Computer and Networking

8/14/2019 Computer and Networking

1/61

ElectronicsPedia

InternetPedia

MP3Pedia

MusicPedia

SearchPedia

SoftwarePedia

VideoPedia

Pedia

PediaCard

ComputerPedia

WWW

Sunday, July 27,

2003

Welcome to

ComputerPedia

-- The

Computer

Encyclopedia

Our Mission:

Provide consumerswith faster, easier

access to the

information,products and

services they want.

We search the

8/14/2019 Computer and Networking

2/61

major search

engines andremove the

duplicates, the

advertising sites,

the pop-up ads, andanything that might

harm your

computer. Then weinclude all the

related products

and services in thiseasy-to-remember

place where you

spend less time

searching, and

more time findingwhat you want.

Computer News

Links:

Nicholson Baker:Scrapping the card

catalogue, from

1994.27 Jul 2009 at

2:39am

Read the full textof this article .ABSTRACT:

ANNALS OF

SCHOLARSHIPabout the scrapping

of libra...

Science whizattends UNM after

Intel win

26 Jul 2009 at

8:00pmWhile most 16-

year-olds play

Minesweeper onthe computer to

cure boredom,

UNM studentRishin Behl...

8/14/2019 Computer and Networking

3/61

Before Fining Your

Ass For Failing toCompost, City

Unveils

Educational

Compu...27 Jul 2009 at

2:34am

With the city nowpoised to extract

green from

residents who failto put rotting

greens in the gr...

Powered by

MediaPedia

Computer:

A computer is a

machine that

manipulates dataaccording to a list

of instructions.

The first devices

that resemble

modern computers

date to the mid-20th century,

although the

computer conceptand various

machines similar to

computers existedearlier. Early

electronic

computers were the

size of a large

room, consumingas much power as

several hundredmodern personal

computers(PC).

Modern computersare based on tiny

integrated circuits

8/14/2019 Computer and Networking

4/61

and are millions to

billions of timesmore capable while

occupying a

fraction of the

space. Today,simple computers

may be made small

enough to fit into awristwatch and be

powered from a

watch battery.Personal

computers, in

various forms, are

icons of the

Information Ageand are what most

people think of as"a computer";

however, the most

common form ofcomputer in use

today is the

embeddedcomputer.

Embedded

computers aresmall, simpledevices that are

used to control

other devices forexample, they may

be found in

machines rangingfrom fighter

aircraft to

industrial robots,

digital cameras,and children's toys.

The ability to storeand execute lists of

instructions called

programs makescomputers

8/14/2019 Computer and Networking

5/61

extremely versatile

and distinguishesthem from

calculators. The

Church Turing

thesis is amathematical

statement of this

versatility: anycomputer with a

certain minimum

capability is, inprinciple, capable

of performing the

same tasks that any

other computer can

perform. Therefore,computers with

capability andcomplexity ranging

from that of a

personal digitalassistant to a

supercomputer are

all able to performthe same

computational

tasks given enoughtime and storagecapacity.

History of

Computer

Hardware:

The history ofcomputer hardware

encompasses the

hardware, its

architecture, and itsimpact on software.

The elements of

computinghardware have

undergone

significantimprovement over

8/14/2019 Computer and Networking

6/61

their history. This

improvement hastriggered

worldwide use of

the technology,

performance hasimproved and the

price has declined.

Computers areaccessible to ever-

increasing sectors

of the world'spopulation.

Computing

hardware has

become a platform

for uses other thancomputation, such

as automation,communication,

control,

entertainment, andeducation. Each

field in turn has

imposed its ownrequirements on

the hardware,

which has evolvedin response to thoserequirements.

The von Neumannarchitecture unifies

our current

computinghardware

implementations.

Since digital

computers rely ondigital storage, and

tend to be limited

by the size andspeed of memory,

the history of

computer datastorage is tied to

8/14/2019 Computer and Networking

7/61

the development of

computers. Themajor elements of

computing

hardware

implementabstractions: input,

output, memory,

and processor. Aprocessor is

composed of

control anddatapath. In the

von Neumann

architecture,

control of the

datapath is storedin memory. This

allowed control tobecome an

automatic process;

the datapath couldbe under software

control, perhaps in

response to events.Beginning with

mechanical

datapaths such asthe abacus andastrolabe, the

hardware first

started usinganalogs for a

computation,

including waterand even air as the

analog quantities:

analog computers

have used lengths,pressures, voltages,

and currents to

represent theresults of

calculations.

Eventually thevoltages or currents

8/14/2019 Computer and Networking

8/61

were standardized,

and then digitized.Digital computing

elements have

ranged from

mechanical gears,to

electromechanical

relays, to vacuumtubes, to

transistors, and to

integrated circuits,all of which are

currently

implementing the

von Neumann

architecture.

