directly to specific locations. A
term IBM has preferred is direct access storage or memory. Note that other
forms of storage such as the hard disk and CDROM are also accessed directly (or
"randomly") but the term random access is not applied to these forms
of storage.
In
addition to disk, floppy disk, and CD-ROM storage, another important form of
storage is read-only memory (ROM), a more expensive kind of memory that retains
data even when the computer is turned off. Every computer comes with a small
amount of ROM that holds just enough programming so that the operating system
can be loaded into RAM each time the computer is turned on.
ROM
ROM is
"built-in" computer memory containing data that normally can only be
read, not written to. ROM contains the programming that allows your computer to
be "booted up" or regenerated each time you turn it on. Unlike a
computer's random access memory (RAM), the data in ROM is not lost when the
computer power is turned off. The ROM is sustained by a small long-life battery
in your computer. If you ever do the hardware setup procedure with your
computer, you effectively will be writing to ROM.
PROM
Programmable
read-only memory (PROM) is read-only memory (ROM) that can be modified once by
a user. PROM is a way of allowing a user to tailor a microcode program using a
special machine called a PROM programmer. This machine supplies an electrical
current to specific cells in the ROM that effectively blows a fuse in them. The
process is known as burning the PROM. Since this process leaves no margin for
error, most ROM chips designed to be modified by users use erasable
programmable read-only memory (EPROM) or electrically erasable programmable read-only
memory (EEPROM).
EPROM
EPROM (erasable programmable
read-only memory) is programmable read-only memory (programmable ROM) that can
be erased and re-used. Erasure is caused by shining an intense ultraviolet
light through a window that is designed into the memory chip. (Although
ordinary room lighting does not contain enough ultraviolet light to cause
erasure, bright sunlight can cause erasure. For this reason, the window is
usually covered with a label when not installed in the computer.)
A different approach to a
modifiable ROM is electrically erasable programmable read-only memory (EEPROM).
EEPROM
EEPROM
(electrically erasable programmable read-only memory) is user-modifiable
read-only memory (ROM) that can be erased and reprogrammed (written to)
repeatedly through the application of higher than normal electrical voltage.
Unlike EPROM chips, EEPROMs do not need to be removed from the computer to be
modified. However, an EEPROM chip has to be erased and reprogrammed in its
entirety, not selectively. It also has a limited life - that is, the number of
times it can be reprogrammed is limited to tens or hundreds of thousands of
times. In an EEPROM that is frequently reprogrammed while the computer is in
use, the life of the EEPROM can be an important design consideration. A special
form of EEPROM is flash memory, which uses normal PC voltages for erasure and
reprogramming.
Cache
A cache
(pronounced CASH) is a place to store something temporarily. The files you
automatically request by looking at a Web page are stored on your hard disk in
a cache subdirectory under the directory for your browser (for example,
Internet Explorer). When you return to a page you've recently looked at, the
browser can get it from the cache rather than the original server, saving you
time and the network the burden of some additional traffic. You can usually
vary the size of your cache, depending on your particular browser.
Computers
include caches at several levels of operation, including cache memory and a
disk cache. Caching can also be implemented for Internet content by
distributing it to multiple servers that are periodically refreshed. (The use
of the term in this context is closely related to the general concept of a
distributed information base.)
Altogether, we are aware of these
types of caches:
- International, national, regional, organizational and other "macro" caches to which highly popular information can be distributed and periodically updated and from which most users would obtain information.
- Local server caches (for example, corporate LAN servers or access provider servers that cache frequently accessed files). This is similar to the previous idea, except that the decision of what data to cache may be entirely local.
- Your Web browser's cache, which contains the most recent Web files that you have downloaded and which is physically located on your hard disk (and possibly some of the following caches at any moment in time)
- A disk cache (either a reserved area of RAM or a special hard disk cache) where a copy of the most recently accessed data and adjacent (most likely to be accessed) data is stored for fast access.
- RAM itself, which can be viewed as a cache for data that is initially loaded in from the hard disk (or other I/O storage systems).
