English for Computer

Students

Edited By Sassan Azad

Islamic Azad University of Roudehen

Fall 2009

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Computer....................................................................................................................... 3

Operating system................................................................................................... 29

Computer networking ......................................................................................... 45

Internet........................................................................................................................ 51

Programming language ...................................................................................... 69

Windows XP............................................................................................................... 84

Windows Vista ......................................................................................................... 95

Algorithm .................................................................................................................. 117

Compiler .................................................................................................................... 137

Network topology ................................................................................................ 147

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Computer

The NASA Columbia Supercomputer.

A computer in a wristwatch.

A computer is a machine which manipulates data according to a list of instructions.

Computers take numerous physical forms. The first devices that resemble modern

computers date to the mid-20th century (around 1940 - 1941), although the

computer concept and various machines similar to computers existed earlier. Early

electronic computers were the size of a large room, consuming as much power as

several hundred modern personal computers. Modern computers are based on

comparatively tiny integrated circuits and are millions to billions of times more

capable while occupying a fraction of the space. Today, simple computers may be

made small enough to fit into a wrist watch and be powered from a watch battery.

Personal computers in various forms are icons of the information age and are what

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most people think of as "a computer". However, the most common form of computer

in use today is by far the embedded computer. Embedded computers are small,

simple devices that are often used to control other devices—for example, they may

be found in machines ranging from fighter aircraft to industrial robots, digital

cameras, and even children's toys.

The ability to store and execute lists of instructions called programs makes

computers extremely versatile and distinguishes them from calculators. The Church–

Turing thesis is a mathematical statement of this versatility: Any computer with a

certain minimum capability is, in principle, capable of performing the same tasks that

any other computer can perform. Therefore, computers with capability and

complexity ranging from that of a personal digital assistant to a supercomputer are

all able to perform the same computational tasks given enough time and storage

capacity.

History of computing

The Jacquard loom was one of the first programmable devices.

It is difficult to identify any one device as the earliest computer, partly because the

term "computer" has been subject to varying interpretations over time.

Originally, the term "computer" referred to a person who performed numerical

calculations (a human computer), often with the aid of a mechanical calculating

device. Examples of early mechanical computing devices included the abacus, the

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slide rule and arguably the astrolabe and the Antikythera mechanism (which dates

from about 150-100 BC). The end of the Middle Ages saw a re-invigoration of

European mathematics and engineering, and Wilhelm Schickard's 1623 device was

the first of a number of mechanical calculators constructed by European engineers.

However, none of those devices fit the modern definition of a computer because they

could not be programmed. In 1801, Joseph Marie Jacquard made an improvement to

the textile loom that used a series of punched paper cards as a template to allow his

loom to weave intricate patterns automatically. The resulting Jacquard loom was an

important step in the development of computers because the use of punched cards

to define woven patterns can be viewed as an early, albeit limited, form of

programmability.

In 1837, Charles Babbage was the first to conceptualize and design a fully

programmable mechanical computer that he called "The Analytical Engine".. Due to

limited finance, and an inability to resist tinkering with the design, Babbage never

actually built his Analytical Engine.

Large-scale automated data processing of punched cards was performed for the U.S.

Census in 1890 by tabulating machines designed by Herman Hollerith and

manufactured by the Computing Tabulating Recording Corporation, which later

became IBM. By the end of the 19th century a number of technologies that would

later prove useful in the realization of practical computers had begun to appear: the

punched card, Boolean algebra, the vacuum tube (thermionic valve) and the

teleprinter.

During the first half of the 20th century, many scientific computing needs were met

by increasingly sophisticated analog computers, which used a direct mechanical or

electrical model of the problem as a basis for computation. However, these were not

programmable and generally lacked the versatility and accuracy of modern digital

computers.

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Defining characteristics of five early digital computers

Computer First

operation Place Decimal/Binary Electronic Programmable

Turing complete

Zuse Z3 May 1941 Germany binary No By punched film stock

Yes (1998)

Atanasoff–Berry Computer

Summer 1941

USA binary Yes No No

Colossus

December 1943 / January 1944

UK binary Yes Partially, by rewiring

No

Harvard Mark I – IBM ASCC

1944 USA decimal No By punched paper tape

Yes (1998)

1944 USA decimal Yes Partially, by rewiring

Yes

ENIAC

1948 USA decimal Yes By Function Table ROM

Yes

A succession of steadily more powerful and flexible computing devices were

constructed in the 1930s and 1940s, gradually adding the key features that are seen

in modern computers. The use of digital electronics (largely invented by Claude

Shannon in 1937) and more flexible programmability were vitally important steps,

but defining one point along this road as "the first digital electronic computer" is

difficult (Shannon 1940). Notable achievements include:

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Konrad Zuse's electromechanical "Z machines". The Z3 (1941) was the first

working machine featuring binary arithmetic, including floating point

arithmetic and a measure of programmability. In 1998 the Z3 was proved to

be Turing complete, therefore being the world's first operational computer.

The non-programmable Atanasoff–Berry Computer (1941) which used

vacuum tube based computation, binary numbers, and regenerative capacitor

memory.

The secret British Colossus computer (1944), which had limited

programmability but demonstrated that a device using thousands of tubes

could be reasonably reliable and electronically reprogrammable. It was used

for breaking German wartime codes.

The Harvard Mark I (1944), a large-scale electromechanical computer with

limited programmability.

The U.S. Army's Ballistics Research Laboratory ENIAC (1946), which used

decimal arithmetic and is sometimes called the first general purpose

electronic computer (since Konrad Zuse's Z3 of 1941 used electromagnets

instead of electronics). Initially, however, ENIAC had an inflexible architecture

which essentially required rewiring to change its programming.

Several developers of ENIAC, recognizing its flaws, came up with a far more flexible

and elegant design, which came to be known as the stored program architecture

or von Neumann architecture. This design was first formally described by John von

Neumann in the paper "First Draft of a Report on the EDVAC", published in 1945. A

number of projects to develop computers based on the stored program architecture

commenced around this time, the first of these being completed in Great Britain. The

first to be demonstrated working was the Manchester Small-Scale Experimental

Machine (SSEM) or "Baby". However, the EDSAC, completed a year after SSEM, was

perhaps the first practical implementation of the stored program design. Shortly

thereafter, the machine originally described by von Neumann's paper—EDVAC—was

completed but did not see full-time use for an additional two years.

Nearly all modern computers implement some form of the stored program

architecture, making it the single trait by which the word "computer" is now defined.

By this standard, many earlier devices would no longer be called computers by

today's definition, but are usually referred to as such in their historical context. While

the technologies used in computers have changed dramatically since the first

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electronic, general-purpose computers of the 1940s, most still use the von Neumann

architecture. The design made the universal computer a practical reality.

Microprocessors are miniaturized devices that often implement stored program CPUs.

Vacuum tube-based computers were in use throughout the 1950s, but were largely

replaced in the 1960s by transistor-based devices, which were smaller, faster,

cheaper, used less power and were more reliable. These factors allowed computers

to be produced on an unprecedented commercial scale. By the 1970s, the adoption

of integrated circuit technology and the subsequent creation of microprocessors such

as the Intel 4004 caused another leap in size, speed, cost and reliability. By the

1980s, computers had become sufficiently small and cheap to replace simple

mechanical controls in domestic appliances such as washing machines. Around the

same time, computers became widely accessible for personal use by individuals in

the form of home computers and the now ubiquitous personal computer. In

conjunction with the widespread growth of the Internet since the 1990s, personal

computers are becoming as common as the television and the telephone and almost

all modern electronic devices contain a computer of some kind.

Stored program architecture

The defining feature of modern computers which distinguishes them from all other

machines is that they can be programmed. That is to say that a list of instructions

(the program) can be given to the computer and it will store them and carry them

out at some time in the future.

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In most cases, computer instructions are simple: add one number to another, move

some data from one location to another, send a message to some external device,

etc. These instructions are read from the computer's memory and are generally

carried out (executed) in the order they were given. However, there are usually

specialized instructions to tell the computer to jump ahead or backwards to some

other place in the program and to carry on executing from there. These are called

"jump" instructions (or branches). Furthermore, jump instructions may be made to

happen conditionally so that different sequences of instructions may be used

depending on the result of some previous calculation or some external event. Many

computers directly support subroutines by providing a type of jump that

"remembers" the location it jumped from and another instruction to return to the

instruction following that jump instruction.

Program execution might be likened to reading a book. While a person will normally

read each word and line in sequence, they may at times jump back to an earlier

place in the text or skip sections that are not of interest. Similarly, a computer may

sometimes go back and repeat the instructions in some section of the program over

and over again until some internal condition is met. This is called the flow of control

within the program and it is what allows the computer to perform tasks repeatedly

without human intervention.

Comparatively, a person using a pocket calculator can perform a basic arithmetic

operation such as adding two numbers with just a few button presses. But to add

together all of the numbers from 1 to 1,000 would take thousands of button presses

and a lot of time—with a near certainty of making a mistake. On the other hand, a

computer may be programmed to do this with just a few simple instructions. For

example:

mov #0,sum ; set sum to 0

mov #1,num ; set num to 1

loop: add num,sum ; add num to sum

add #1,num ; add 1 to num

cmp num,#1000 ; compare num to 1000

ble loop ; if num <= 1000, go back to 'loop'

halt ; end of program. stop running

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Once told to run this program, the computer will perform the repetitive addition task

without further human intervention. It will almost never make a mistake and a

modern PC can complete the task in about a millionth of a second.

However, computers cannot "think" for themselves in the sense that they only solve

problems in exactly the way they are programmed to. An intelligent human faced

with the above addition task might soon realize that instead of actually adding up all

the numbers, one can simply use the equation and arrive at the correct answer

(500,500) with little work.. In other words, a computer programmed to add up the

numbers one by one as in the example above would do exactly that without regard

to efficiency or alternative solutions.

Programs

A 1970s punched card containing one line from a FORTRAN program. The card reads:

"Z(1) = Y + W(1)" and is labelled "PROJ039" for identification purposes.

In practical terms, a computer program might include anywhere from a dozen

instructions to many millions of instructions for something like a word processor or a

web browser. A typical modern computer can execute billions of instructions every

second and nearly never make a mistake over years of operation.

Large computer programs may take teams of computer programmers years to write

and the probability of the entire program having been written completely in the

manner intended is unlikely. Errors in computer programs are called bugs.

Sometimes bugs are benign and do not affect the usefulness of the program, in other

cases they might cause the program to completely fail (crash), in yet other cases

there may be subtle problems. Sometimes otherwise benign bugs may be used for

malicious intent, creating a security exploit. Bugs are usually not the fault of the

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computer. Since computers merely execute the instructions they are given, bugs are

nearly always the result of programmer error or an oversight made in the program's

design.

In most computers, individual instructions are stored as machine code with each

instruction being given a unique number (its operation code or opcode for short).

The command to add two numbers together would have one opcode, the command

to multiply them would have a different opcode and so on. The simplest computers

are able to perform any of a handful of different instructions, the more complex

computers have several hundred to choose from—each with a unique numerical

code. Since the computer's memory is able to store numbers, it can also store the

instruction codes. This leads to the important fact that entire programs (which are

just lists of instructions) can be represented as lists of numbers and can themselves

be manipulated inside the computer just as if they were numeric data. The

fundamental concept of storing programs in the computer's memory alongside the

data they operate on is the crux of the von Neumann, or stored program,

architecture. In some cases, a computer might store some or all of its program in

memory that is kept separate from the data it operates on. This is called the Harvard

architecture after the Harvard Mark I computer. Modern von Neumann computers

display some traits of the Harvard architecture in their designs, such as in CPU

caches.

While it is possible to write computer programs as long lists of numbers (machine

language) and this technique was used with many early computers,. it is extremely

tedious to do so in practice, especially for complicated programs. Instead, each basic

instruction can be given a short name that is indicative of its function and easy to

remember—a mnemonic such as ADD, SUB, MULT or JUMP. These mnemonics are

collectively known as a computer's assembly language. Converting programs written

in assembly language into something the computer can actually understand

(machine language) is usually done by a computer program called an assembler.

Machine languages and the assembly languages that represent them (collectively

termed low-level programming languages) tend to be unique to a particular type of

computer. For instance, an ARM architecture computer (such as may be found in a

PDA or a hand-held videogame) cannot understand the machine language of an Intel

Pentium or the AMD Athlon 64 computer that might be in a PC.

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Though considerably easier than in machine language, writing long programs in

assembly language is often difficult and error prone. Therefore, most complicated

programs are written in more abstract high-level programming languages that are

able to express the needs of the computer programmer more conveniently (and

thereby help reduce programmer error). High level languages are usually "compiled"

into machine language (or sometimes into assembly language and then into machine

language) using another computer program called a compiler.. Since high level

languages are more abstract than assembly language, it is possible to use different

compilers to translate the same high level language program into the machine

language of many different types of computer. This is part of the means by which

software like video games may be made available for different computer

architectures such as personal computers and various video game consoles.

The task of developing large software systems is an immense intellectual effort. It

has proven, historically, to be very difficult to produce software with an acceptably

high reliability, on a predictable schedule and budget. The academic and professional

discipline of software engineering concentrates specifically on this problem.