It is difficult toidentify any one

device as the

earliest computer,partly because the

term "computer"

has been subject tovarying

interpretations over

time. Originally,the term"computer"

referred to a person

who performednumerical

calculations (a

human computer),often with the aid

of a mechanical

calculating device.

The history of themodern computer

begins with twoseparate

technologies - that

of automatedcalculation and that

of

8/14/2019 Computer and Networking

9/61

programmability.

Examples of early

mechanical

calculating devices

included theabacus, the slide

rule and arguably

the astrolabe andthe Antikythera

mechanism (which

dates from about150-100 BC). Hero

of Alexandria built

a mechanical

theater which

performed a playlasting 10 minutes

and was operatedby a complex

system of ropes

and drums thatmight be

considered to be a

means of decidingwhich parts of the

mechanism

performed whichactions and when.This is the essence

of

programmability.

The "castle clock",

an astronomicalclock invented by

Al-Jazari in 1206,

is considered to be

the earliest

programmableanalog computer. It

displayed thezodiac, the solar

and lunar orbits, a

crescent moon-shaped pointer

travelling across a

8/14/2019 Computer and Networking

10/61

gateway causing

automatic doors toopen every hour,

and five robotic

musicians who

play music whenstruck by levers

operated by a

camshaft attachedto a water wheel.

The length of day

and night could bere-programmed

every day in order

to account for the

changing lengths of

day and nightthroughout the

year.

The end of the

Middle Ages saw are-invigoration of

European

mathematics andengineering, and

Wilhelm

Schickard's 1623device was the firstof a number of

mechanical

calculatorsconstructed by

European

engineers.However, none of

those devices fit

the modern

definition of acomputer because

they could not be

programmed.

In 1801, Joseph

Marie Jacquardmade an

improvement to the

8/14/2019 Computer and Networking

11/61

textile loom that

used a series ofpunched paper

cards as a template

to allow his loom

to weave intricatepatterns

automatically. The

resulting Jacquardloom was an

important step in

the development ofcomputers because

the use of punched

cards to define

woven patterns can

be viewed as anearly, albeit

limited, form ofprogrammability.

It was the fusion ofautomatic

calculation with

programmabilitythat produced the

first recognizable

computers. In1837, CharlesBabbage was the

first to

conceptualize anddesign a fully

programmable

mechanicalcomputer that he

called "The

Analytical Engine".

Due to limitedfinances, and an

inability to resist

tinkering with thedesign, Babbage

never actually built

his AnalyticalEngine.

8/14/2019 Computer and Networking

12/61

Large-scale

automated dataprocessing of

punched cards was

performed for the

U.S. Census in1890 by tabulating

machines designed

by HermanHollerith and

manufactured by

the ComputingTabulating

Recording

Corporation, which

later became IBM.

By the end of the19th century a

number oftechnologies that

would later prove

useful in therealization of

practical computers

had begun toappear: the

punched card,

Boolean algebra,the vacuum tube(thermionic valve)

and the teleprinter.

During the first

half of the 20th

century, manyscientific

computing needs

were met by

increasinglysophisticated

analog computers,

which used a directmechanical or

electrical model of

the problem as abasis for

8/14/2019 Computer and Networking

13/61

computation.

However, thesewere not

programmable and

generally lacked

the versatility andaccuracy of

modern digital

computers.

A succession of

steadily morepowerful and

flexible computing

devices were

constructed in the

1930s and 1940s,gradually adding

the key featuresthat are seen in

modern computers.

The use of digitalelectronics (largely

invented by Claude

Shannon in 1937)and more flexible

programmability

were vitallyimportant steps, butdefining one point

along this road as

"the first digitalelectronic

computer" is

difficult (Shannon1940). Notable

achievements

include:

Konrad Zuse'selectromechanical

"Z machines". TheZ3 (1941) was the

first working

machine featuringbinary arithmetic,

including floating

8/14/2019 Computer and Networking

14/61

point arithmetic

and a measure ofprogrammability.

In 1998 the Z3 was

proved to be

Turing complete,therefore being the

world's first

operationalcomputer.

The non-programmable

AtanasoffBerry

Computer (1941)

which used

vacuum tube basedcomputation,

binary numbers,and regenerative

capacitor memory.

The secret British

Colossus

computers (1943),which had limited

programmability

but demonstrated

that a device usingthousands of tubes

could be

reasonably reliableand electronically

reprogrammable. It

was used forbreaking German

wartime codes.

The Harvard Mark

I (1944), a large-scale

electromechanicalcomputer with

limited

programmability.

The U.S. Army's

8/14/2019 Computer and Networking

15/61

Ballistics Research

Laboratory ENIAC(1946), which used

decimal arithmetic

and is sometimes

called the firstgeneral purpose

electronic

computer (sinceKonrad Zuse's Z3

of 1941 used

electromagnetsinstead of

electronics).