- L2 cache memory, which is on a separate chip from the microprocessor but faster to access than regular RAM.
- L1 cache memory on the same chip as the microprocessor.
Secondary
Storage
Secondary storage is all
addressable data storage that is not currently in the computer's main storage
or memory. Synonyms are external storage and auxiliary storage.
Floppy
Disks
A
diskette is a random access, removable data storage medium that can be used
with personal computers. The term usually refers to the magnetic medium housed
in a rigid plastic cartridge measuring 3.5 inches square and about 2
millimeters thick. Also called a "3.5-inch diskette," it can store up
to 1.44 megabytes (MB) of data. Although many personal computers today come
with a 3.5-inch diskette drive preinstalled, some notebook computers and
centrally-administered desktop computers omit them.
Some older computers provide drives
for magnetic diskettes that are 5.25 inches square, about 1 millimeter thick,
and capable of holding 1.2 megabytes of data. These were sometimes called
"floppy disks" or "floppies" because their housings are
flexible. In recent years, 5.25-inch diskettes have been largely replaced by
3.5-inch diskettes, which are physically more rugged. Many people also call the
newer hardcased diskette a "floppy."
Magnetic diskettes are convenient
for storing individual files and small programs. However, the magneto-optical
(MO) disk is more popular for mass storage, backup, and archiving. An MO
diskette is only a little larger, physically, than a conventional 3.5-inch
magnetic diskette. But because of the sophisticated read/write technology, the
MO diskette can store many times more data.
Disk
Drives
Hard
Disks
A hard disk is part of a unit, often
called a "disk drive," "hard drive," or "hard disk
drive," that store and provides relatively quick access to large amounts
of data on an electromagnetically charged surface or set of surfaces. Today's
computers typically come with a hard disk that contains several billion bytes
(gigabytes) of storage.
A hard disk is really a set of
stacked "disks," each of which, like phonograph records, has data
recorded electromagnetically in concentric circles or "tracks" on the
disk. A "head" (something like a phonograph arm but in a relatively
fixed position) records (writes) or reads the information on the tracks. Two
heads, one on each side of a disk, read or write the data as the disk spins.
Each read or write operation requires that data be located, which is an
operation called a "seek." (Data already in a disk cache, however,
will be located more quickly.)
A hard
disk/drive unit comes with a set rotation speed varying from 4500 to 7200 rpm.
Disk access time is measured in milliseconds. Although the physical location
can be identified with cylinder, track, and sector locations, these are
actually mapped to a logical block address (LBA) that works with the larger
address range on today's hard disks.
Optical
Disks
An optical disc is an electronic
data storage medium that can be written to and read using a low-powered laser
beam. Originally developed in the late 1960s, the first optical disc, created
by James T. Russell, stored data as micron-wide dots of light and dark. A laser
read the dots, and the data was converted to an electrical signal, and finally
to audio or visual output. However, the technology didn't appear in the marketplace
until Philips and Sony came out with the compact disc (CD) in 1982.
Since
then, there has been a constant succession of optical disc formats, first in CD
formats, followed by a number of DVD formats. Optical disc offers a number of
advantages over magnetic storage media. An optical disc holds much more data.
The greater control and focus possible with laser beams (in comparison to tiny
magnetic heads) means that more data can be written into a smaller space.
Storage capacity increases with each new generation of optical media. Emerging
standards, such as Blu-ray, offer up to 27 gigabytes (GB) on a single-sided
12-centimeter disc. In comparison, a diskette, for example, can hold 1.44
megabytes (MB). Optical discs are inexpensive to manufacture and data stored on
them is relatively impervious to most environmental threats, such as power
surges, or magnetic disturbances.