Example

A traffic light showing red.

Suppose a computer is being employed to drive a traffic light. A simple stored

program might say:

1. Turn off all of the lights

2. Turn on the red light

3. Wait for sixty seconds

4. Turn off the red light

5. Turn on the green light

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6. Wait for sixty seconds

7. Turn off the green light

8. Turn on the yellow light

9. Wait for two seconds

10. Turn off the yellow light

11. Jump to instruction number (2)

With this set of instructions, the computer would cycle the light continually through

red, green, yellow and back to red again until told to stop running the program.

However, suppose there is a simple on/off switch connected to the computer that is

intended be used to make the light flash red while some maintenance operation is

being performed. The program might then instruct the computer to:

1. Turn off all of the lights

2. Turn on the red light

3. Wait for sixty seconds

4. Turn off the red light

5. Turn on the green light

6. Wait for sixty seconds

7. Turn off the green light

8. Turn on the yellow light

9. Wait for two seconds

10. Turn off the yellow light

11. If the maintenance switch is NOT turned on then jump to instruction number2

12. Turn on the red light

13. Wait for one second

14. Turn off the red light

15. Wait for one second

16. Jump to instruction number 11

In this manner, the computer is either running the instructions from number (2) to

(11) over and over or its running the instructions from (11) down to (16) over and

over, depending on the position of the switch.

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How computers work

A general purpose computer has four main sections: the arithmetic and logic unit

(ALU), the control unit, the memory, and the input and output devices (collectively

termed I/O). These parts are interconnected by busses, often made of 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 central processing unit (CPU). Early

CPUs were composed of many separate components but since the mid-1970s CPUs

have typically been constructed on a single integrated circuit called a

microprocessor.

Control unit

The control unit (often called a control system or central controller) directs the

various components of a computer. It reads and interprets (decodes) instructions in

the program one by one. The control system decodes each instruction and turns it

into a series of control signals that operate the other parts of the computer.. Control

systems in advanced computers may change the order of some instructions so as to

improve performance.

A key component common to all CPUs is the program counter, a special memory cell

(a register) that keeps track of which location in memory the next instruction is to be

read from.

Diagram showing how a particular MIPS architecture instruction would be decoded by the control system.

The control system's function is as follows—note that this is a simplified description

and some of these steps may be performed concurrently or in a different order

depending on the type of CPU:

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1. Read the code for the next instruction from the cell indicated by the program

counter.

2. Decode the numerical code for the instruction into a set of commands or

signals for each of the other systems.

3. Increment the program counter so it points to the next instruction.

4. Read whatever data the instruction requires from cells in memory (or perhaps

from an input device). The location of this required data is typically stored

within the instruction code.

5. Provide the necessary data to an ALU or register.

6. If the instruction requires an ALU or specialized hardware to complete,

instruct the hardware to perform the requested operation.

7. Write the result from the ALU back to a memory location or to a register or

perhaps an output device.

8. Jump back to step (1).

Since the program counter is (conceptually) just another set of memory cells, it can

be changed by calculations done in the ALU. Adding 100 to the program counter

would cause the next instruction to be read from a place 100 locations further down

the program. Instructions that modify the program counter are often known as

"jumps" and allow for loops (instructions that are repeated by the computer) and

often conditional instruction execution (both examples of control flow).

It is noticeable that the sequence of operations that the control unit goes through to

process an instruction is in itself like a short computer program - and indeed, in

some more complex CPU designs, there is another yet smaller computer called a

microsequencer that runs a microcode program that causes all of these events to

happen.

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Arithmetic/logic unit (ALU)

The ALU is capable of performing two classes of operations: arithmetic and logic.

The set of arithmetic operations that a particular ALU supports may be limited to

adding and subtracting or might include multiplying or dividing, trigonometry

functions (sine, cosine, etc) and square roots. Some can only operate on whole

numbers (integers) whilst others use floating point to represent real numbers—albeit

with limited precision. However, any computer that is capable of performing just the

simplest operations can be programmed to break down the more complex operations

into simple steps that it can perform. Therefore, any computer can be programmed

to perform any arithmetic operation—although it will take more time to do so if its

ALU does not directly support the operation. An ALU may also compare numbers and

return boolean truth values (true or false) depending on whether one is equal to,

greater than or less than the other ("is 64 greater than 65?").

Logic operations involve Boolean logic: AND, OR, XOR and NOT. These can be useful

both for creating complicated conditional statements and processing boolean logic.

Superscalar computers contain multiple ALUs so that they can process several

instructions at the same time. Graphics processors and computers with SIMD and

MIMD features often provide ALUs that can perform arithmetic on vectors and

matrices.

Memory

Magnetic core memory was popular main memory for computers through the 1960s

until it was completely replaced by semiconductor memory.

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A computer's memory can be viewed as a list of cells into which numbers can be

placed or read. Each cell has a numbered "address" and can store a single number.

The computer can be instructed to "put the number 123 into the cell numbered

1357" or to "add the number that is in cell 1357 to the number that is in cell 2468

and put the answer into cell 1595". The information stored in memory may represent

practically anything. Letters, numbers, even computer instructions can be placed into

memory with equal ease. Since the CPU does not differentiate between different

types of information, it is up to the software to give significance to what the memory

sees as nothing but a series of numbers.

In almost all modern computers, each memory cell is set up to store binary 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 are required, they are usually stored in two's complement notation. Other

arrangements are possible, but are usually not seen outside of specialized

applications or historical contexts. A computer can store any kind of information in

memory as long as it can be somehow represented in numerical form. Modern

computers have billions or even trillions of bytes of memory.

The CPU contains a special set of memory cells called registers that can be read and

written to much more rapidly than the main memory area. There are typically

between two and one hundred registers depending on the type of CPU. Registers are

used for the most frequently needed data items to avoid having to access main

memory every time data is needed. Since data is constantly being worked on,

reducing the need to access main memory (which is often slow compared to the ALU

and control units) greatly increases the computer's speed.

Computer main memory comes in two principal varieties: random access memory or

RAM and read-only memory or ROM. RAM can be read and written to anytime the

CPU commands it, but ROM is pre-loaded with data and software that never changes,

so the CPU can only read from it. ROM is typically used to store the computer's initial

start-up instructions. In general, the contents of RAM is erased when the power to

the computer is turned off while ROM retains its data indefinitely. In a PC, the ROM

contains a specialized program called the BIOS that orchestrates loading the

computer's operating system from the hard disk drive into RAM whenever the

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computer is turned on or reset. In embedded computers, which frequently do not

have disk drives, all of the software required to perform the task may be stored in

ROM. Software that is stored in ROM is often called firmware because it is notionally

more like hardware than software. Flash memory blurs the distinction between ROM

and RAM by retaining data when turned off but being rewritable like RAM. However,

flash memory is typically much slower than conventional ROM and RAM so its use is

restricted to applications where high speeds are not required.

In more sophisticated computers there may be one or more RAM cache memories

which are slower than registers but faster than main memory. Generally computers

with this sort of cache are designed to move frequently needed data into the cache

automatically, often without the need for any intervention on the programmer's part.

Input/output (I/O)

Hard disks are common I/O devices used with computers.

I/O is the means by which a computer receives information from the outside world

and sends results back. Devices that provide input or output to the computer are

called peripherals. On a typical personal computer, peripherals include input devices

like the keyboard and mouse, and output devices such as the display and printer.

Hard disk drives, floppy disk drives and optical disc drives serve as both input and

output devices. Computer networking is another form of I/O.

Often, I/O devices are complex computers in their own right with their own CPU and

memory. A graphics processing unit might contain fifty or more tiny computers that

perform the calculations necessary to display 3D graphics. Modern desktop

computers contain many smaller computers that assist the main CPU in performing

I/O.

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Multitasking

While a computer may be viewed as running one gigantic program stored in its main

memory, in some systems it is necessary to give the appearance of running several

programs simultaneously. This is achieved by having the computer switch rapidly

between running each program in turn. One means by which this is done is with a

special signal called an interrupt which can periodically cause the computer to stop

executing instructions where it was and do something else instead. By remembering

where it was executing prior to the interrupt, the computer can return to that task

later. If several programs are running "at the same time", then the interrupt

generator might be causing several hundred interrupts per second, causing a

program switch each time. Since modern computers typically execute instructions

several orders of magnitude faster than human perception, it may appear that many

programs are running at the same time even though only one is ever executing in

any given instant. This method of multitasking is sometimes termed "time-sharing"

since each program is allocated a "slice" of time in turn.

Before the era of cheap computers, the principle use for multitasking was to allow

many people to share the same computer.

Seemingly, multitasking would cause a computer that is switching between several

programs to run more slowly - in direct proportion to the number of programs it is

running. However, most programs spend much of their time waiting for slow

input/output devices to complete their tasks. If a program is waiting for the user to

click on the mouse or press a key on the keyboard, then it will not take a "time slice"

until the event it is waiting for has occurred. This frees up time for other programs to

execute so that many programs may be run at the same time without unacceptable

speed loss.

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Multiprocessing

Cray designed many supercomputers that used multiprocessing heavily.

Some computers may divide their work between one or more separate CPUs,

creating a multiprocessing configuration. Traditionally, this technique was utilized

only in large and powerful computers such as supercomputers, mainframe computers

and servers. However, multiprocessor and multi-core (multiple CPUs on a single

integrated circuit) personal and laptop computers have become widely available and

are beginning to see increased usage in lower-end markets as a result.

Supercomputers in particular often have highly unique architectures that differ

significantly from the basic stored-program architecture and from general purpose

computers. They often feature thousands of CPUs, customized high-speed

interconnects, and specialized computing hardware. Such designs tend to be useful

only for specialized tasks due to the large scale of program organization required to

successfully utilize most of a the available resources at once. Supercomputers

usually see usage in large-scale simulation, graphics rendering, and cryptography

applications, as well as with other so-called "embarrassingly parallel" tasks.

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Networking and the Internet

Visualization of a portion of the routes on the Internet.

Computers have been used to coordinate information in multiple locations since the

1950s, with the U.S. military's SAGE system the first large-scale example of such a

system, which led to a number of special-purpose commercial systems like Sabre.

In the 1970s, computer engineers at research institutions throughout the United

States began to link their computers together using telecommunications technology.

This effort was funded by ARPA (now DARPA), and the computer network that it

produced was called the ARPANET. The technologies that made the Arpanet possible

spread and evolved. In time, the network spread beyond academic and military

institutions and became known as the Internet. The emergence of networking

involved a redefinition of the nature and boundaries of the computer. Computer

operating systems and applications were modified to include the ability to define and

access the resources of other computers on the network, such as peripheral devices,

stored information, and the like, as extensions of the resources of an individual

computer. Initially these facilities were available primarily to people working in high-

tech environments, but in the 1990s the spread of applications like e-mail and the

World Wide Web, combined with the development of cheap, fast networking

technologies like Ethernet and ADSL saw computer networking become almost

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ubiquitous. In fact, the number of computers that are networked is growing

phenomenally. A very large proportion of personal computers regularly connect to

the Internet to communicate and receive information. "Wireless" networking, often

utilizing mobile phone networks, has meant networking is becoming increasingly

ubiquitous even in mobile computing environments.

Hardware

The term hardware covers all of those parts of a computer that are tangible objects.

Circuits, displays, power supplies, cables, keyboards, printers and mice are all

hardware.

History of computing hardware

Calculators Antikythera mechanism, Difference Engine, Norden bombsight First Generation

(Mechanical/Electromechanical) Programmable Devices

Jacquard loom, Analytical Engine, Harvard Mark I, Z3

Calculators Atanasoff–Berry Computer, IBM 604, UNIVAC 60, UNIVAC 120 Second Generation (Vacuum

Tubes)

Programmable Devices ENIAC, EDSAC, EDVAC, UNIVAC I, IBM 701, IBM 702, IBM 650, Z22

Mainframes IBM 7090, IBM 7080, System/360, BUNCH Third Generation (Discrete

transistors and SSI, MSI, LSI Integrated circuits) Minicomputer

PDP-8, PDP-11, System/32, System/36

Minicomputer VAX, AS/400

4-bit microcomputer Intel 4004, Intel 4040

8-bit microcomputer

Intel 8008, Intel 8080, Motorola 6800, Motorola 6809, MOS Technology 6502, Zilog Z80

16-bit microcomputer 8088, Zilog Z8000, WDC 65816/65802

32-bit microcomputer 80386, Pentium, 68000, ARM architecture

64-bit microcomputer. x86-64, PowerPC, MIPS, SPARC

Fourth Generation (VLSI integrated circuits)

Embedded computer 8048, 8051

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Personal computer

Desktop computer, Home computer, Laptop computer, Personal digital assistant (PDA), Portable computer, Tablet computer, Wearable computer

Theoretical/experimental

Quantum computer, Chemical computer, DNA computing, Optical computer, Spintronics based computer

Other Hardware Topics

Input Mouse, Keyboard, Joystick, Image scanner

Output Monitor, Printer Peripheral device (Input/output)

Both Floppy disk drive, Hard disk, Optical disc drive, Teleprinter

Short range RS-232, SCSI, PCI, USB Computer busses Long range (Computer

networking) Ethernet, ATM, FDDI

Software

Software refers to parts of the computer which do not have a material form, such

as programs, data, protocols, etc. When software is stored in hardware that cannot

easily be modified (such as BIOS ROM in an IBM PC compatible), it is sometimes

called "firmware" to indicate that it falls into an uncertain area somewhere between

hardware and software.