Initially, however,

ENIAC had an

inflexiblearchitecture which

essentially requiredrewiring to change

its programming.

Several developers

of ENIAC,

recognizing itsflaws, came up

with a far more

flexible and elegantdesign, which cameto be known as the

"stored program

architecture" orvon Neumann

architecture. This

design was firstformally described

by John von

Neumann in the

paper First Draft ofa Report on the

EDVAC,

distributed in 1945.A number of

projects to develop

computers basedon the stored-

8/14/2019 Computer and Networking

16/61

program

architecturecommenced around

this time, the first

of these being

completed in GreatBritain. The first to

be demonstrated

working was theManchester Small-

Scale Experimental

Machine (SSEM or"Baby"), while the

EDSAC,

completed a year

after SSEM, was

the first practicalimplementation of

the stored programdesign. Shortly

thereafter, the

machine originallydescribed by von

Neumann's paper

was completed butdid not see full-

time use for an

additional twoyears.

Nearly all modern

computersimplement some

form of the stored-

programarchitecture,

making it the single

trait by which the

word "computer" isnow defined. While

the technologies

used in computershave changed

dramatically since

the first electronic,general-purpose

8/14/2019 Computer and Networking

17/61

computers of the

1940s, most stilluse the von

Neumann

architecture.

Computers that

used vacuum tubes

as their electronicelements were in

use throughout the

1950s. Vacuumtube electronics

were largely

replaced in the

1960s by

transistor-basedelectronics, which

are smaller, faster,cheaper to produce,

require less power,

and are morereliable. In the

1970s, integrated

circuit technologyand the subsequent

creation of

microprocessors,such as the Intel4004, further

decreased size and

cost and furtherincreased speed

and reliability of

computers. By the1980s, computers

became sufficiently

small and cheap to

replace simplemechanical

controls in

domesticappliances such as

washing machines.

The 1980s alsowitnessed home

8/14/2019 Computer and Networking

18/61

computers and the

now ubiquitouspersonal computer.

With the evolution

of the Internet,

personal computersare becoming as

common as the

television and thetelephone in the

household.

Stored Program

Architecture:

Computer

Program:

Computerprograms (also

software programs,or just programs)

are instructions for

a computer. Acomputer requires

programs to

function.Moreover, a

computer program

does not run unlessits instructions areexecuted by a

central processor;

however, aprogram may

communicate an

algorithm to peoplewithout running.

Computer

programs are

usually executableprograms or the

source code from

which executableprograms are

derived.

Computer source

code is often

8/14/2019 Computer and Networking

19/61

written by

professionalcomputer

programmers.

Source code is

written in aprogramming

language that

usually follows oneof two main

paradigms:

imperative ordeclarative

programming.

Source code may

be converted into

an executable file(sometimes called

an executableprogram or a

binary) by a

compiler.Alternatively,

computer programs

may be executedby a central

processing unit

with the aid of aninterpreter, or maybe embedded

directly into

hardware.

Computer

programs may becategorized along

functional lines:

system software

and applicationsoftware. And

many computer

programs may runsimultaneously on

a single computer,

a process known asmultitasking.

8/14/2019 Computer and Networking

20/61

Computer

Programming:

Computer

programming

(often shortened to

programming orcoding) is the

process of writing,

testing,debugging/troubles

hooting, and

maintaining thesource code of

computer

programs. This

source code is

written in aprogramming

language. The codemay be a

modification of an

existing source orsomething

completely new.

The purpose ofprogramming is to

create a program

that exhibits acertain desiredbehavior

(customization).

The process ofwriting source code

requires expertise

in many differentsubjects, including

knowledge of the

application

domain,specialized

algorithms and

formal logic.

The defining

feature of moderncomputers which

8/14/2019 Computer and Networking

21/61

distinguishes them

from all othermachines is that

they can be

programmed. That

is to say that a listof instructions (the

program) can be

given to thecomputer and it

will store them and

carry them out atsome time in the

future.

In most cases,

computerinstructions are

simple: add onenumber to another,

move some data

from one locationto another, send a

message to some

external device,etc. These

instructions are

read from thecomputer'smemory and are

generally carried

out (executed) inthe order they were

given. However,

there are usuallyspecialized

instructions to tell

the computer to

jump ahead orbackwards to some

other place in the

program and tocarry on executing

from there. These

are called "jump"instructions (or

8/14/2019 Computer and Networking

22/61

branches).

Furthermore, jumpinstructions may be

made to happen

conditionally so

that differentsequences of

instructions may be

used depending onthe result of some

previous

calculation or someexternal event.