CD-ROM
CD-ROM
(Compact Disc, read-only-memory) is an adaptation of the CD that is designed to
store computer data in the form of text and graphics, as well as hi-fi stereo
sound. The original data format standard was defined by Philips and Sony in the
1983 Yellow Book. Other standards are used in conjunction with it to define
directory and file structures, including ISO 9660, HFS (Hierarchal File System,
for Macintosh computers), and Hybrid HFS-ISO. Format of the CD-ROM is the same
as for audio CDs: a standard CD is 120 mm (4.75 inches) in diameter and 1.2 mm
(0.05 inches) thick and is composed of a polycarbonate plastic substrate (underlayer
– this is the main body of the disc), one or more thin reflective metal
(usually aluminum) layers, and a lacquer coating. The Yellow Book
specifications were so general that there was some fear in the industry that
multiple incompatible and proprietary formats would be created. In order to
prevent such an occurrence, representatives from industry leaders met at the
High Sierra Hotel in Lake Tahoe to collaborate on a common standard. Nicknamed
the High Sierra Format, this version was later modified to become ISO 9660.
Today, CDROMs are standardized and will work in any standard CD-ROM drive.
CD-ROM drives can also read audio compact discs for music, although CD players
cannot read CD-ROM discs.
Magnetic
Tape
The use
of magnetic media to record and store numeric and textual information, sound,
motion, and still images has presented librarians and archivists with
opportunities and challenges. On the one hand, magnetic media increase the
kinds of artifacts and events we can capture and store. On the other hand,
their special long-term storage needs are different from traditional library
materials, confusing to those in charge of their care, and demanding of
resources not always available to libraries and archives. Audio and video
collections require specific care and handling to ensure that the recorded
information will be preserved. Special storage environments may be required if
the recorded information is to be preserved for longer than ten years. For
information that must be preserved indefinitely, periodic transcription from
old media to new media will be necessary, not only because the media are
unstable, but because the recording technology will become obsolete. As an
information storage medium, magnetic tape is not as stable as film or paper. Properly
cared for, film and nonacidic paper can last for centuries, whereas magnetic
tape will only last a few decades. Use of magnetic media for storage is further
confounded by the prevalence of several formats (e.g., U-matic, VHS, S-VHS,
8mm, and BetaCam for video), media types (iron oxide, chromium dioxide, barium
ferrite, metal particulate, and metal evaporated), and by rapid advances in
media technology. On the other hand, books have virtually maintained the same
format for centuries, have almost exclusively used ink on paper as the
information storage medium, and require no special technology to access the
recorded information.
Likewise,
newer microfilm, microfiche, and movie film are known for their stability when
kept in proper environments, and viewing formats have not changed significantly
over the years. (The breakdown of acetate backing that plagues older film
materials is discussed in Section 2.3: Substrate Deformation.) This report will
compare care and handling procedures for tapes with procedures for paper and
film whenever possible.
Zip
Drives
A Zip
drive is a small, portable disk drive used primarily for backing up and
archiving personal computer files. The trademarked Zip drive was developed and
is sold by Iomega Corporation. Zip drives and disks come in two sizes. The 100
megabyte size actually holds 100,431,872 bytes of data or the equivalent of 70
floppy diskettes. There is also a 250 megabyte drive and disk. The Iomega Zip
drive comes with a software utility that lets you copy the entire contents of
your hard drive to one or more Zip disks.
In addition to data backup, Iomega
suggests these additional uses:
·
Archiving old e-mail or other files you
don't use any more but may want to access someday
·
Storing unusually large files, such as
graphic images that you need infrequently
·
Exchanging large files with someone
·
Putting your system on another
computer, perhaps a portable computer
·
Keeping certain files separate from
files on your hard disk (for example, personal finance files)
The Zip
drive can be purchased in either a parallel or a Small Computer System
Interface (SCSI) version. In the parallel version, a printer can be chained off
the Zip drive so that both can be plugged into your computer's parallel port.
DVD
DVD
(digital versatile disc) is an optical disc technology that is expected to
rapidly replace the CD-ROM disc (as well as the audio compact disc) over the
next few years. The digital versatile disc (DVD) holds 4.7 gigabyte of
information on one of its two sides, or enough for a 133-minute movie. With two
layers on each of its two sides, it will hold up to 17 gigabytes of video,
audio, or other information. (Compare this to the current CD-ROM disc of the
same physical size, holding 600 megabyte. The DVD can hold more than 28 times as
much information!)