Computer software

Unix/BSD UNIX System V, AIX, HP-UX, Solaris (SunOS), IRIX, List of BSD operating systems

GNU/Linux List of Linux distributions, Comparison of Linux distributions

Microsoft Windows

Windows 95, Windows 98, Windows NT, Windows XP, Windows Vista, Windows CE

DOS 86-DOS (QDOS), PC-DOS, MS-DOS, FreeDOS

Operating system

Mac OS Mac OS classic, Mac OS X

24

Embedded and real-time

List of embedded operating systems

Experimental Amoeba, Oberon/Bluebottle, Plan 9 from Bell Labs

Multimedia DirectX, OpenGL, OpenAL Library Programming

library C standard library, Standard template library

Protocol TCP/IP, Kermit, FTP, HTTP, SMTP Data

File format HTML, XML, JPEG, MPEG, PNG

Graphical user interface (WIMP)

Microsoft Windows, GNOME, KDE, QNX Photon, CDE, GEM

Text user interface

Command line interface, shells User interface

Other

Office suite

Word processing, Desktop publishing, Presentation program, Database management system, Scheduling & Time management, Spreadsheet, Accounting software

Internet Access Browser, E-mail client, Web server, Mail transfer agent, Instant messaging

Design and manufacturing

Computer-aided design, Computer-aided manufacturing, Plant management, Robotic manufacturing, Supply chain management

Graphics Raster graphics editor, Vector graphics editor, 3D modeler, Animation editor, 3D computer graphics, Video editing, Image processing

Audio Digital audio editor, Audio playback, Mixing, Audio synthesis, Computer music

Software Engineering

Compiler, Assembler, Interpreter, Debugger, Text Editor, Integrated development environment, Performance analysis, Revision control, Software configuration management

Educational Edutainment, Educational game, Serious game, Flight simulator

Games Strategy, Arcade, Puzzle, Simulation, First-person shooter, Platform, Massively multiplayer, Interactive fiction

Application

Misc Artificial intelligence, Antivirus software, Malware scanner, Installer/Package management systems, File manager

25

Programming languages

Programming languages provide various ways of specifying programs for computers

to run. Unlike natural languages, programming languages are designed to permit no

ambiguity and to be concise. They are purely written languages and are often

difficult to read aloud. They are generally either translated into machine language by

a compiler or an assembler before being run, or translated directly at run time by an

interpreter. Sometimes programs are executed by a hybrid method of the two

techniques. There are thousands of different programming languages—some

intended to be general purpose, others useful only for highly specialized applications.

Programming Languages

Lists of programming languages

Timeline of programming languages, Categorical list of programming languages, Generational list of programming languages, Alphabetical list of programming languages, Non-English-based programming languages

Commonly used Assembly languages

ARM, MIPS, x86

Commonly used High level languages

BASIC, C, C++, C#, COBOL, Fortran, Java, Lisp, Pascal

Commonly used Scripting languages

Bourne script, JavaScript, Python, Ruby, PHP, Perl

Professions and organizations

As the use of computers has spread throughout society, there are an increasing

number of careers involving computers. Following the theme of hardware, software

and firmware, the brains of people who work in the industry are sometimes known

irreverently as wetware or "meatware".

Computer-related professions

Hardware-related

Electrical engineering, Electronics engineering, Computer engineering, Telecommunications engineering, Optical engineering, Nanoscale engineering

Software-related

Human-computer interaction, Information technology, Software engineering, Scientific computing, Web design, Desktop publishing

26

The need for computers to work well together and to be able to exchange

information has spawned the need for many standards organizations, clubs and

societies of both a formal and informal nature.

Organizations

Standards groups ANSI, IEC, IEEE, IETF, ISO, W3C

Professional Societies ACM, ACM Special Interest Groups, IET, IFIP

Free/Open source software groups

Free Software Foundation, Mozilla Foundation, Apache Software Foundation

Notes

1. ^ In 1946, ENIAC consumed an estimated 174 kW. By comparison, a typical

personal computer may use around 400 W; over four hundred times less.

(Kempf 1961)

2. ^ Early computers such as Colossus and ENIAC were able to process between

5 and 100 operations per second. A modern "commodity" microprocessor (as

of 2007) can process billions of operations per second, and many of these

operations are more complicated and useful than early computer operations.

3. ^ The Analytical Engine should not be confused with Babbage's difference

engine which was a non-programmable mechanical calculator.

4. ^ This program was written similarly to those for the PDP-11 minicomputer

and shows some typical things a computer can do. All the text after the

semicolons are comments for the benefit of human readers. These have no

significance to the computer and are ignored. (Digital Equipment Corporation

1972)

5. ^ Attempts are often made to create programs that can overcome this

fundamental limitation of computers. Software that mimics learning and

adaptation is part of artificial intelligence.

6. ^ It is not universally true that bugs are solely due to programmer oversight.

Computer hardware may fail or may itself have a fundamental problem that

produces unexpected results in certain situations. For instance, the Pentium

FDIV bug caused some Intel microprocessors in the early 1990s to produce

inaccurate results for certain floating point division operations. This was

caused by a flaw in the microprocessor design and resulted in a partial recall

of the affected devices.

27

7. ^ Even some later computers were commonly programmed directly in

machine code. Some minicomputers like the DEC PDP-8 could be

programmed directly from a panel of switches. However, this method was

usually used only as part of the booting process. Most modern computers

boot entirely automatically by reading a boot program from some non-volatile

memory.

8. ^ However, there is sometimes some form of machine language compatibility

between different computers. An x86-64 compatible microprocessor like the

AMD Athlon 64 is able to run most of the same programs that an Intel Core 2

microprocessor can, as well as programs designed for earlier microprocessors

like the Intel Pentiums and Intel 80486. This contrasts with very early

commercial computers, which were often one-of-a-kind and totally

incompatible with other computers.

9. ^ High level languages are also often interpreted rather than compiled.

Interpreted languages are translated into machine code on the fly by another

program called an interpreter.

10. ̂ Although this is a simple program, it contains a software bug. If the traffic

signal is showing red when someone switches the "flash red" switch, it will

cycle through green once more before starting to flash red as instructed. This

bug is quite easy to fix by changing the program to repeatedly test the switch

throughout each "wait" period—but writing large programs that have no bugs

is exceedingly difficult.

11. ̂ The control unit's rule in interpreting instructions has varied somewhat in

the past. While the control unit is solely responsible for instruction

interpretation in most modern computers, this is not always the case. Many

computers include some instructions that may only be partially interpreted by

the control system and partially interpreted by another device. This is

especially the case with specialized computing hardware that may be partially

self-contained. For example, EDVAC, the first modern stored program

computer to be designed, used a central control unit that only interpreted

four instructions. All of the arithmetic-related instructions were passed on to

its arithmetic unit and further decoded there.

12. ̂ Instructions often occupy more than one memory address, so the program

counters usually increases by the number of memory locations required to

store one instruction.

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13. ̂ Flash memory also may only be rewritten a limited number of times before

wearing out, making it less useful for heavy random access usage. (Verma

1988)

14. ̂ However, it is also very common to construct supercomputers out of many

pieces of cheap commodity hardware; usually individual computers connected

by networks. These so-called computer clusters can often provide

supercomputer performance at a much lower cost than customized designs.

While custom architectures are still used for most of the most powerful

supercomputers, there has been a proliferation of cluster computers in recent

years. (TOP500 2006)

15. ̂ Most major 64-bit instruction set architectures are extensions of earlier

designs. All of the architectures listed in this table existed in 32-bit forms

before their 64-bit incarnations were introduced.

29

Operating system

An operating system (OS) is the software that manages the sharing of the

resources of a computer and provides programmers with an interface used to access

those resources. An operating system processes system data and user input, and

responds by allocating and managing tasks and internal system resources as a

service to users and programs of the system. At the foundation of all system

software, an operating system performs basic tasks such as controlling and

allocating memory, prioritizing system requests, controlling input and output devices,

facilitating networking and managing file systems. Most operating systems come with

an application that provides a user interface for managing the operating system,

such as a command line interpreter or graphical user interface. The operating system

forms a platform for other system software and for application software.

The most commonly-used contemporary desktop and laptop (notebook) OS is

Microsoft Windows. More powerful servers often employ Linux, FreeBSD, and other

Unix-like systems. However, these operating systems, especially Mac OS X, are also

used on personal computers.

Services

Process management

Every program running on a computer, be it a service or an application, is a process.

As long as a von Neumann architecture is used to build computers, only one process

per CPU can be run at a time. Older microcomputer OSes such as MS-DOS did not

attempt to bypass this limit, with the exception of interrupt processing, and only one

process could be run under them (although DOS itself featured TSR as a very partial

and not too easy to use solution).

Most operating systems enable concurrent execution of many processes and

programs at once via multitasking, even with one CPU. The mechanism was used in

mainframes since the early 1960s, but in the personal computers it became available

in 1990s. Process management is an operating system's way of dealing with running

30

those multiple processes. On the most fundamental of computers (those containing

one processor with one core) multitasking is done by simply switching processes

quickly. Depending on the operating system, as more processes run, either each

time slice will become smaller or there will be a longer delay before each process is

given a chance to run. Process management involves computing and distributing CPU

time as well as other resources. Most operating systems allow a process to be

assigned a priority which affects its allocation of CPU time. Interactive operating

systems also employ some level of feedback in which the task with which the user is

working receives higher priority. Interrupt driven processes will normally run at a

very high priority. In many systems there is a background process, such as the

System Idle Process in Windows, which will run when no other process is waiting for

the CPU.

Memory management

Current computer architectures arrange the computer's memory in a hierarchical

manner, starting from the fastest registers, CPU cache, random access memory and

disk storage. An operating system's memory manager coordinates the use of these

various types of memory by tracking which one is available, which is to be allocated

or deallocated and how to move data between them. This activity, usually referred to

as virtual memory management, increases the amount of memory available for each

process by making the disk storage seem like main memory. There is a speed

penalty associated with using disks or other slower storage as memory – if running

processes require significantly more RAM than is available, the system may start

thrashing. This can happen either because one process requires a large amount of

RAM or because two or more processes compete for a larger amount of memory than

is available. This then leads to constant transfer of each process's data to slower

storage.

Another important part of memory management is managing virtual addresses. If

multiple processes are in memory at once, they must be prevented from interfering

with each other's memory (unless there is an explicit request to utilise shared

memory). This is achieved by having separate address spaces. Each process sees the

whole virtual address space, typically from address 0 up to the maximum size of

virtual memory, as uniquely assigned to it. The operating system maintains a page

table that match virtual addresses to physical addresses. These memory allocations

31

are tracked so that when a process terminates, all memory used by that process can

be made available for other processes.

The operating system can also write inactive memory pages to secondary storage.

This process is called "paging" or "swapping" – the terminology varies between

operating systems.

It is also typical for operating systems to employ otherwise unused physical memory

as a page cache; requests for data from a slower device can be retained in memory

to improve performance. The operating system can also pre-load the in-memory

cache with data that may be requested by the user in the near future; SuperFetch is

an example of this.

Disk and file systems

Generally, operating systems include support for file systems.

Modern file systems comprise a hierarchy of directories. While the idea is

conceptually similar across all general-purpose file systems, some differences in

implementation exist. Two noticeable examples of this are the character used to

separate directories, and case sensitivity.

Unix demarcates its path components with a slash (/), a convention followed by

operating systems that emulated it or at least its concept of hierarchical directories,

such as Linux, Amiga OS and Mac OS X. MS-DOS also emulated this feature, but had

already also adopted the CP/M convention of using slashes for additional options to

commands, so instead used the backslash (\) as its component separator. Microsoft

Windows continues with this convention; Japanese editions of Windows use ¥, and

Korean editions use ₩. Prior to Mac OS X, versions of Mac OS use a colon (:) for a

path separator. RISC OS uses a period (.).

Unix and Unix-like operating systems allow for any character in file names other than

the slash and NUL characters (including line feed (LF) and other control characters).

Unix file names are case sensitive, which allows multiple files to be created with

names that differ only in case. By contrast, Microsoft Windows file names are not

case sensitive by default. Windows also has a larger set of punctuation characters

that are not allowed in file names.

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File systems may provide journaling, which provides safe recovery in the event of a

system crash. A journaled file system writes information twice: first to the journal,

which is a log of file system operations, then to its proper place in the ordinary file

system. In the event of a crash, the system can recover to a consistent state by

replaying a portion of the journal. In contrast, non-journaled file systems typically

need to be examined in their entirety by a utility such as fsck or chkdsk. Soft

updates is an alternative to journalling that avoids the redundant writes by carefully

ordering the update operations. Log-structured file systems and ZFS also differ from

traditional journaled file systems in that they avoid inconsistencies by always writing

new copies of the data, eschewing in-place updates.