Many computers

directly support

subroutines by

providing a type ofjump that

"remembers" thelocation it jumped

from and another

instruction toreturn to the

instruction

following thatjump instruction.

Program executionmight be likened toreading a book.

While a person will

normally read eachword and line in

sequence, they may

at times jump backto an earlier place

in the text or skip

sections that are

not of interest.Similarly, a

computer may

sometimes go backand repeat the

instructions in

some section of theprogram over and

8/14/2019 Computer and Networking

23/61

over again until

some internalcondition is met.

This is called the

flow of control

within the programand it is what

allows the

computer toperform tasks

repeatedly without

humanintervention.

Comparatively, a

person using a

pocket calculatorcan perform a basic

arithmeticoperation such as

adding two

numbers with just afew button presses.

But to add together

all of the numbersfrom 1 to 1,000

would take

thousands of buttonpresses and a lot oftimewith a near

certainty of making

a mistake. On theother hand, a

computer may be

programmed to dothis with just a few

simple instructions.

Programs:

In practical terms,a computer

program may runfrom just a few

instructions to

many millions ofinstructions, as in a

program for a word

8/14/2019 Computer and Networking

24/61

processor or a web

browser. A typicalmodern computer

can execute

billions of

instructions persecond (gigahertz

or GHz) and rarely

make a mistakeover many years of

operation. Large

computer programscomprising several

million instructions

may take teams of

programmers years

to write, thus theprobability of the

entire programhaving been

written without

error is highlyunlikely.

Errors in computerprograms are called

"bugs". Bugs may

be benign and notaffect theusefulness of the

program, or have

only subtle effects.But in some cases

they may cause the

program to "hang"- become

unresponsive to

input such as

mouse clicks orkeystrokes, or to

completely fail or

"crash". Otherwisebenign bugs may

sometimes may be

harnessed formalicious intent by

8/14/2019 Computer and Networking

25/61

an unscrupulous

user writing an"exploit" - code

designed to take

advantage of a bug

and disrupt aprogram's proper

execution. Bugs are

usually not thefault of the

computer. Since

computers merelyexecute the

instructions they

are given, bugs are

nearly always the

result ofprogrammer error

or an oversightmade in the

program's design.

In most computers,

individual

instructions arestored as machine

code with each

instruction beinggiven a uniquenumber (its

operation code or

opcode for short).The command to

add two numbers

together wouldhave one opcode,

the command to

multiply them

would have adifferent opcode

and so on. The

simplest computersare able to perform

any of a handful of

differentinstructions; the

8/14/2019 Computer and Networking

26/61

8/14/2019 Computer and Networking

27/61

computer. Modern

von Neumanncomputers display

some traits of the

Harvard

architecture in theirdesigns, such as in

CPU caches.

While it is possible

to write computer

programs as longlists of numbers

(machine language)

and this technique

was used with

many earlycomputers, it is

extremely tediousto do so in practice,

especially for

complicatedprograms. Instead,

each basic

instruction can begiven a short name

that is indicative of

its function andeasy to remember amnemonic such as

ADD, SUB,

MULT or JUMP.These mnemonics

are collectively

known as acomputer's

assembly language.

Converting

programs written inassembly language

into something the

computer canactually understand

(machine language)

is usually done bya computer

8/14/2019 Computer and Networking

28/61

program called an

assembler.Machine languages

and the assembly

languages that

represent them(collectively

termed low-level

programminglanguages) tend to

be unique to a

particular type ofcomputer. For

instance, an ARM

architecture

computer (such as

may be found in aPDA or a hand-

held videogame)cannot understand

the machine

language of anIntel Pentium or

the AMD Athlon

64 computer thatmight be in a PC.

Thoughconsiderably easierthan in machine

language, writing

long programs inassembly language

is often difficult

and error prone.Therefore, most

complicated

programs are

written in moreabstract high-level

programming

languages that areable to express the

needs of the

computerprogrammer more

8/14/2019 Computer and Networking

29/61

conveniently (and

thereby help reduceprogrammer error).

High level

languages are

usually "compiled"into machine

language (or

sometimes intoassembly language

and then into

machine language)using another

computer program

called a compiler.

Since high level

languages are moreabstract than

assembly language,it is possible to use

different compilers

to translate thesame high level

language program

into the machinelanguage of many

different types of

computer. This ispart of the meansby which software

like video games

may be madeavailable for

different computer

architectures suchas personal

computers and

various video game

consoles.

The task of

developing largesoftware systems is

an immense

intellectual effort.Producing software

8/14/2019 Computer and Networking

30/61

with an acceptably

high reliability on apredictable

schedule and

budget has proved

historically to be agreat challenge; the

academic and

professionaldiscipline of

software

engineeringconcentrates

specifically on this

problem.