DVD-Video is the usual name for the DVD
format designed for full-length movies and is a box that will work with your
television set. DVD-ROM is the name of the player that will (sooner or later)
replace your computer's CD-ROM. It will play regular CDROM discs as well as
DVD-ROM discs. DVD-RAM is the writeable version. DVDAudio is a player designed
to replace your compact disc player.
DVD uses the MPEG-2 file and compression
standard. MPEG-2 images have four times the resolution of MPEG-1 images and can
be delivered at 60 interlaced fields per second where two fields constitute one
image frame. (MPEG-1 can deliver 30 nointerlaced frames per second.) Audio
quality on DVD is comparable to that of current audio compact discs.
Main
Circuit Board of a PC
A
motherboard is the physical arrangement in a computer that contains the
computer's basic circuitry and components. On the typical motherboard, the
circuitry is imprinted or affixed to the surface of a firm planar surface and
usually manufactured in a single step. The most common motherboard design in
desktop computers today is the AT, based on the IBM AT motherboard. A more
recent motherboard specification, ATX, improves on the AT design. In both the
AT and ATX designs, the computer components included in the motherboard are:
The microprocessor
(Optionally) coprocessors
Memory
basic input/output system (BIOS)
Expansion slot
Interconnecting circuitry
Additional
components can be added to a motherboard through its expansion slot. The
electronic interface between the motherboard and the smaller boards or cards in
the expansion slots is called the bus.
Bus
In a computer or on a network, a bus is a
transmission path on which signals are dropped off or picked up at every device
attached to the line. Only devices addressed by the signals pay attention to
them; the others discard the signals. According to Winn L. Rosch, the term
derives from its similarity to autobuses that stop at every town or block to drop
off or take on riders
In general, the term is used in
two somewhat different contexts:
- A bus is a network topology or circuit arrangement in which all devices are attached to a line directly and all signals pass through each of the devices. Each device has a unique identity and can recognize those signals intended for it.
- In a computer, a bus is the data path on the computer's motherboard that interconnects the microprocessor with attachments to the motherboard in expansion slots (such as hard disk drives, CD-ROM drives, and graphics adapters).
Chips
Chip"
is short for microchip, the incredibly complex yet tiny modules that store
computer memory or provide logic circuitry for microprocessors. Perhaps the
best known chips are the Pentium microprocessors from Intel. The PowerPC
microprocessor, developed by Apple, Motorola, and IBM, is used in Macintosh
personal computers and some workstations. AMD and Cyrix also make popular
microprocessor chips.
There
are quite a few manufacturers of memory chips. Many special-purpose chips,
known as application-specific integrated circuits, are being made today for
automobiles, home appliances, telephones, and other devices. A chip is
manufactured from a silicon (or, in some special cases, a sapphire) wafer,
which is first cut to size and then etched with circuits and electronic
devices. The electronic devices use complementary metal-oxide semiconductor
technology. The current stage of micro-integration is known as Very Large-Scale
Integration (VLSI). A chip is also sometimes called an IC or integrated
circuit.
Ports
On
computer and telecommunication devices, a port (noun) is generally a specific
place for being physically connected to some other device, usually with a
socket and plug of some kind. Typically, a personal computer is provided with
one or more serial ports and usually one parallel port. The serial port
supports sequential, one bit-at-atime transmission to peripheral devices such
as scanners and the parallel port supports multiple-bit-at-a-time transmission
to devices such as printers.
Expansion
Slots
In
computers, a slot, or expansion slot, is an engineered technique for adding
capability to a computer in the form of connection pinholes (typically, in the
range of 16 to 64 closely-spaced holes) and a place to fit an expansion card
containing the circuitry that provides some specialized capability, such as
video acceleration, sound, or disk drive control.