Many Linux distributions support some or all of ext2, ext3, ReiserFS, Reiser4, GFS,

GFS2, OCFS, OCFS2, and NILFS. Linux also has full support for XFS and JFS, along

with the FAT file systems, and NTFS.

Microsoft Windows includes support for FAT12, FAT16, FAT32, and NTFS. The NTFS

file system is the most efficient and reliable of the four Windows file systems, and as

of Windows Vista, is the only file system which the operating system can be installed

on. Windows Embedded CE 6.0 introduced ExFAT, a file system suitable for flash

drives.

Mac OS X supports HFS+ with journaling as its primary file system. It is derived from

the Hierarchical File System of the earlier Mac OS. Mac OS X has facilities to read

and write FAT16, FAT32, NTFS and other file systems, but cannot be installed to

them.

Common to all these (and other) operating systems is support for file systems

typically found on removable media. FAT12 is the file system most commonly found

on floppy discs. ISO 9660 and Universal Disk Format are two common formats that

target Compact Discs and DVDs, respectively. Mount Rainier is a newer extension to

UDF supported by Linux 2.6 kernels and Windows Vista that facilitates rewriting to

DVDs in the same fashion as has been possible with floppy disks.

Networking

Most current operating systems are capable of using the TCP/IP networking

protocols. This means that computers running dissimilar operating systems can

33

participate in a common network for sharing resources such as computing, files,

printers, and scanners using either wired or wireless connections.

Many operating systems also support one or more vendor-specific legacy networking

protocols as well, for example, SNA on IBM systems, DECnet on systems from Digital

Equipment Corporation, and Microsoft-specific protocols on Windows. Specific

protocols for specific tasks may also be supported such as NFS for file access.

Security

Many operating systems include some level of security. Security is based on the two

ideas that:

The operating system provides access to a number of resources, directly or

indirectly, such as files on a local disk, privileged system calls, personal

information about users, and the services offered by the programs running on

the system;

The operating system is capable of distinguishing between some requesters of

these resources who are authorized (allowed) to access the resource, and

others who are not authorized (forbidden). While some systems may simply

distinguish between "privileged" and "non-privileged", systems commonly

have a form of requester identity, such as a user name. Requesters, in turn,

divide into two categories:

o Internal security: an already running program. On some systems, a

program once it is running has no limitations, but commonly the

program has an identity which it keeps and is used to check all of its

requests for resources.

o External security: a new request from outside the computer, such as a

login at a connected console or some kind of network connection. To

establish identity there may be a process of authentication. Often a

username must be quoted, and each username may have a password.

Other methods of authentication, such as magnetic cards or biometric

data, might be used instead. In some cases, especially connections

from the network, resources may be accessed with no authentication

at all.

34

In addition to the allow/disallow model of security, a system with a high level of

security will also offer auditing options. These would allow tracking of requests for

access to resources (such as, "who has been reading this file?").

Security of operating systems has long been a concern because of highly sensitive

data held on computers, both of a commercial and military nature. The United States

Government Department of Defense (DoD) created the Trusted Computer System

Evaluation Criteria (TCSEC) which is a standard that sets basic requirements for

assessing the effectiveness of security. This became of vital importance to operating

system makers, because the TCSEC was used to evaluate, classify and select

computer systems being considered for the processing, storage and retrieval of

sensitive or classified information.

In December 2007, Apple Inc. released a security update for Mac OS X and Mac OS X

Server to address vulnerabilities in Flash and Shockwave, products of Adobe

Systems, and Tar, a GNU utility. Among the problems addressed are the quite

ordinary ability to "execute arbitrary code" and to "gain access to sensitive

information" or "cause a denial of service". More surprisingly, the update addressed

attackers abilitiy to "surreptitiously initiate a video conference".

Internal security

Internal security can be thought of as protecting the computer's resources from the

programs concurrently running on the system. Most operating systems set programs

running natively on the computer's processor, so the problem arises of how to stop

these programs doing the same task and having the same privileges as the operating

system (which is after all just a program too). Processors used for general purpose

operating systems generally have a hardware concept of privilege. Generally less

privileged programs are automatically blocked from using certain hardware

instructions, such as those to read or write from external devices like disks. Instead,

they have to ask the privileged program (operating system kernel) to read or write.

The operating system therefore gets the chance to check the program's identity and

allow or refuse the request.

An alternative strategy, and the only sandbox strategy available in systems that do

not meet the Popek and Goldberg virtualization requirements, is the operating

35

system not running user programs as native code, but instead either emulates a

processor or provides a host for a p-code based system such as Java.

Internal security is especially relevant for multi-user systems; it allows each user of

the system to have private files that the other users cannot tamper with or read.

Internal security is also vital if auditing is to be of any use, since a program can

potentially bypass the operating system, inclusive of bypassing auditing.

External security

Typically an operating system offers (or hosts) various services to other network

computers and users. These services are usually provided through ports or

numbered access points beyond the operating systems network address. Services

include offerings such as file sharing, print services, email, web sites, and file

transfer protocols (FTP), most of which can have compromised security.

At the front line of security are hardware devices known as firewalls or intrusion

detection/prevention systems. At the operating system level, there are a number of

software firewalls available, as well as intrusion detection/prevention systems. Most

modern operating systems include a software firewall, which is enabled by default. A

software firewall can be configured to allow or deny network traffic to or from a

service or application running on the operating system. Therefore, one can install

and be running an insecure service, such as Telnet or FTP, and not have to be

threatened by a security breach because the firewall would deny all traffic trying to

connect to the service on that port.

Graphical user interfaces

Today, most modern operating systems contain Graphical User Interfaces. A few

older operating systems tightly integrated the GUI into the kernel—for example, in

the original implementations of Microsoft Windows and Mac OS, the graphical

subsystem was actually part of the kernel. More modern operating systems are

modular, separating the graphics subsystem from the kernel (as is now done in

Linux, and Mac OS X).

36

Many operating systems allow the user to install or create any user interface they

desire. The X Window System in conjunction with GNOME or KDE is a commonly

found setup on most Unix and Unix-like (BSD, Linux, Minix) systems.

Graphical user interfaces evolve over time. For example, Windows has modified its

user interface almost every time a new major version of Windows is released, and

the Mac OS GUI changed dramatically with the introduction of Mac OS X in 2001.

Device drivers

A device driver is a specific type of computer software developed to allow interaction

with hardware devices. Typically this constitutes an interface for communicating with

the device, through the specific computer bus or communications subsystem that the

hardware is connected to, providing commands to and/or receiving data from the

device, and on the other end, the requisite interfaces to the operating system and

software applications. It is a specialized hardware-dependent computer program

which is also operating system specific that enables another program, typically an

operating system or applications software package or computer program running

under the operating system kernel, to interact transparently with a hardware device,

and usually provides the requisite interrupt handling necessary for any necessary

asynchronous time-dependent hardware interfacing needs.

The key design goal of device drivers is abstraction. Every model of hardware (even

within the same class of device) is different. Newer models also are released by

manufacturers that provide more reliable or better performance and these newer

models are often controlled differently. Computers and their operating systems

cannot be expected to know how to control every device, both now and in the future.

To solve this problem, OSes essentially dictate how every type of device should be

controlled. The function of the device driver is then to translate these OS mandated

function calls into device specific calls. In theory a new device, which is controlled in

a new manner, should function correctly if a suitable driver is available. This new

driver will ensure that the device appears to operate as usual from the operating

systems' point of view for any person.

37

History

The first computers did not have operating systems. By the early 1960s, commercial

computer vendors were supplying quite extensive tools for streamlining the

development, scheduling, and execution of jobs on batch processing systems.

Examples were produced by UNIVAC and Control Data Corporation, amongst others.

Mainframes

Through the 1960s, many major features were pioneered in the field of operating

systems. The development of the IBM System/360 produced a family of mainframe

computers available in widely differing capacities and price points, for which a single

operating system OS/360 was planned (rather than developing ad-hoc programs for

every individual model). This concept of a single OS spanning an entire product line

was crucial for the success of System/360 and, in fact, IBM's current mainframe

operating systems are distant descendants of this original system; applications

written for the OS/360 can still be run on modern machines. OS/360 also contained

another important advance: the development of the hard disk permanent storage

device (which IBM called DASD).

Control Data Corporation developed the SCOPE operating system in the 1960s, for

batch processing. In cooperation with the University of Minnesota, the KRONOS and

later the NOS operating systems were developed during the 1970s, which supported

simultaneous batch and timesharing use. Like many commercial timesharing

systems, its interface was an extension of the Dartmouth BASIC operating systems,

one of the pioneering efforts in timesharing and programming languages. In the late

1970s, Control Data and the University of Illinois developed the PLATO operating

system, which used plasma panel displays and long-distance time sharing networks.

Plato was remarkably innovative for its time, featuring real-time chat, and multi-user

graphical games.

Burroughs Corporation introduced the B5000 in 1961 with the MCP, (Master Control

Program) operating system. The B5000 was a stack machine designed to exclusively

support high-level languages with no machine language or assembler and indeed the

MCP was the first OS to be written exclusively in a high-level language (ESPOL, a

dialect of ALGOL). MCP also introduced many other ground-breaking innovations,

38

such as being the first commercial implementation of virtual memory. MCP is still in

use today in the Unisys ClearPath/MCP line of computers.

UNIVAC, the first commercial computer manufacturer, produced a series of EXEC

operating systems. Like all early main-frame systems, this was a batch-oriented

system that managed magnetic drums, disks, card readers and line printers. In the

1970s, UNIVAC produced the Real-Time Basic (RTB) system to support large-scale

time sharing, also patterned after the Dartmouth BASIC system.

General Electric and MIT developed General Electric Comprehensive Operating

Supervisor (GECOS), which introduced the concept of ringed security privilege levels.

After acquisition by Honeywell it was renamed to General Comprehensive Operating

System (GCOS).

Digital Equipment Corporation developed many operating systems for its various

computer lines, including TOPS-10 and TOPS-20 time sharing systems for the 36-bit

PDP-10 class systems. Prior to the widespread use of UNIX, TOPS-10 was a

particularly popular system in universities, and in the early ARPANET community.

In the late 1960s through the late 1970s, several hardware capabilities evolved that

allowed similar or ported software to run on more than one system. Early systems

had utilized microprogramming to implement features on their systems in order to

permit different underlying architecture to appear to be the same as others in a

series. In fact most 360's after the 360/40 (except the 360/165 and 360/168) were

microprogrammed implementations. But soon other means of achieving application

compatibility were proven to be more significant.

The enormous investment in software for these systems made since 1960s caused

most of the original computer manufacturers to continue to develop compatible

operating systems along with the hardware. The notable supported mainframe

operating systems include:

Burroughs MCP -- B5000,1961 to Unisys Clearpath/MCP, present.

IBM OS/360 -- IBM System/360, 1964 to IBM z/OS, present.

IBM CP-67 -- IBM System/360, 1967 to IBM z/VM, present.

UNIVAC EXEC 8 -- UNIVAC 1108, 1964, to Unisys Clearpath IX, present.

39

Microcomputers

The first microcomputers did not have the capacity or need for the elaborate

operating systems that had been developed for mainframes and minis; minimalistic

operating systems were developed, often loaded from ROM and known as Monitors.

One notable early disk-based operating system was CP/M, which was supported on

many early microcomputers and was closely imitated in MS-DOS, which became

wildly popular as the operating system chosen for the IBM PC (IBM's version of it

was called IBM-DOS or PC-DOS), its successors making Microsoft one of the world's

most profitable companies. In the 80's Apple Computer Inc. (now Apple Inc.)

abandoned its popular Apple II series of microcomputers to introduce the Apple

Macintosh computer with the an innovative Graphical User Interface (GUI) to the Mac

OS.

The introduction of the Intel 80386 CPU chip with 32-bit architecture and paging

capabilities, provided personal computers with the ability to run multitasking

operating systems like those of earlier minicomputers and mainframes. Microsoft's

responded to this progress by hiring Dave Cutler, who had developed the VMS

operating system for Digital Equipment Corporation. He would lead the development

of the Windows NT operating system, which continues to serve as the basis for

Microsoft's operating systems line. Steve Jobs, a co-founder of Apple, started NeXT

Computer Inc., which developed the Unix-like NEXTSTEP operating system.

NEXTSTEP would later be acquired by Apple Inc. and used, along with code from

FreeBSD as the core of Mac OS X.

Minix, an academic teaching tool which could be run on early PCs, would inspire

another reimplementation of Unix, called Linux. Started by computer student Linus

Torvalds with cooperation from volunteers over the internet, developed a kernel

which was combined with the tools from the GNU Project. The Berkeley Software

Distribution, known as BSD, is the UNIX derivative distributed by the University of

California, Berkeley, starting in the 1970s. Freely distributed and ported to many

minicomputers, it eventually also gained a following for use on PCs, mainly as

FreeBSD, NetBSD and OpenBSD.

40

Unix-like operating systems

A customized KDE desktop running under Linux.