How ComputersWork:

A general purposecomputer has four

main sections: the

arithmetic andlogic unit (ALU),

the control unit, the

memory, and theinput and output

devices

(collectivelytermed I/O). Theseparts are

interconnected by

busses, often madeof groups of wires.

The control unit,ALU, registers, and

basic I/O (and

often other

hardware closely

linked with these)are collectively

known as a centralprocessing unit

(CPU). Early CPUs

were composed ofmany separate

components but

8/14/2019 Computer and Networking

31/61

since the mid-

1970s CPUs havetypically been

constructed on a

single integrated

circuit called amicroprocessor.

Control Unit:

The control unit

(often called a

control system orcentral controller)

directs the various

components of a

computer. It reads

and interprets(decodes)

instructions in theprogram one by

one. The control

system decodeseach instruction

and turns it into a

series of controlsignals that operate

the other parts of

the computer.Control systems inadvanced

computers may

change the order ofsome instructions

so as to improve

performance.

A key component

common to all

CPUs is the

program counter, aspecial memory

cell (a register) thatkeeps track of

which location in

memory the nextinstruction is to be

8/14/2019 Computer and Networking

32/61

read from.

The control

system's function is

as follows note that

this is a simplifieddescription, and

some of these steps

may be performedconcurrently or in a

different order

depending on thetype of CPU:

1. Read the code

for the next

instruction from

the cell indicatedby the program

counter.2. Decode the

numerical code for

the instruction intoa set of commands

or signals for each

of the othersystems.

3. Increment the

program counter soit points to the nextinstruction.

4. Read whatever

data the instructionrequires from cells

in memory (or

perhaps from aninput device). The

location of this

required data is

typically storedwithin the

instruction code.

5. Provide thenecessary data to

an ALU or register.

6. If the instruction

8/14/2019 Computer and Networking

33/61

requires an ALU or

specializedhardware to

complete, instruct

the hardware to

perform therequested

operation.

7. Write the resultfrom the ALU back

to a memory

location or to aregister or perhaps

an output device. 8.

Jump back to step

(1).

Since the program

counter is(conceptually) just

another set of

memory cells, itcan be changed by

calculations done

in the ALU.Adding 100 to the

program counter

would cause thenext instruction tobe read from a

place 100 locations

further down theprogram.

Instructions that

modify theprogram counter

are often known as

"jumps" and allow

for loops(instructions that

are repeated by the

computer) andoften conditional

instruction

execution (bothexamples of

8/14/2019 Computer and Networking

34/61

control flow).

It is noticeable that

the sequence of

operations that the

control unit goesthrough to process

an instruction is in

itself like a shortcomputer program

- and indeed, in

some morecomplex CPU

designs, there is

another yet smaller

computer called a

microsequencerthat runs a

microcode programthat causes all of

these events to

happen.

Arithmetic/Logic

Unit (ALU):

The ALU is

capable of

performing two

classes ofoperations:

arithmetic and

logic.

The set of

arithmeticoperations that a

particular ALU

supports may be

limited to adding

and subtracting ormight include

multiplying ordividing,

trigonometry

functions (sine,cosine, etc) and

square roots. Some

8/14/2019 Computer and Networking

35/61

8/14/2019 Computer and Networking

36/61

These can be useful

both for creatingcomplicated

conditional

statements and

processing booleanlogic.

Superscalarcomputers contain

multiple ALUs so

that they canprocess several

instructions at the

same time.

Graphics

processors andcomputers with

SIMD and MIMDfeatures often

provide ALUs that

can performarithmetic on

vectors and

matrices.

Memory:

A computer's

memory can beviewed as a list of

cells into which

numbers can beplaced or read.

Each cell has a

numbered"address" and can

store a single

number. The

computer can be

instructed to "putthe number 123

into the cellnumbered 1357" or

to "add the number

that is in cell 1357to the number that

is in cell 2468 and

8/14/2019 Computer and Networking

37/61

put the answer into

cell 1595". Theinformation stored

in memory may

represent

practicallyanything. Letters,

numbers, even

computerinstructions can be

placed into

memory with equalease. Since the

CPU does not

differentiate

between different

types ofinformation, it is

up to the softwareto give significance

to what the

memory sees asnothing but a series

of numbers.

In almost all

modern computers,

each memory cellis set up to storebinary numbers in

groups of eight bits

(called a byte).Each byte is able to

represent 256

different numbers;either from 0 to

255 or -128 to

+127. To store

larger numbers,several consecutive

bytes may be used

(typically, two,four or eight).

When negative

numbers arerequired, they are

8/14/2019 Computer and Networking

38/61

8/14/2019 Computer and Networking

39/61

reducing the need

to access mainmemory (which is

often slow

compared to the

ALU and controlunits) greatly

increases the

computer's speed.