Almost all desktop computers come
with a set of expansion slots. These help ensure that you'll be able to add new
hardware capabilities in the future.
SIMM
A SIMM
(single in-line memory module) is a module containing one or several random
access memory (RAM) chips on a small circuit board with PINs that connect to
the computer motherboard. Since the more RAM your computer has, the less
frequently it will need to access your secondary storage (for example, hard
disk or CD-ROM), PC owners sometimes expand RAM by installing additional SIMMs.
SIMMs typically come with a 32 data bit (36 bits counting parity bits) path to
the computer that requires a 72-pin connector. SIMMs usually come in memory
chip multiples of four megabytes.
The memory chips on a SIMM are
typically dynamic RAM (DRAM) chips. An improved form of RAM called Synchronous
DRAM (SDRAM) can also be used. Since SDRAM provides a 64 data bit path, it
requires at least two SIMMs or a dual in-line memory module (DIMM).
RIMM
In a
computer, a RIMM is a memory module developed by Kingston Technology Corp. that
takes up less space inside the computer than the older DIMM module and has
different PIN characteristics. A RIMM has a 184-pin connector and an SO-RIMM
module has a 160-pin connector. An SO-RIMM is smaller and is used in systems
that require smaller form factors. While RIMM is commonly believed to stand for
"Rambus inline memory module," Kingston Technology has trademarked
"RIMM" and uses only that term.
A RIMM
module consists of RDRAM chips that are attached using a thin layer of solder,
a metal alloy that, when melted, fuses metals to each other. Solder balls on
each chip create a metal pathway used to conduct electricity.
DIMM
A DIMM
(dual in-line memory module) is a double SIMM (single in-line memory module).
Like a SIMM, it's a module containing one or several random access memory (RAM)
chips on a small circuit board with pins that connect it to the computer
motherboard. A SIMM typically has a 32 data bit (36 bits counting parity bits)
path to the computer that requires a 72-pin connector. For synchronous dynamic
RAM (SDRAM) chips, which have a 64 data bit connection to the computer, SIMMs
must be installed in in-line pairs (since each supports a 32 bit path). A
single DIMM can be used instead. A DIMM has a 168-pin connector and supports
64-bit data transfer. It is considered likely that future computers will
standardize on the DIMM.
VDU
Interface
A device
used for the real-time temporary display of computer output data. Note:
Monitors usually use cathode-ray-tube or liquid-crystal technology. Synonyms
video display terminal, video display unit, visual display unit.
VDU, or
"video terminal", "video display terminal", VDT,
"display terminal") A device incorporating a cathode ray tube (CRT)
display, a keyboard and a serial port. A VDU usually also includes its own
display electronics which store the received data and convert it into
electrical waveforms to drive the CRT.
VDUs
fall into two categories: dumb terminals and intelligent terminals (sometimes
called "programmable terminals"). Early VDUs could only display
characters in a single preset font, and these were confined to being layed out
in a rectangular grid, reproducing the functionality of the paper-based
teletypes they were designed to replace.
Later
models added graphics facilities but were still driven via serial
communications, typically with several VDUs attached to a single multi-user
computer. This contrasts with the much faster single bitmap displays integrated
into most modern single-user personal computers and workstations.
The term "Display Screen
Equipment" (DSE) is used almost exclusively in connection with the health
and safety issues concerning VDUs.
The Number Systems
The reason that "hex" and
octal are popular in computing is that it's easy to translate to and from the
binary system that computers really use. People use decimal primarily because
they have ten fingers, but it's just not that convenient to switch back and
forth from 10011010010 to 1234. It IS convenient to convert to 4D2 (hex) or
2322 (octal).
The numbers from decimal 0 through
15 in decimal, binary, octal, and hexadecimal form are listed below.
Binary
Binary
describes a numbering scheme in which there are only two possible values for
each digit: 0 and 1. The term also refers to any digital encoding/decoding
system in which there are exactly two possible states. In digital data memory,
storage, processing, and communications, the 0 and 1 values are sometimes
called "low" and "high," respectively.