The Unix-like family is a diverse group of operating systems, with several major sub-

categories including System V, BSD, and Linux. The name "UNIX" is a trademark of

The Open Group which licenses it for use with any operating system that has been

shown to conform to their definitions. "Unix-like" is commonly used to refer to the

large set of operating systems which resemble the original Unix.

Unix systems run on a wide variety of machine architectures. They are used heavily

as server systems in business, as well as workstations in academic and engineering

environments. Free software Unix variants, such as Linux and BSD, are popular in

these areas. The market share for Linux is divided between many different

distributions. Enterprise class distributions by Red Hat or SuSe are used by

corporations, but some home users may use those products. Historically home users

typically installed a distribution themselves, but in 2007 Dell began to offer the

Ubuntu Linux distribution on home PCs. Linux on the desktop is also popular in the

developer and hobbyist operating system development communities.

Market share statistics for freely available operating systems are usually inaccurate

since most free operating systems are not purchased, making usage under-

represented. On the other hand, market share statistics based on total downloads of

free operating systems are often inflated, as there is no economic disincentive to

acquire multiple operating systems so users can download multiple, test them, and

decide which they like best.

Some Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that

vendor's hardware. Others, such as Solaris, can run on multiple types of hardware,

including x86 servers and PCs. Apple's Mac OS X, a hybrid kernel-based BSD variant

41

derived from NeXTSTEP, Mach, and FreeBSD, has replaced Apple's earlier (non-Unix)

Mac OS.

Unix interoperability was sought by establishing the POSIX standard. The POSIX

standard can be applied to any operating system, although it was originally created

for various unix varients.

Open source

Over the past several years, the trend in the Unix and Unix-like space has been to

freely provide the software programming code. This process, is called open source,

although there was initially confusion as "open" previously meant sharing code and

standards only with the representative of select companies.

Ken Thompson, Dennis Ritchie and Douglas McIlroy at Bell Labs designed and

developed the C programming language to build the operating system Unix.

Programmers at Bell Labs went on to develop Plan 9 and Inferno, which were

engineered for modern distributed environments. They had graphics built-in, unlike

Unix counterparts that added it to the design later. Plan 9 did not become popular

because, unlike many Unix distributions, it was not originally free. It has since been

released under Free Software and Open Source Lucent Public License, and has an

expanding community of developers. Inferno was sold to Vita Nuova Holdings and

has been released under a GPL/MIT license.

Mac OS X

Mac OS X is a line of proprietary, graphical operating systems developed, marketed,

and sold by Apple Inc., the latest of which is pre-loaded on all currently shipping

Macintosh computers. Mac OS X is the successor to the original Mac OS, which had

been Apple's primary operating system since 1984. Unlike its predecessor, Mac OS X

is a UNIX operating system built on technology that had been developed at NeXT

through the second half of the 1980s and up until Apple purchased the company in

early 1997.

The operating system was first released in 1999 as Mac OS X Server 1.0, with a

desktop-oriented version (Mac OS X v10.0) following in March 2001. Since then, five

more distinct "end-user" and "server" editions of Mac OS X have been released, the

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most recent being Mac OS X v10.5, which was first made available in October 2007.

Releases of Mac OS X are named after big cats; Mac OS X v10.5 is usually referred

to by Apple and users as "Leopard".

The server edition, Mac OS X Server, is architecturally identical to its desktop

counterpart but usually runs on Apple's line of Macintosh server hardware. Mac OS X

Server includes workgroup management and administration software tools that

provide simplified access to key network services, including a mail transfer agent, a

Samba server, an LDAP server, a domain name server, and others.

Microsoft Windows

The Microsoft Windows family of operating systems originated as an add-on to the

older MS-DOS environment for the IBM PC. Modern versions are based on the newer

Windows NT core that was originally intended for OS/2 and borrowed from VMS.

Windows runs on x86, x86-64 and Itanium processors. Earlier versions also ran on

the DEC Alpha, MIPS, Fairchild (later Intergraph) Clipper and PowerPC architectures

(some work was done to port it to the SPARC architecture).

As of September 2007, Microsoft Windows holds a large amount of the worldwide

desktop market share. Windows is also used on servers, supporting applications such

as web servers and database servers. In recent years, Microsoft has spent significant

marketing and research & development money to demonstrate that Windows is

capable of running any enterprise application, which has resulted in consistent

price/performance records (see the TPC) and significant acceptance in the enterprise

market.

The most widely used version of the Microsoft Windows family is Windows XP,

released on October 25, 2001.

In November 2006, after more than five years of development work, Microsoft

released Windows Vista, a major new version of Microsoft Windows which contains a

large number of new features and architectural changes. Chief amongst these are a

new user interface and visual style called Windows Aero, a number of new security

features such as User Account Control, and new multimedia applications such as

Windows DVD Maker.

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Microsoft has announced a new version codenamed Windows 7 will be released in

2010 or later.

Embedded systems

Embedded systems use a variety of dedicated operating systems. In some cases, the

"operating system" software is directly linked to the application to produce a

monolithic special-purpose program. In the simplest embedded systems, there is no

distinction between the OS and the application. Embedded systems that have certain

time requirements are known as real-time operating systems.

Operating systems such as VxWorks, eCos, and Palm OS, are unrelated to Unix and

Windows. Windows CE is descendant of Windows, and several embedded BSD and

Linux distributions exist.

Hobby operating system development

Operating system development, or OSDev for short, as a hobby has a large cult

following. As such, operating systems, such as Linux, have derived from hobby

operating system projects. The design and implementation of an operating system

requires skill and determination, and the term can cover anything from a basic "Hello

World" boot loader to a fully featured kernel. One classical example of this is the

Minix Operating System -- an OS that was designed as a teaching tool but was

heavily used by hobbyists before Linux eclipsed it in popularity.

Other

Older operating systems which are still used in niche markets include OS/2 from

IBM; Mac OS, the non-Unix precursor to Apple's Mac OS X; BeOS; XTS-300. Some,

most notably AmigaOS and RISC OS, continue to be developed as minority platforms

for enthusiast communities and specialist applications. OpenVMS formerly from DEC,

is still under active development by Hewlett-Packard.

Research and development of new operating systems continues. GNU Hurd is

designed to be backwards compatible with Unix, but with enhanced functionality and

a microkernel architecture. Singularity is a project at Microsoft Research to develop

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an operating system with better memory protection based on the .Net managed code

model.

45

Computer networking

Computer networking is the engineering discipline concerned with communication

between computer systems or devices. Networking, routers, routing protocols, and

networking over the public Internet have their specifications defined in documents

called RFCs. Computer networking is sometimes considered a sub-discipline of

telecommunications, computer science, information technology and/or computer

engineering. Computer networks rely heavily upon the theoretical and practical

application of these scientific and engineering disciplines.

A computer network is any set of computers or devices connected to each other with

the ability to exchange data. Examples of networks are:

local area network (LAN), which is usually a small network constrained to a

small geographic area.

wide area network (WAN) that is usually a larger network that covers a large

geographic area.

wireless LANs and WANs (WLAN & WWAN) is the wireless equivalent of the

LAN and WAN

All networks are interconnected to allow communication with a varity of different

kinds of media, which including twisted-pair copper wire cable, coaxial cable, optical

fiber, and various wireless technologies. The devices can be separated by a few

meters (e.g. via Bluetooth) or nearly unlimited distances (e.g. via the

interconnections of the Internet).

Views of networks

Users and network administrators often have different views of their networks.

Often, users that share printers and some servers form a workgroup, which usually

46

means they are in the same geographic location and are on the same LAN. A

community of interest has less of a connotation of being in a local area, and should

be thought of as a set of arbitrarily located users who share a set of servers, and

possibly also communicate via peer-to-peer technologies.

Network administrators see networks from both physical and logical perspectives.

The physical perspective involves geographic locations, physical cabling, and the

network elements (e.g., routers, bridges and application layer gateways that

interconnect the physical media. Logical networks, called, in the TCP/IP architecture,

subnets , map onto one or more physical media. For example, a common practice in

a campus of buildings is to make a set of LAN cables in each building appear to be a

common subnet, using virtual LAN (VLAN) technology.

Both users and administrators will be aware, to varying extents, of the trust and

scope characteristics of a network. Again using TCP/IP architectural terminology, an

intranet is a community of interest under private administration usually by an

enterprise, and is only accessible by authorized users (e.g. employees) (RFC 2547).

Intranets do not have to be connected to the Internet, but generally have a limited

connection. An extranet is an extension of an intranet that allows secure

communications to users outside of the intranet (e.g. business partners,

customers)RFC 3547.

Informally, the Internet is the set of users, enterprises, and content providers that

are interconnected by Internet Service Providers (ISP). From an engineering

standpoint, the Internet is the set of subnets, and aggregates of subnets, which

share the registered IP address space and exchange information about the

reachability of those IP addresses using the Border Gateway Protocol. Typically, the

human-readable names of servers are translated to IP addresses, transparently to

users, via the directory function of the Domain Name System (DNS).

Over the Internet, there can be business-to-business (B2B), business-to-consumer

(B2C) and consumer-to-consumer (C2C) communications. Especially when money or

sensitive information is exchanged, the communications are apt to be secured by

some form of communications security mechanism. Intranets and extranets can be

securely superimposed onto the Internet, without any access by general Internet

users, using secure Virtual Private Network (VPN) technology.

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History

Before the advent of computer networks that were based upon some type of

telecommunications system, communication between calculation machines and early

computers was performed by human users by carrying instructions between them.

Many of the social behavior seen in today's Internet was demonstrably present in

nineteenth-century telegraph networks, and arguably in even earlier networks using

visual signals.

In September 1940 George Stibitz used a teletype machine to send instructions for a

problem set from his Model K at Dartmouth College in New Hampshire to his

Complex Number Calculator in New York and received results back by the same

means. Linking output systems like teletypes to computers was an interest at the

Advanced Research Projects Agency (ARPA) when, in 1962, J.C.R. Licklider was hired

and developed a working group he called the "Intergalactic Network", a precursor to

the ARPANet.

In 1964, researchers at Dartmouth developed the Dartmouth Time Sharing System

for distributed users of large computer systems. The same year, at MIT, a research

group supported by General Electric and Bell Labs used a computer (DEC's PDP-8) to

route and manage telephone connections.

Throughout the 1960s Leonard Kleinrock, Paul Baran and Donald Davies

independently conceptualized and developed network systems which used datagrams

or packets that could be used in a packet switched network between computer

systems.

The first widely used PSTN switch that used true computer control was the Western

Electric 1ESS switch, introduced in 1965.

In 1969 the University of California at Los Angeles, SRI (in Stanford), University of

California at Santa Barbara, and the University of Utah were connected as the

beginning of the ARPANet network using 50 kbit/s circuits. Commercial services

using X.25, an alternative architecture to the TCP/IP suite, were deployed in 1972.

Computer networks, and the technologies needed to connect and communicate

through and between them, continue to drive computer hardware, software, and

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peripherals industries. This expansion is mirrored by growth in the numbers and

types of users of networks from the researcher to the home user.

Today, computer networks are the core of modern communication. For example, all

modern aspects of the Public Switched Telephone Network (PSTN) are computer-

controlled, and telephony increasingly runs over the Internet Protocol, although not

necessarily the public Internet. The scope of communication has increased

significantly in the past decade and this boom in communications would not have

been possible without the progressively advancing computer network.

Networking methods

Networking is a complex part of computing that makes up most of the IT Industry.

Without networks, almost all communication in the world would cease to happen. It

is because of networking that telephones, televisions, the internet, etc. work.

One way to categorize computer networks are by their geographic scope, although

many real-world networks interconnect Local Area Networks (LAN) via Wide Area

Networks (WAN). These two (broad) types are:

Local area network (LAN)

A local area network is a network that spans a relatively small space and provides

services to a small number of people. Depending on the number of people that use a

Local Area Network, a peer-to-peer or client-server method of networking may be

used. A peer-to-peer network is where each client shares their resources with other

workstations in the network. Examples of peer-to-peer networks are: Small office

networks where resource use is minimal and a home network. A client-server

network is where every client is connected to the server and each other. Client-

server networks use servers in different capacities. These can be classified into two

types: Single-service servers, where the server performs one task such as file

server, print server, etc.; while other servers can not only perform in the capacity of

file servers and print servers, but they also conduct calculations and use these to

provide information to clients (Web/Intranet Server). Computers are linked via

Ethernet Cable, can be joined either directly (one computer to another), or via a

network hub that allows multiple connections.

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Historically, LANs have featured much higher speeds than WANs. This is not

necessarily the case when the WAN technology appears as Metro Ethernet,

implemented over optical transmission systems.

Wide area network (WAN)

A wide area network is a network where a wide variety of resources are deployed

across a large domestic area or internationally. An example of this is a multinational

business that uses a WAN to interconnect their offices in different countries. The

largest and best example of a WAN is the Internet, which is a network comprised of

many smaller networks. The Internet is considered the largest network in the world.

The PSTN (Public Switched Telephone Network) also is an extremely large network

that is converging to use Internet technologies, although not necessarily through the

public Internet.