Computer main

memory comes intwo principal

varieties: random

access memory or

RAM and read-

only memory orROM. RAM can be

read and written toanytime the CPU

commands it, but

ROM is pre-loadedwith data and

software that never

changes, so theCPU can only read

from it. ROM is

typically used tostore thecomputer's initial

start-up

instructions. Ingeneral, the

contents of RAM is

erased when thepower to the

computer is turned

off while ROM

retains its dataindefinitely. In a

PC , the ROM

contains aspecialized

program called the

BIOS thatorchestrates

8/14/2019 Computer and Networking

40/61

loading the

computer'soperating system

from the hard disk

drive into RAM

whenever thecomputer is turned

on or reset. In

embeddedcomputers, which

frequently do not

have disk drives,all of the software

required to perform

the task may be

stored in ROM.

Software that isstored in ROM is

often calledfirmware because it

is notionally more

like hardware thansoftware. Flash

memory blurs the

distinction betweenROM and RAM by

retaining data when

turned off butbeing rewritablelike RAM.

However, flash

memory istypically much

slower than

conventional ROMand RAM so its use

is restricted to

applications where

high speeds are notrequired.

In moresophisticated

computers there

may be one ormore RAM cache

8/14/2019 Computer and Networking

41/61

memories which

are slower thanregisters but faster

than main memory.

Generally

computers with thissort of cache are

designed to move

frequently neededdata into the cache

automatically,

often without theneed for any

intervention on the

programmer's part.

Input/Output(I/O):

I/O is the means bywhich a computer

receives

information fromthe outside world

and sends results

back. Devices thatprovide input or

output to the

computer are calledperipherals. On atypical personal

computer,

peripherals includeinput devices like

the keyboard and

mouse, and outputdevices such as the

display and printer.

Hard disk drives,

floppy disk drivesand optical disc

drives serve as both

input and outputdevices. Computer

networking is

another form ofI/O.

8/14/2019 Computer and Networking

42/61

Often, I/O devices

are complexcomputers in their

own right with

their own CPU and

memory. Agraphics processing

unit might contain

fifty or more tinycomputers that

perform the

calculationsnecessary to

display 3D

graphics. Modern

desktop computers

contain manysmaller computers

that assist the mainCPU in performing

I/O.

Multitasking:

While a computer

may be viewed asrunning one

gigantic program

stored in its mainmemory, in somesystems it is

necessary to give

the appearance ofrunning several

programs

simultaneously.This is achieved by

having the

computer switch

rapidly betweenrunning each

program in turn.

One means bywhich this is done

is with a special

signal called aninterrupt which can

8/14/2019 Computer and Networking

43/61

periodically cause

the computer tostop executing

instructions where

it was and do

something elseinstead. By

remembering

where it wasexecuting prior to

the interrupt, the

computer canreturn to that task

later. If several

programs are

running "at the

same time", thenthe interrupt

generator might becausing several

hundred interrupts

per second, causinga program switch

each time. Since

modern computerstypically execute

instructions several

orders ofmagnitude fasterthan human

perception, it may

appear that manyprograms are

running at the same

time even thoughonly one is ever

executing in any

given instant. This

method ofmultitasking is

sometimes termed

"time-sharing"since each program

is allocated a

"slice" of time inturn.

8/14/2019 Computer and Networking

44/61

Before the era of

cheap computers,the principle use

for multitasking

was to allow many

people to share thesame computer.

Seemingly,multitasking would

cause a computer

that is switchingbetween several

programs to run

more slowly - in

direct proportion to

the number ofprograms it is

running. However,most programs

spend much of

their time waitingfor slow

input/output

devices tocomplete their

tasks. If a program

is waiting for theuser to click on themouse or press a

key on the

keyboard, then itwill not take a

"time slice" until

the event it iswaiting for has

occurred. This

frees up time for

other programs toexecute so that

many programs

may be run at thesame time without

unacceptable speed

loss.

Multiprocessing:

8/14/2019 Computer and Networking

45/61

Some computers

may divide theirwork between one

or more separate

CPUs, creating a

multiprocessingconfiguration.

Traditionally, this

technique wasutilized only in

large and powerful

computers such assupercomputers,

mainframe

computers and

servers. However,

multiprocessor andmulti-core

(multiple CPUs ona single integrated

circuit) personal

and laptopcomputers have

become widely

available and arebeginning to see

increased usage in

lower-end marketsas a result.

Supercomputers in

particular oftenhave highly unique

architectures that

differ significantlyfrom the basic

stored-program

architecture and

from generalpurpose computers.