Binary
numbers look strange when they are written out directly. This is because the
digits' weight increases by powers of 2, rather than by powers of 10. In a
digital numeral, the digit furthest to the right is the "ones" digit;
the next digit to the left is the "twos" digit; next comes the
"fours" digit, then the "eights" digit, then the
"16s" digit, then the "32s" digit, and so on. The decimal
equivalent of a binary number can be found by summing all the digits. For
example, the binary 10101 is equivalent to the decimal 1 + 4 + 16 = 21:
Hexadecimal
Hexadecimal
describes a base-16 number system. That is, it describes a numbering system
containing 16 sequential numbers as base units (including 0) before adding a
new position for the next number. (Note that we're using "16" here as
a decimal number to explain a number that would be "10" in hexadecimal.)
The hexadecimal numbers are 0-9 and then use the letters A-F. We show the
equivalence of binary, decimal, and hexadecimal numbers in the table below.
Hexadecimal is a convenient way to
express binary numbers in modern computers in which a byte is almost always
defined as containing eight binary digits. When showing the contents of
computer storage (for example, when getting a core dump of storage in order to
debug a new computer program or when expressing a string of text characters or
a string of binary values in coding a program or HTML page), one hexadecimal
digit can represent the arrangement of four binary digits. Two hexadecimal
digits can represent eight binary digits, or a byte.
Octal
Octal
(pronounced AHK-tuhl, from Latin octo or "eight") is a term that
describes a base-8 number system. An octal number system consists of eight
single-digit numbers: 0, 1, 2, 3, 4, 5, 6, and 7. The number after 7 is 10. The
number after 17 is 20 and so forth.
In
computer programming, the octal equivalent of a binary number is sometimes used
to represent it because it is shorter.
Coding
Systems
ASCII
ASCII
(American Standard Code for Information Interchange) is the most common format
for text files in computers and on the Internet. In an ASCII file, each
alphabetic, numeric, or special character is represented with a 7-bit binary
number (a string of seven 0s or 1s). 128 possible characters are defined.
Unix and
DOS-based operating systems use ASCII for text files. Windows NT and 2000 uses
a newer code, Unicode. IBM's S/390 systems use a proprietary 8-bit code called
EBCDIC. Conversion programs allow different operating systems to change a file
from one code to another.
ASCII was developed by the
American National Standards Institute (ANSI).
EBCDIC
EBCDIC
(pronounced either "ehb-suh-dik" or "ehb-kuh-dik") is a
binary code for alphabetic and numeric characters that IBM developed for its
larger operating systems. It is the code for text files that is used in IBM's
OS/390 operating system for its S/390 servers and that thousands of
corporations use for their legacy applications and databases. In an EBCDIC
file, each alphabetic or numeric character is represented with an 8-bit binary
number (a string of eight 0's or 1's). 256 possible characters (letters of the
alphabet, numerals, and special characters) are defined. IBM's PC and
workstation operating systems do not use IBM's proprietary EBCDIC. Instead,
they use the industry standard code for text, ASCII. Conversion programs allow
different operating systems to change a file from one code to another.
Logic Gates: AND Gate, OR Gate, NOR Gate
A logic gate is an elementary building block
of a digital circuit. Most logic gates have two inputs and one output. At any
given moment, every terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage
levels. The logic state of a terminal can, and
generally does, change often, as the circuit processes data. In most logic gates, the low state is approximately
zero volts (0 V), while the high state is
approximately five volts positive (+5 V).
There are seven basic
logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR.
Using combinations of
logic gates, complex operations can be performed. In theory, there is no limit
to the number of gates that can be arrayed together in a single device. But in
practice, there is a limit to the number of gates that can be packed into a
given physical space. Arrays of logic gates are found in digital integrated
circuits (ICs). As IC technology advances, the required physical volume for
each individual logic gate decreases and digital devices of the same or smaller
size become capable of performing ever-more-complicated operations at
ever-increasing speeds.