A Wide Area Network involves communication through the use of a wide range of

different technologies. These technologies include Point-to-Point WANs such as Point-

to-Point Protocol (PPP) and High-Level Data Link Control (HDLC), Frame Relay, ATM

(Asynchronous Transfer Mode) and Sonet (Synchronous Optical Network). The

difference between the WAN technologies is based on the switching capabilities they

perform and the speed at which sending and receiving bits of information (data)

occur.

For more information on WANs, see Frame Relay, ATM and Sonet.

Wireless networks (WLAN, WWAN)

A wireless network is basically the same as a LAN or a WAN but there are no wires

between hosts and servers. The data is transferred over sets of radio transceivers.

These types of networks are beneficial when it is too costly or inconvenient to run

the necessary cables. For more information, see Wireless LAN and Wireless wide area

network. The media access protocols for LANs come from the IEEE.

The most common IEEE 802.11 WLANs cover, depending on antennas, ranges from

hundreds of meters to low kilometers. For larger areas, either communications

satellites of various types, cellular radio, or wireless local loop (IEEE 802.16) all have

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advantages and disadvantages. Depending on the type of mobility needed, the

relevant standards may come from the IETF or the ITU.

Network topology

The network topology defines the way in which computers, printers, and other

devices are connected, physically and logically. A network topology describes the

layout of the wire and devices as well as the paths used by data transmissions.

Commonly used topologies include:

Bus

Star

Tree (hierarchical)

Linear

Ring

Mesh

o partially connected

o fully connected (sometimes known as fully redundant)

The network topologies mentioned above are only a general representation of the

kinds of topologies used in computer network and are considered basic topologies.

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Internet

Visualization of the various routes through a portion of the Internet.

The Internet is a worldwide, publicly accessible series of interconnected computer

networks that transmit data by packet switching using the standard Internet Protocol

(IP). It is a "network of networks" that consists of millions of smaller domestic,

academic, business, and government networks, which together carry various

information and services, such as electronic mail, online chat, file transfer, and the

interlinked web pages and other resources of the World Wide Web (WWW).

Terminology

The Internet protocol suite is a collection of standards and protocols organized into

layers so that each layer provides the foundation and the services required by the

layer above. In this scheme, the Internet consists of the computers and networks

that handle Internet Protocol (IP) data packets. Transmission Control Protocol (TCP)

depends on IP and solves problems like data packets arriving out of order or not at

all. Next comes Hypertext Transfer Protocol (HTTP), which is an application layer

protocol. It runs on top of TCP/IP and provides user agents, such as web browsers,

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with access to the files, documents and other resources of the World Wide Web

(WWW). More generally, the Internet is the world-wide network holding most data

communications together, while the World-Wide Web is just one of many applications

that the Internet can be used for.

History

Growth

Although the basic applications and guidelines that make the Internet possible had

existed for almost a decade, the network did not gain a public face until the 1990s.

On August 6, 1991, CERN, which straddles the border between France and

Switzerland, publicized the new World Wide Web project, two years after British

scientist Tim Berners-Lee had begun creating HTML, HTTP and the first few Web

pages at CERN.

An early popular web browser was ViolaWWW based upon HyperCard. It was

eventually replaced in popularity by the Mosaic web browser. In 1993, the National

Center for Supercomputing Applications at the University of Illinois released version

1.0 of Mosaic, and by late 1994 there was growing public interest in the previously

academic/technical Internet. By 1996 usage of the word "Internet" had become

commonplace, and consequently, so had its misuse as a reference to the World Wide

Web.

Meanwhile, over the course of the decade, the Internet successfully accommodated

the majority of previously existing public computer networks (although some

networks, such as FidoNet, have remained separate). During the 1990s, it was

estimated that the Internet grew by 100% per year, with a brief period of explosive

growth in 1996 and 1997. This growth is often attributed to the lack of central

administration, which allows organic growth of the network, as well as the non-

proprietary open nature of the Internet protocols, which encourages vendor

interoperability and prevents any one company from exerting too much control over

the network.

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University Students Appreciation and Contributions

New findings in the field of communications during the 1960’s and 1970’s were

quickly adopted by universities across the United States. Their openness for

technology and new ideas saw many of them amongst the first to appreciate this

new Cultural Revolution – in most cases seeking technological innovation for the

pure joy of discovery, and seeing the potential for a tool of liberation.

Examples of early university Internet communities are Cleveland FreeNet, Blacksburg

Electronic Village and Nova Scotia. Students took up the opportunity of free

communications and saw this new phenomenon as a tool of liberation. Personal

computers and the Internet would free them from corporations and governments

(Nelson, Jennings, Stallman).

‘The culture of individual freedom sprouting in the university campuses of the 1960’s

and 1970’s used computer networking to it’s own ends’ (Castells 2001)

The students appreciated it through a cultural revolutionary way of thinking, similar

to that of Ted Nelson or Douglas Engelbart. Students agreed with the ideas of free

software and cooperative use of resources, which was always the early hacker

conduct. (Castells 2001)

Graduate students also played a huge part in the creation of ARPANET. In the

1960’s, the network working group, which did most of the design for ARPANET’s

protocols was composed mainly of graduate students.

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Today's Internet

The Opera Community rack. From the top, user file storage (content of

files.myopera.com), "bigma" (the master MySQL database server), and two IBM

blade centers containing multi-purpose machines (Apache front ends, Apache back

ends, slave MySQL database servers, load balancers, file servers, cache servers and

sync masters.

Aside from the complex physical connections that make up its infrastructure, the

Internet is facilitated by bi- or multi-lateral commercial contracts (e.g., peering

agreements), and by technical specifications or protocols that describe how to

exchange data over the network. Indeed, the Internet is essentially defined by its

interconnections and routing policies.

As of September 30, 2007, 1.244 billion people use the Internet according to

Internet World Stats. Writing in the Harvard International Review, philosopher

N.J.Slabbert, a writer on policy issues for the Washington DC-based Urban Land

Institute, has asserted that the Internet is fast becoming a basic feature of global

civilization, so that what has traditionally been called "civil society" is now becoming

identical with information technology society as defined by Internet use. Only 2% of

the World's population regularly accesses the internet.

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Internet protocols

In this context, there are three layers of protocols:

At the lower level (OSI layer 3) is IP (Internet Protocol), which defines the

datagrams or packets that carry blocks of data from one node to another. The

vast majority of today's Internet uses version four of the IP protocol (i.e.

IPv4), and although IPv6 is standardized, it exists only as "islands" of

connectivity, and there are many ISPs without any IPv6 connectivity. ICMP

(Internet Control Message Protocol) also exists at this level. ICMP is

connectionless; it is used for control, signaling, and error reporting purposes.

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) exist

at the next layer up (OSI layer 4); these are the protocols by which data is

transmitted. TCP makes a virtual 'connection', which gives some level of

guarantee of reliability. UDP is a best-effort, connectionless transport, in

which data packets that are lost in transit will not be re-sent.

The application protocols sit on top of TCP and UDP and occupy layers 5, 6,

and 7 of the OSI model. These define the specific messages and data formats

sent and understood by the applications running at each end of the

communication. Examples of these protocols are HTTP, FTP, and SMTP.

Internet structure

There have been many analyses of the Internet and its structure. For example, it has

been determined that the Internet IP routing structure and hypertext links of the

World Wide Web are examples of scale-free networks.

Similar to the way the commercial Internet providers connect via Internet exchange

points, research networks tend to interconnect into large subnetworks such as:

GEANT

GLORIAD

The Internet2 Network (formally known as the Abilene Network)

JANET (the UK's national research and education network)

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These in turn are built around relatively smaller networks. See also the list of

academic computer network organizations.

In network diagrams, the Internet is often represented by a cloud symbol, into and

out of which network communications can pass.

ICANN

ICANN headquarters in Marina Del Rey, California, United States

.

The Internet Corporation for Assigned Names and Numbers (ICANN) is the authority

that coordinates the assignment of unique identifiers on the Internet, including

domain names, Internet Protocol (IP) addresses, and protocol port and parameter

numbers. A globally unified namespace (i.e., a system of names in which there is at

most one holder for each possible name) is essential for the Internet to function.

ICANN is headquartered in Marina del Rey, California, but is overseen by an

international board of directors drawn from across the Internet technical, business,

academic, and non-commercial communities. The US government continues to have

the primary role in approving changes to the root zone file that lies at the heart of

the domain name system. Because the Internet is a distributed network comprising

many voluntarily interconnected networks, the Internet, as such, has no governing

body. ICANN's role in coordinating the assignment of unique identifiers distinguishes

it as perhaps the only central coordinating body on the global Internet, but the scope

of its authority extends only to the Internet's systems of domain names, IP

addresses, protocol ports and parameter numbers.

On November 16, 2005, the World Summit on the Information Society, held in Tunis,

established the Internet Governance Forum (IGF) to discuss Internet-related issues.

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Language

The prevalent language for communication on the Internet is English. This may be a

result of the Internet's origins, as well as English's role as the lingua franca. It may

also be related to the poor capability of early computers, largely originating in the

United States, to handle characters other than those in the English variant of the

Latin alphabet.

After English (31% of Web visitors) the most-requested languages on the World

Wide Web are Chinese 16%, Spanish 9%, Japanese 7%, German 5% and French 5%

(from Internet World Stats, updated June 30, 2007).

By continent, 37% of the world's Internet users are based in Asia, 27% in Europe,

19% in North America, and 9% in Latin America and the Carribean ([2] updated

September 30, 2007).

The Internet's technologies have developed enough in recent years, especially in the

use of Unicode, that good facilities are available for development and communication

in most widely used languages. However, some glitches such as mojibake (incorrect

display of foreign language characters, also known as kryakozyabry) still remain.

Internet and the workplace

The Internet is allowing greater flexibility in working hours and location, especially

with the spread of unmetered high-speed connections and Web applications.

The Internet viewed on mobile devices

The Internet can now be accessed virtually anywhere by numerous means. Mobile

phones, datacards, handheld game consoles and cellular routers allow users to

connect to the Internet from anywhere there is a cellular network supporting that

device's technology.

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Common uses of the Internet

E-mail

The concept of sending electronic text messages between parties in a way analogous

to mailing letters or memos predates the creation of the Internet. Even today it can

be important to distinguish between Internet and internal e-mail systems. Internet

e-mail may travel and be stored unencrypted on many other networks and machines

out of both the sender's and the recipient's control. During this time it is quite

possible for the content to be read and even tampered with by third parties, if

anyone considers it important enough. Purely internal or intranet mail systems,

where the information never leaves the corporate or organization's network, are

much more secure, although in any organization there will be IT and other personnel

whose job may involve monitoring, and occasionally accessing, the email of other

employees not addressed to them.

The World Wide Web

Graphic representation of less than 0.0001% of the WWW, representing some of the

hyperlinks

Many people use the terms Internet and World Wide Web (or just the Web)

interchangeably, but, as discussed above, the two terms are not synonymous.

The World Wide Web is a huge set of interlinked documents, images and other

resources, linked by hyperlinks and URLs. These hyperlinks and URLs allow the web-

servers and other machines that store originals, and cached copies, of these

resources to deliver them as required using HTTP (Hypertext Transfer Protocol).

HTTP is only one of the communication protocols used on the Internet.

Web services also use HTTP to allow software systems to communicate in order to

share and exchange business logic and data.

Software products that can access the resources of the Web are correctly termed

user agents. In normal use, web browsers, such as Internet Explorer and Firefox

access web pages and allow users to navigate from one to another via hyperlinks.

Web documents may contain almost any combination of computer data including

59

photographs, graphics, sounds, text, video, multimedia and interactive content

including games, office applications and scientific demonstrations.

Through keyword-driven Internet research using search engines, like Yahoo!, and

Google, millions of people worldwide have easy, instant access to a vast and diverse

amount of online information. Compared to encyclopedias and traditional libraries,

the World Wide Web has enabled a sudden and extreme decentralization of

information and data.

It is also easier, using the Web, than ever before for individuals and organisations to

publish ideas and information to an extremely large audience. Anyone can find ways

to publish a web page or build a website for very little initial cost. Publishing and

maintaining large, professional websites full of attractive, diverse and up-to-date

information is still a difficult and expensive proposition, however.

Many individuals and some companies and groups use "web logs" or blogs, which are

largely used as easily-updatable online diaries. Some commercial organizations

encourage staff to fill them with advice on their areas of specialization in the hope

that visitors will be impressed by the expert knowledge and free information, and be

attracted to the corporation as a result. One example of this practice is Microsoft,

whose product developers publish their personal blogs in order to pique the public's

interest in their work.

Collections of personal web pages published by large service providers remain

popular, and have become increasingly sophisticated. Whereas operations such as

Angelfire and GeoCities have existed since the early days of the Web, newer

offerings from, for example, Facebook and MySpace currently have large followings.

These operations often brand themselves as social network services rather than

simply as web page hosts.

Advertising on popular web pages can be lucrative, and e-commerce or the sale of

products and services directly via the Web continues to grow.