They often feature

thousands ofCPUs, customized

high-speed

interconnects, andspecialized

8/14/2019 Computer and Networking

46/61

8/14/2019 Computer and Networking

47/61

institutions

throughout theUnited States

began to link their

computers together

usingtelecommunication

s technology. This

effort was fundedby ARPA (now

DARPA), and the

computer networkthat it produced

was called the

ARPANET. The

technologies that

made the Arpanetpossible spread and

evolved. In time,the network spread

beyond academic

and militaryinstitutions and

became known as

the Internet. Theemergence of

networking

involved aredefinition of thenature and

boundaries of the

computer.Computer

operating systems

and applicationswere modified to

include the ability

to define and

access theresources of other

computers on the

network, such asperipheral devices,

stored information,

and the like, asextensions of the

8/14/2019 Computer and Networking

48/61

resources of an

individualcomputer. Initially

these facilities

were available

primarily to peopleworking in high-

tech environments,

but in the 1990s thespread of

applications like e-

mail and the WorldWide Web,

combined with the

development of

cheap, fast

networkingtechnologies like

Ethernet andADSL saw

computer

networking becomealmost ubiquitous.

In fact, the number

of computers thatare networked is

growing

phenomenally. Avery largeproportion of

personal computers

regularly connectto the Internet to

communicate and

receiveinformation.

"Wireless"

networking, often

utilizing mobilephone networks,

has meant

networking isbecoming

increasingly

ubiquitous even inmobile computing

8/14/2019 Computer and Networking

49/61

environments.

Other Topics:

Hardware:

A typical personal

computer consistsof a case or chassis

in a tower shape

(desktop) and thefollowing parts:

* Motherboard -

It is the "body" ormainframe of the

computer, through

which all other

components

interface.* Central

Processing Unit

(CPU) - Performs

most of the

calculations whichenable a computer

to function,

sometimes referredto as the "backbone

or brain" of the

computer.* Computer Fan -Used to lower the

temperature of the

computer; a fan isalmost always

attached to the

CPU, and thecomputer case will

generally have

several fans to

maintain a constantairflow. Liquid

cooling can also be

used to cool acomputer, though it

focuses more on

individual partsrather than the

8/14/2019 Computer and Networking

50/61

overall temperature

inside the chassis.* Random Access

Memory (RAM) -

It is also known as

the physicalmemory of the

computer. Fast-

access memory thatis cleared when the

computer is

powered-down.RAM attaches

directly to the

motherboard, and

is used to store

programs that arecurrently running.

* Firmware isloaded from the

Read only memory

eg. ROM run fromthe Basic Input-

Output System

(BIOS) or in newersystems Extensible

Firmware Interface

(EFI) compliant+ Internal Buses -Connections to

various internal

components.+ PCI (being

phased out for

graphic cards butstill used for other

uses)

+ PCI-E

+ ISA (obsolete inPCs, but still used

in industrial

computers)+ USB

+ HyperTransport

+ CSI (expected in2008)

8/14/2019 Computer and Networking

51/61

+ AGP (being

phased out)+ VLB (outdated)

* External Bus

Controllers - usedto connect to

external

peripherals, such asprinters and input

devices. These

ports may also bebased upon

expansion cards,

attached to the

internal buses.

Power Supply:

A case control, and(usually) a cooling

fan, and supplies

power to run therest of the

computer, the most

common types ofpower supplies are

AT and BabyAT

(old) but the

standard for PCsactually are ATX

and Micro ATX.

Storage

Controllers:

Controllers forhard disk, CD-

ROM and other

drives like internal

Zip and Jaz

conventionally fora PC are IDE/ATA;

the controllers sitdirectly on the

motherboard (on-

board) or onexpansion cards,

such as a Disk

8/14/2019 Computer and Networking

52/61

array controller.

IDE is usuallyintegrated, unlike

SCSI Small

Computer System

Interface which canbe found in some

servers. The floppy

drive interface is alegacy MFM

interface which is

now slowlydisappearing. All

these interfaces are

gradually being

phased out to be

replaced by SATAand SAS.

Video Display

Controller:

Produces theoutput for the

visual display unit.

This will either bebuilt into the

motherboard or

attached in its ownseparate slot (PCI,PCI-E, PCI-E 2.0,

or AGP), in the

form of a GraphicsCard.

Removable Media

Devices:

* CD (CompactDisc) - the most

common type of

removable media,inexpensive but has

a short life-span.* CD-ROM Drive- a device used for

reading data from aCD.

* CD Writer - a

8/14/2019 Computer and Networking

53/61

device used for

both reading andwriting data to and

from a CD.

* DVD (Digital

Versatile Disc) - apopular type of

removable media

that is the samedimensions as a

CD but stores up to

* 6 times as muchinformation. It is

the most common

way of transferring

digital video.

* DVD-ROMDrive - a device

used for readingdata from a DVD.