AND Gate
The AND gate is so named because, if 0 is called "false" and 1 is called "true," the gate acts in the same way as the logical "and" operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is "true" when both inputs are "true." Otherwise, the output is "false."
The AND gate is so named because, if 0 is called "false" and 1 is called "true," the gate acts in the same way as the logical "and" operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is "true" when both inputs are "true." Otherwise, the output is "false."
OR Gate
The OR gate gets its name from the fact that it behaves after the fashion of the logical inclusive "or." The output is "true" if either or both of the inputs are "true." If both inputs are "false," then the output is "false."
NOR Gate
The OR gate gets its name from the fact that it behaves after the fashion of the logical inclusive "or." The output is "true" if either or both of the inputs are "true." If both inputs are "false," then the output is "false."
NOR Gate
The NOR
gate is a combination OR gate followed by an inverter. Its output is
"true" if both inputs are "false." Otherwise, the output is
"false."
Introduction
to Computer Viruses
A virus
is a piece of programming code usually disguised as something else that causes
some unexpected and usually undesirable event. A virus is often designed so
that it is automatically spread to other computer users. Viruses can be
transmitted as attachments to an e-mail note, as downloads, or be present on a
diskette or CD. The source of the e-mail note, downloaded file, or diskette
you've received is often unaware of the virus. Some viruses wreak their effect
as soon as their code is executed; other viruses lie dormant until
circumstances cause their code to be executed by the computer. Some viruses are
playful in intent and effect ("Happy Birthday, Ludwig!") and some can
be quite harmful, erasing data or causing your hard disk to require
reformatting.
Generally, there are three main
classes of viruses:
Boot
Sector
These
viruses infect executable code found in certain system areas on a disk. They
attach to the DOS boot sector on diskettes or the Master Boot Record on hard
disks. A typical scenario (familiar to the author) is to receive a diskette
from an innocent source that contains a boot disk virus. When your operating
system is running, files on the diskette can be read without triggering the
boot disk virus. However, if you leave the diskette in the drive, and then turn
the computer off or reload the operating system, the computer will look first
in your A drive, find the diskette with its boot disk virus, load it, and make
it temporarily impossible to use your hard disk. (Allow several days for
recovery.) This is why you should make sure you have a bootable floppy.
File
Some
file infector viruses attach themselves to program files, usually selected .COM
or .EXE files. Some can infect any program for which execution is requested,
including .SYS, .OVL, .PRG, and .MNU files. When the program is loaded, the
virus is loaded as well. Other file infector viruses arrive as wholly-contained
programs or scripts sent as an attachment to an e-mail note.
Trojan
Horse
Trojan
horse attacks pose one of the most serious threats to computer security. If you
were referred here, you may have not only been attacked but may also be
attacking others unknowingly. This page will teach you how to avoid falling
prey to them, and how to repair the damage if you already did. According to
legend, the Greeks won the Trojan war by hiding in a huge, hollow wooden horse
to sneak into the fortified city of Troy. In today's computer world, a Trojan horse
is defined as a "malicious, security-breaking program that is disguised as
something benign". For example, you download what appears to be a movie or
music file, but when you click on it, you unleash a dangerous program that
erases your disk, sends your credit card numbers and passwords to a stranger,
or lets that stranger hijack your computer to commit illegal denial of service
attacks like those that have virtually crippled the DALnet IRC network for
months on end.
The following general information applies
to all operating systems, but by far most of the damage is done to/with Windows
users due to its vast popularity and many weaknesses.
(Note: Many people use terms like Trojan
horse, virus, worm, hacking and cracking all interchangeably, but they really
don't mean the same thing. If you're curious, here's a quick primer defining
and distinguishing them. Let's just say that once you are "infected",
trojans are just as dangerous as viruses and can spread to hurt others just as
easily!)
Methods of
Activation
You can open a file by double
clicking on it, but you can’t see the behind process. Most of the times these
files can be .exe files which are in floppy diskettes or in Compack Disks.
You might have down load something
from the internet which you don’t trust 100%.