In the early days, web pages were usually created as sets of complete and isolated

HTML text files stored on a web server. More recently, web sites are more often

created using content management system (CMS) or wiki software with, initially,

very little content. Users of these systems, who may be paid staff, members of a

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club or other organization or members of the public, fill the underlying databases

with content using editing pages designed for that purpose, while casual visitors view

and read this content in its final HTML form. There may or may not be editorial,

approval and security systems built into the process of taking newly entered content

and making it available to the target visitors.

Remote access

The Internet allows computer users to connect to other computers and information

stores easily, wherever they may be across the world. They may do this with or

without the use of security, authentication and encryption technologies, depending

on the requirements.

This is encouraging new ways of working from home, collaboration and information

sharing in many industries. An accountant sitting at home can audit the books of a

company based in another country, on a server situated in a third country that is

remotely maintained by IT specialists in a fourth. These accounts could have been

created by home-working book-keepers, in other remote locations, based on

information e-mailed to them from offices all over the world. Some of these things

were possible before the widespread use of the Internet, but the cost of private,

leased lines would have made many of them infeasible in practice.

An office worker away from his desk, perhaps the other side of the world on a

business trip or a holiday, can open a remote desktop session into their normal office

PC using a secure Virtual Private Network (VPN) connection via the Internet. This

gives the worker complete access to all of their normal files and data, including e-

mail and other applications, while away from the office.

This concept is also referred to by some network security people as the Virtual

Private Nightmare, because it extends the secure perimeter of a corporate network

into its employees' homes; this has been the source of some notable security

breaches, but also provides security for the workers.

Collaboration

The low cost and nearly instantaneous sharing of ideas, knowledge, and skills has

made collaborative work dramatically easier. Not only can a group cheaply

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communicate and test, but the wide reach of the Internet allows such groups to

easily form in the first place, even among niche interests. An example of this is the

free software movement in software development which produced GNU and Linux

from scratch and has taken over development of Mozilla and OpenOffice.org

(formerly known as Netscape Communicator and StarOffice). Films such as Zeitgeist,

Loose Change and Endgame have had extensive coverage on the internet, while

being virtually ignored in the mainstream media.

Internet 'chat', whether in the form of IRC 'chat rooms' or channels, or via instant

messaging systems allow colleagues to stay in touch in a very convenient way when

working at their computers during the day. Messages can be sent and viewed even

more quickly and conveniently than via e-mail. Extension to these systems may

allow files to be exchanged, 'whiteboard' drawings to be shared as well as voice and

video contact between team members.

Version control systems allow collaborating teams to work on shared sets of

documents without either accidentally overwriting each other's work or having

members wait until they get 'sent' documents to be able to add their thoughts and

changes.

File sharing

A computer file can be e-mailed to customers, colleagues and friends as an

attachment. It can be uploaded to a Web site or FTP server for easy download by

others. It can be put into a "shared location" or onto a file server for instant use by

colleagues. The load of bulk downloads to many users can be eased by the use of

"mirror" servers or peer-to-peer networks.

In any of these cases, access to the file may be controlled by user authentication;

the transit of the file over the Internet may be obscured by encryption and money

may change hands before or after access to the file is given. The price can be paid

by the remote charging of funds from, for example a credit card whose details are

also passed—hopefully fully encrypted—across the Internet. The origin and

authenticity of the file received may be checked by digital signatures or by MD5 or

other message digests.

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These simple features of the Internet, over a world-wide basis, are changing the

basis for the production, sale, and distribution of anything that can be reduced to a

computer file for transmission. This includes all manner of print publications,

software products, news, music, film, video, photography, graphics and the other

arts. This in turn has caused seismic shifts in each of the existing industries that

previously controlled the production and distribution of these products.

Internet collaboration technology enables business and project teams to share

documents, calendars and other information. Such collaboration occurs in a wide

variety of areas including scientific research, software development, conference

planning, political activism and creative writing.

Streaming media

Many existing radio and television broadcasters provide Internet 'feeds' of their live

audio and video streams (for example, the BBC). They may also allow time-shift

viewing or listening such as Preview, Classic Clips and Listen Again features. These

providers have been joined by a range of pure Internet 'broadcasters' who never had

on-air licenses. This means that an Internet-connected device, such as a computer

or something more specific, can be used to access on-line media in much the same

way as was previously possible only with a television or radio receiver. The range of

material is much wider, from pornography to highly specialized technical Web-casts.

Podcasting is a variation on this theme, where—usually audio—material is first

downloaded in full and then may be played back on a computer or shifted to a digital

audio player to be listened to on the move. These techniques using simple

equipment allow anybody, with little censorship or licensing control, to broadcast

audio-visual material on a worldwide basis.

Webcams can be seen as an even lower-budget extension of this phenomenon. While

some webcams can give full frame rate video, the picture is usually either small or

updates slowly. Internet users can watch animals around an African waterhole, ships

in the Panama Canal, the traffic at a local roundabout or their own premises, live and

in real time. Video chat rooms, video conferencing, and remote controllable webcams

are also popular. Many uses can be found for personal webcams in and around the

home, with and without two-way sound.

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Voice telephony (VoIP)

VoIP stands for Voice over IP, where IP refers to the Internet Protocol that underlies

all Internet communication. This phenomenon began as an optional two-way voice

extension to some of the Instant Messaging systems that took off around the year

2000. In recent years many VoIP systems have become as easy to use and as

convenient as a normal telephone. The benefit is that, as the Internet carries the

actual voice traffic, VoIP can be free or cost much less than a normal telephone call,

especially over long distances and especially for those with always-on Internet

connections such as cable or ADSL.

Thus VoIP is maturing into a viable alternative to traditional telephones.

Interoperability between different providers has improved and the ability to call or

receive a call from a traditional telephone is available. Simple inexpensive VoIP

modems are now available that eliminate the need for a PC.

Voice quality can still vary from call to call but is often equal to and can even exceed

that of traditional calls.

Remaining problems for VoIP include emergency telephone number dialing and

reliability. Currently a few VoIP providers provide an emergency service but it is not

universally available. Traditional phones are line powered and operate during a

power failure, VoIP does not do so without a backup power source for the

electronics.

Most VoIP providers offer unlimited national calling but the direction in VoIP is clearly

toward global coverage with unlimited minutes for a low monthly fee.

VoIP has also become increasingly popular within the gaming world, as a form of

communication between players. Popular gaming VoIP clients include Ventrilo and

Teamspeak, and there are others available also. The PlayStation 3 also features a

VoIP chat feature.

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Internet access

Common methods of home access include dial-up, landline broadband (over coaxial

cable, fiber optic or copper wires), Wi-Fi, satellite and 3G technology cell phones.

Public places to use the Internet include libraries and Internet cafes, where

computers with Internet connections are available. There are also Internet access

points in many public places such as airport halls and coffee shops, in some cases

just for brief use while standing. Various terms are used, such as "public Internet

kiosk", "public access terminal", and "Web payphone". Many hotels now also have

public terminals, though these are usually fee-based. These terminals are widely

accessed for various usage like ticket booking, bank deposit, online payment etc. Wi-

Fi provides wireless access to computer networks, and therefore can do so to the

Internet itself. Hotspots providing such access include Wi-Fi-cafes, where a would-be

user needs to bring their own wireless-enabled devices such as a laptop or PDA.

These services may be free to all, free to customers only, or fee-based. A hotspot

need not be limited to a confined location. The whole campus or park, or even the

entire city can be enabled. Grassroots efforts have led to wireless community

networks. Commercial WiFi services covering large city areas are in place in London,

Vienna, Toronto, San Francisco, Philadelphia, Chicago and Pittsburgh. The Internet

can then be accessed from such places as a park bench.

Apart from Wi-Fi, there have been experiments with proprietary mobile wireless

networks like Ricochet, various high-speed data services over cellular phone

networks, and fixed wireless services.

High-end mobile phones such as smartphones generally come with Internet access

through the phone network. Web browsers such as Opera are available on these

advanced handsets, which can also run a wide variety of other Internet software.

More mobile phones have Internet access than PCs, though this is not as widely

used. An Internet access provider and protocol matrix differentiates the methods

used to get online.

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Leisure

The Internet has been a major source of leisure since before the World Wide Web,

with entertaining social experiments such as MUDs and MOOs being conducted on

university servers, and humor-related Usenet groups receiving much of the main

traffic. Today, many Internet forums have sections devoted to games and funny

videos; short cartoons in the form of Flash movies are also popular. Over 6 million

people use blogs or message boards as a means of communication and for the

sharing of ideas.

The pornography and gambling industries have both taken full advantage of the

World Wide Web, and often provide a significant source of advertising revenue for

other Web sites. Although many governments have attempted to put restrictions on

both industries' use of the Internet, this has generally failed to stop their widespread

popularity.

One main area of leisure on the Internet is multiplayer gaming. This form of leisure

creates communities, bringing people of all ages and origins to enjoy the fast-paced

world of multiplayer games. These range from MMORPG to first-person shooters,

from role-playing games to online gambling. This has revolutionized the way many

people interact and spend their free time on the Internet.

While online gaming has been around since the 1970s, modern modes of online

gaming began with services such as GameSpy and MPlayer, which players of games

would typically subscribe to. Non-subscribers were limited to certain types of

gameplay or certain games.

Many use the Internet to access and download music, movies and other works for

their enjoyment and relaxation. As discussed above, there are paid and unpaid

sources for all of these, using centralized servers and distributed peer-to-peer

technologies. Discretion is needed as some of these sources take more care over the

original artists' rights and over copyright laws than others.

Many use the World Wide Web to access news, weather and sports reports, to plan

and book holidays and to find out more about their random ideas and casual

interests.

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People use chat, messaging and email to make and stay in touch with friends

worldwide, sometimes in the same way as some previously had pen pals. Social

networking Web sites like Myspace and Facebook many others like them also put and

keep people in contact for their enjoyment.

The Internet has seen a growing number of Internet operating systems, where users

can access their files, folders, and settings via the Internet. An example of an

opensource webOS is Eyeos.

Cyberslacking has become a serious drain on corporate resources; the average UK

employee spends 57 minutes a day surfing the Web at work, according to a study by

Peninsula Business Services.

A survey performed by JWT advertising agency showed that most Americans say

they cannot live without use of the Internet for more than a week. 1,011 Americans

participated in the survey answering such questions as how long they can do without

Internet. As much as 15 percent mentioned that they would be able to live without

the Internet for not more than a day or less, 21% mentioned they could be Internet-

free no more than a couple of days, 19% said mentioned a few days. Only 1/3 of

respondents mentioned they will be able to live without Internet for a week.

Complex architecture

Many computer scientists see the Internet as a "prime example of a large-scale,

highly engineered, yet highly complex system". The Internet is extremely

heterogeneous. (For instance, data transfer rates and physical characteristics of

connections vary widely.) The Internet exhibits "emergent phenomena" that depend

on its large-scale organization. For example, data transfer rates exhibit temporal

self-similarity. Further adding to the complexity of the Internet is the ability of more

than one computer to use the Internet through only one node, thus creating the

possibility for a very deep and hierarchal based sub-network that can theoretically be

extended infinitely (disregarding the programmatic limitations of the IPv4 protocol).

However, since principles of this architecture date back to the 1960s, it might not be

a solution best suited to modern needs, and thus the possibility of developing

alternative structures is currently being looked into.

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According to a June 2007 article in Discover Magazine, the combined weight of all the

electrons moved within the the internet in a day is 0.2 millionths of an ounce. Others

have estimated this at nearer 2 ounces (50 grams).

Marketing

The Internet has also become a large market for companies; some of the biggest

companies today have grown by taking advantage of the efficient nature of low-cost

advertising and commerce through the Internet, also known as e-commerce. It is the

fastest way to spread information to a vast number of people simultaneously. The

Internet has also subsequently revolutionized shopping—for example; a person can

order a CD online and receive it in the mail within a couple of days, or download it

directly in some cases. The Internet has also greatly facilitated personalized

marketing which allows a company to market a product to a specific person or a

specific group of people more so than any other advertising medium.

Examples of personalized marketing include online communities such as MySpace,

Friendster, Orkut, Facebook and others which thousands of Internet users join to

advertise themselves and make friends online. Many of these users are young teens

and adolescents ranging from 13 to 25 years old. In turn, when they advertise

themselves they advertise interests and hobbies, which online marketing companies

can use as information as to what those users will purchase online, and advertise

their own companies' products to those users.

The name Internet

Internet is traditionally written with a capital first letter, as it is a proper noun. The

Internet Society, the Internet Engineering Task Force, the Internet Corporation for

Assigned Names and Numbers, the World Wide Web Consortium, and several other

Internet-related organizations use this convention in their publications.

Many newspapers, newswires, periodicals, and technical journals capitalize the term

(Internet). Examples include The New York Times, the Associated Press, Time, The

Times of India, Hindustan Times, and Communications of the ACM.

Others assert that the first letter should be in lower case (internet), and that the

specific article “the” is sufficient to distinguish “the internet” from other internets. A

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significant number of publications use this form, including The Economist, the

Canadian Broadcasting Corporation, the Financial Times, The Guardian, The Times,

and The Sydney Morning Herald. As of 2005, many publications using internet

appear to be located outside of North America—although one U.S. news source,

Wired News, has adopted the lower-case spelling.