* DVD Writer - a

device used forboth reading and

writing data to and

from a DVD.* DVD-RAM

Drive - a device

used for rapidwriting and readingof data from a

special type of

DVD.* Blu-Ray - a

high-density

optical disc formatfor the storage of

digital information,

including high-

definition video.* BD-ROM Drive- a device used for

reading data from aBlu-Ray disc.

* BD Writer - a

device used forboth reading and

8/14/2019 Computer and Networking

54/61

writing data to and

from a Blu-Raydisc.

* HD DVD - a

high-density

optical disc formatand successor to

the standard DVD.

It was adiscontinued

competitor to the

Blu-Ray format.* Floppy Disk- an

outdated storage

device consisting

of a thin disk of a

flexible magneticstorage medium.

* Zip Drive - anoutdated medium-

capacity removable

disk storagesystem, first

introduced by

Iomega in 1994.* USB Flash

Drive - a flash

memory datastorage deviceintegrated with a

USB interface,

typically small,lightweight,

removable, and

rewritable.* Tape Drive - a

device that reads

and writes data on

a magnetictape,used for long

term storage.

Internal Storage:

Hardware that

keeps data insidethe computer for

8/14/2019 Computer and Networking

55/61

later use and

remains persistenteven when the

computer has no

power.

* Hard Disk- formedium-term

storage of data.

* Solid-State

Drive - a device

similar to hard

disk, but containingno moving parts. V

* Disk ArrayController - a

device to manage

several hard disks,to achieve

performance orreliability

improvement.

Sound Card:

Enables the

computer to outputsound to audio

devices, as well as

accept input from amicrophone. Mostmodern computers

have sound cards

built-in to themotherboard,

though it is

common for a userto install a separate

sound card as an

upgrade.

Networking:Connects the

computer to theInternet and/or

other computers.

* Modem - fordial-up

connections.

8/14/2019 Computer and Networking

56/61

* Network Card -

for DSL/Cableinternet, and/or

connecting to other

computers.

* Direct CableConnection - Use

of a null modem,

connecting twocomputers together

using their serial

ports or a Laplink.* Cable,

connecting two

computers together

with their parallel

ports.* Dial Up

Connections. *

Broad Band

Connections.

Other

Peripherals:

A peripheral is adevice attached to a

host computer

behind the chipsetwhose primaryfunctionality is

dependent upon the

host, and cantherefore be

considered as

expanding thehosts capabilities,

while not forming

part of the system's

core architecture.Some of the more

common peripheraldevices are

printers, scanners,

disk drives, tapedrives,

microphones,

8/14/2019 Computer and Networking

57/61

speakers, and

cameras. Peripheraldevices can also

include other

computers on a

network system. Adevice can also

refer to a non-

physical item, suchas a pseudo

terminal, a RAM

drive, or a virtualnetwork adapter.

Some people do

not consider

internal devicessuch as video

capture cards to beperipherals because

they are added

inside the computercase; for them, the

term peripherals is

reservedexclusively for

devices that are

hooked upexternally to thecomputer. It is

debatable however

whether PCMCIAcards qualify as

peripherals under

this restrictivedefinition, because

some of them go

fully inside the

laptop, while some,like WiFi cards,

have external

appendages.

The term is

different fromcomputer

accessories:

8/14/2019 Computer and Networking

58/61

8/14/2019 Computer and Networking

59/61

English-language

key layout is theQWERTY layout.

* Pointing Devices* Mouse - a

pointing devicethat detects two

dimensional

motion relative toits supporting

surface.

* Trackball - apointing device

consisting of an

exposed protruding

ball housed in a

socket that detectsrotation about two

axes.* Gaming Devices* Joystick- a

general controldevice that consists

of a handheld stick

that pivots aroundone end, to detect

angles in two or

three dimensions.* Gamepad - ageneral handheld

game controller

that relies on thedigits (especially

thumbs) to provide

input.* Game

Controller - a

specific type of

controllerspecialized for

certain gaming

purposes.* Image, VideoInput Devices

* Image Scanner -a device that

8/14/2019 Computer and Networking

60/61

provides input by

analyzing images,printed text,

handwriting, or an

object.

* Webcam - a lowresolution video

camera used to

provide visualinput that can be

easily transferred

over the internet.* Audio Input

Devices

* Microphone - an

acoustic sensor that

provides input byconverting sound

into electricalsignals

Output:

* Image, Video

Output Devices

* Printer* Monitor

* Audio Output

Devices* Speakers* Headset

If you haveinformation or

links that you

would likeincluded in

ComputerPedia,

please email us at:

info@computerpe

dia.com

Terms of UsePrivacyStatementContact UsRecommend aSite

8/14/2019 Computer and Networking

61/61

Copyright 1995-2008 byInformationSuperbrand, Inc.All rights

reserved.