You might open Email attachments
without knowing.
Virus
Effects
- If your computer is running down slowly more than earlier.
- If the machine shut downs abnormally.
- If you can see unknown files or folders have been saved in to the hard disk.
- If anything abnormally happens in your machine than earlier it might be to a virus effect.
Preventive
Steps
1. Above
all check every data medium you get. Even if it belongs to your best
friend, it does not give you the guarantee of security. It concerns not only
floppy disks, but also CD-ROMs, CD-RWs and ZIPs. Do not even open floppy disk
if you get it from an unknown person. Every data medium obtained from an
anonymous source is potentially infected.
2. Block
the possibility of system boot from a floppy disk. Most of the latest BIOSes
have a function, which enables to do it. In that simple way you can avoid the
risk of having your computer infected by boot-sector viruses.
3. Do not
open any letter with attached files received from an unknown sender.
4. Do not
run macros in documents of office packages if an anti-virus monitor is off.
5. Use an
anti-virus monitor working constantly at the background. For the best security
the monitor should check the incoming mail and every file downloaded
from the Internet.
6. You should
systematically update definitions of viruses used by anti-virus software.
Updating should be done at least once a month (the best once a week).
7. In a
situation when you chat in the Internet (especially using mIRC) you have to
follow the same rules as in case of received e-mails. Do not open any received
files, if you do know who sent it to you and why. Some viruses are sent
automatically, so same information about sender does not give you guarantee of
security.
8. NEVER
download blindly from people or sites which you aren't 100% sure about. In
other words, as the old saying goes, don't accept candy from strangers. If you
do a lot of file downloading, it's often just a matter of time before you fall
victim to a trojan.
9. Even if
the file comes from a friend, you still must be sure what the file is before
opening it, because many trojans will automatically try to spread themselves to
friends in an email address book or on an IRC channel. There is seldom reason
for a friend to send you a file that you didn't ask for. When in doubt, ask
them first, and scan the attachment with a fully updated anti-virus program.
10. Beware
of hidden file extensions! Windows by default hides the last extension of a
file, so that innocuous-looking "susie.jpg" might really be
"susie.jpg.exe" - an executable trojan! To reduce the chances of
being tricked, unhide those pesky extensions.
11. NEVER
use features in your programs that automatically get or preview files. Those
features may seem convenient, but they let anybody send you anything which is
extremely reckless. For example, never turn on "auto DCC get" in
mIRC, instead ALWAYS screen every single file you get manually. Likewise,
disable the preview mode in Outlook and other email programs.
12. Never blindly type commands that others tell
you to type, or go to web addresses mentioned by strangers, or run
pre-fabricated programs or scripts (not even popular ones). If you do so, you
are potentially trusting a stranger with control over your computer, which can
lead to trojan infection or other serious harm.
13. Don't be
lulled into a false sense of security just because you run anti-virus programs.
Those do not protect perfectly against many viruses and trojans, even when
fully up to date. Anti-virus programs should not be your front line of
security, but instead they serve as a backup in case something sneaks onto your
computer.
14. Finally,
don't download an executable program just to "check it out" - if it's
a trojan, the first time you run it, you're already infected!
Introduction to Anti-virus Software
Antivirus (or
"anti-virus") software is a class of program that searches your hard
drive and floppy disks for any known or potential viruses. The market for this
kind of program has expanded because of Internet growth and the increasing use
of the Internet by businesses concerned about protecting their computer assets.
Following are some
URLs where you can find more information about antivirus software.
International Computer
Security Association:
http://www.icsa.net
Virus Bulletin
http://www.virusbtn.com
Dr Salomon's page
http://www.drsalomon.com
CERT - Computer
Emergency Response Team
http://www.cert.org
McAfee anti-virus
http://vil.mcafee.com
Symantec
http://www.symantec.com/avcenter/
Other pages about
virus:
http://www.mks.com.pl
http://www.antywirusy.pl
http://www.wirusy.pl
http://www.wirusy.onet.pl
http://www.viruslist.com
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