Historically, Internet and internet have had different meanings, with internet

meaning “an interconnected set of distinct networks,” and Internet referring to the

world-wide, publicly-available IP internet. Under this distinction, "the Internet" is the

familiar network via which websites exist, however "an internet" can exist between

any two remote locations. Any group of distinct networks connected together is an

internet; each of these networks may or may not be part of the Internet. The

distinction was evident in many RFCs, books, and articles from the 1980s and early

1990s (some of which, such as RFC 1918, refer to "internets" in the plural), but has

recently fallen into disuse. Instead, the term intranet is generally used for private

networks, whether they are connected to the Internet or not. See also: extranet.

Some people use the lower-case term as a medium (like radio or newspaper, e.g.

I've found it on the internet), and first letter capitalized as the global network.

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Programming language

A programming language is an artificial language that can be used to control the

behavior of a machine, particularly a computer. Programming languages, like natural

languages, are defined by syntactic and semantic rules which describe their structure

and meaning respectively. Many programming languages have some form of written

specification of their syntax and semantics; some are defined only by an official

implementation.

Programming languages are used to facilitate communication about the task of

organizing and manipulating information, and to express algorithms precisely. Some

authors restrict the term "programming language" to those languages that can

express all possible algorithms; sometimes the term "computer language" is used for

more limited artificial languages.

Thousands of different programming languages have been created, and new

languages are created every year.

Definitions

Traits often considered important for constituting a programming language:

Function: A programming language is a language used to write computer

programs, which involve a computer performing some kind of computation or

algorithm and possibly control external devices such as printers, robots, and

so on.

Target: Programming languages differ from natural languages in that natural

languages are only used for interaction between people, while programming

languages also allow humans to communicate instructions to machines. Some

programming languages are used by one device to control another. For

example PostScript programs are frequently created by another program to

control a computer printer or display.

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Constructs: Programming languages may contain constructs for defining and

manipulating data structures or controlling the flow of execution.

Expressive power: The theory of computation classifies languages by the

computations they can express (see Chomsky hierarchy). All Turing complete

languages can implement the same set of algorithms. ANSI/ISO SQL and

Charity are examples of languages that are not Turing complete yet often

called programming languages.

Non-computational languages, such as markup languages like HTML or formal

grammars like BNF, are usually not considered programming languages. Often a

programming language is embedded in the non-computational (host) language.

Purpose

A prominent purpose of programming languages is to provide instructions to a

computer. As such, programming languages differ from most other forms of human

expression in that they require a greater degree of precision and completeness.

When using a natural language to communicate with other people, human authors

and speakers can be ambiguous and make small errors, and still expect their intent

to be understood. However, computers do exactly what they are told to do, and

cannot understand the code the programmer "intended" to write. The combination of

the language definition, the program, and the program's inputs must fully specify the

external behavior that occurs when the program is executed.

Many languages have been designed from scratch, altered to meet new needs,

combined with other languages, and eventually fallen into disuse. Although there

have been attempts to design one "universal" computer language that serves all

purposes, all of them have failed to be accepted in this role. The need for diverse

computer languages arises from the diversity of contexts in which languages are

used:

Programs range from tiny scripts written by individual hobbyists to huge

systems written by hundreds of programmers.

Programmers range in expertise from novices who need simplicity above all

else, to experts who may be comfortable with considerable complexity.

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Programs must balance speed, size, and simplicity on systems ranging from

microcontrollers to supercomputers.

Programs may be written once and not change for generations, or they may

undergo nearly constant modification.

Finally, programmers may simply differ in their tastes: they may be

accustomed to discussing problems and expressing them in a particular

language.

One common trend in the development of programming languages has been to add

more ability to solve problems using a higher level of abstraction. The earliest

programming languages were tied very closely to the underlying hardware of the

computer. As new programming languages have developed, features have been

added that let programmers express ideas that are more removed from simple

translation into underlying hardware instructions. Because programmers are less tied

to the needs of the computer, their programs can do more computing with less effort

from the programmer. This lets them write more programs in the same amount of

time.

Natural language processors have been proposed as a way to eliminate the need for

a specialized language for programming. However, this goal remains distant and its

benefits are open to debate. Edsger Dijkstra took the position that the use of a

formal language is essential to prevent the introduction of meaningless constructs,

and dismissed natural language programming as "foolish." Alan Perlis was similarly

dismissive of the idea.

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Elements

Syntax

Parse tree of Python code with inset tokenization

Parse tree of Python code with inset tokenization

Syntax highlighting is often used to aid programmers in the recognition of elements of source code. The

language you see here is Python

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A programming language's surface form is known as its syntax. Most programming

languages are purely textual; they use sequences of text including words, numbers,

and punctuation, much like written natural languages. On the other hand, there are

some programming languages which are more graphical in nature, using spatial

relationships between symbols to specify a program.

The syntax of a language describes the possible combinations of symbols that form a

syntactically correct program. The meaning given to a combination of symbols is

handled by semantics. Since most languages are textual, this article discusses

textual syntax.

Programming language syntax is usually defined using a combination of regular

expressions (for lexical structure) and Backus-Naur Form (for grammatical

structure). Below is a simple grammar, based on Lisp:

expression ::= atom | list

atom ::= number | symbol

number ::= [+-]?['0'-'9']+

symbol ::= ['A'-'Z' 'a'-'z'].*

list ::= '(' expression* ')'

This grammar specifies the following:

an expression is either an atom or a list;

an atom is either a number or a symbol;

a number is an unbroken sequence of one or more decimal digits, optionally

preceded by a plus or minus sign;

a symbol is a letter followed by zero or more of any characters (excluding

whitespace); and

a list is a matched pair of parentheses, with zero or more expressions inside

it.

The following are examples of well-formed token sequences in this grammar:

'12345', '()', '(a b c232 (1))'

Not all syntactically correct programs are semantically correct. Many syntactically

correct programs are nonetheless ill-formed, per the language's rules; and may

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(depending on the language specification and the soundness of the implementation)

result in an error on translation or execution. In some cases, such programs may

exhibit undefined behavior. Even when a program is well-defined within a language,

it may still have a meaning that is not intended by the person who wrote it.

Using natural language as an example, it may not be possible to assign a meaning to

a grammatically correct sentence or the sentence may be false:

"Colorless green ideas sleep furiously." is grammatically well-formed but has

no generally accepted meaning.

"John is a married bachelor." is grammatically well-formed but expresses a

meaning that cannot be true.

The following C language fragment is syntactically correct, but performs an operation

that is not semantically defined (because p is a null pointer, the operations p->real

and p->im have no meaning):

complex *p = NULL;

complex abs_p = sqrt (p->real * p->real + p->im * p->im);

The grammar needed to specify a programming language can be classified by its

position in the Chomsky hierarchy. The syntax of most programming languages can

be specified using a Type-2 grammar, i.e., they are context-free grammars.

Type system

For more details on this topic, see Type system.

For more details on this topic, see Type safety.

A type system defines how a programming language classifies values and

expressions into types, how it can manipulate those types and how they interact.

This generally includes a description of the data structures that can be constructed in

the language. The design and study of type systems using formal mathematics is

known as type theory.

Internally, all data in modern digital computers are stored simply as zeros or ones

(binary).

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Typed versus untyped languages

A language is typed if operations defined for one data type cannot be performed on

values of another data type. For example, "this text between the quotes" is a string.

In most programming languages, dividing a number by a string has no meaning.

Most modern programming languages will therefore reject any program attempting

to perform such an operation. In some languages, the meaningless operation will be

detected when the program is compiled ("static" type checking), and rejected by the

compiler, while in others, it will be detected when the program is run ("dynamic"

type checking), resulting in a runtime exception.

A special case of typed languages are the single-type languages. These are often

scripting or markup languages, such as Rexx or SGML, and have only one data type

— most commonly character strings which are used for both symbolic and numeric

data.

In contrast, an untyped language, such as most assembly languages, allows any

operation to be performed on any data, which are generally considered to be

sequences of bits of various lengths. High-level languages which are untyped include

BCPL and some varieties of Forth.

In practice, while few languages are considered typed from the point of view of type

theory (verifying or rejecting all operations), most modern languages offer a degree

of typing. Many production languages provide means to bypass or subvert the type

system.

Static versus dynamic typing

In static typing all expressions have their types determined prior to the program

being run (typically at compile-time). For example, 1 and (2+2) are integer

expressions; they cannot be passed to a function that expects a string, or stored in a

variable that is defined to hold dates.

Statically-typed languages can be manifestly typed or type-inferred. In the first case,

the programmer must explicitly write types at certain textual positions (for example,

at variable declarations). In the second case, the compiler infers the types of

expressions and declarations based on context. Most mainstream statically-typed

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languages, such as C++ and Java, are manifestly typed. Complete type inference

has traditionally been associated with less mainstream languages, such as Haskell

and ML. However, many manifestly typed languages support partial type inference;

for example, Java and C# both infer types in certain limited cases. Dynamic typing,

also called latent typing, determines the type-safety of operations at runtime; in

other words, types are associated with runtime values rather than textual

expressions. As with type-inferred languages, dynamically typed languages do not

require the programmer to write explicit type annotations on expressions. Among

other things, this may permit a single variable to refer to values of different types at

different points in the program execution. However, type errors cannot be

automatically detected until a piece of code is actually executed, making debugging

more difficult. Ruby, Lisp, JavaScript, and Python are dynamically typed.

Weak and strong typing

Weak typing allows a value of one type to be treated as another, for example

treating a string as a number. This can occasionally be useful, but it can also allow

some kinds of program faults to go undetected at compile time.

Strong typing prevents the above. Attempting to mix types raises an error. Strongly-

typed languages are often termed type-safe or safe. Type safety can prevent

particular kinds of program faults occurring (because constructs containing them are

flagged at compile time).

An alternative definition for "weakly typed" refers to languages, such as Perl,

JavaScript, and C++ which permit a large number of implicit type conversions; Perl

in particular can be characterized as a dynamically typed programming language in

which type checking can take place at runtime. See type system. This capability is

often useful, but occasionally dangerous; as it would permit operations whose

objects can change type on demand.

Strong and static are generally considered orthogonal concepts, but usage in the

literature differs. Some use the term strongly typed to mean strongly, statically

typed, or, even more confusingly, to mean simply statically typed. Thus C has been

called both strongly typed and weakly, statically typed.

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Execution semantics

Once data has been specified, the machine must be instructed to perform operations

on the data. The execution semantics of a language defines how and when the

various constructs of a language should produce a program behavior.

For example, the semantics may define the strategy by which expressions are

evaluated to values, or the manner in which control structures conditionally execute

statements.

Core library

Most programming languages have an associated core library (sometimes known as

the 'Standard library', especially if it is included as part of the published language

standard), which is conventionally made available by all implementations of the

language. Core libraries typically include definitions for commonly used algorithms,

data structures, and mechanisms for input and output.

A language's core library is often treated as part of the language by its users,

although the designers may have treated it as a separate entity. Many language

specifications define a core that must be made available in all implementations, and

in the case of standardized languages this core library may be required. The line

between a language and its core library therefore differs from language to language.

Indeed, some languages are designed so that the meanings of certain syntactic

constructs cannot even be described without referring to the core library. For

example, in Java, a string literal is defined as an instance of the java.lang.String

class; similarly, in Smalltalk, an anonymous function expression (a "block")

constructs an instance of the library's BlockContext class. Conversely, Scheme

contains multiple coherent subsets that suffice to construct the rest of the language

as library macros, and so the language designers do not even bother to say which

portions of the language must be implemented as language constructs, and which

must be implemented as parts of a library.

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Practice

A language's designers and users must construct a number of artifacts that govern

and enable the practice of programming. The most important of these artifacts are

the language specification and implementation.

Specification

The specification of a programming language is intended to provide a definition

that language users and implementers can use to determine the behavior of a

program, given its source code.

A programming language specification can take several forms, including the

following:

An explicit definition of the syntax and semantics of the language. While

syntax is commonly specified using a formal grammar, semantic definitions

may be written in natural language (e.g., the C language), or a formal

semantics (e.g., the Standard MLand Scheme specifications).

A description of the behavior of a translator for the language (e.g., the C++

and Fortran specifications). The syntax and semantics of the language have to

be inferred from this description, which may be written in natural or a formal

language.

A reference or model implementation, sometimes written in the language

being specified (e.g., Prolog or ANSI REXX). The syntax and semantics of the

language are explicit in the behavior of the reference implementation.

Implementation

An implementation of a programming language provides a way to execute that

program on one or more configurations of hardware and software. There are,

broadly, two approaches to programming language implementation: compilation and

interpretation. It is generally possible to implement a language using either

technique.

The output of a compiler may be executed by hardware or a program called an

interpreter. In some implementations that make use of the interpreter approach

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there is no distinct boundary between compiling and interpreting. For instance, some

implementations of the BASIC programming language compile and then execute the

source a line at a time.

Programs that are executed directly on the hardware usually run several orders of

magnitude faster than those that are interpreted in software.

One technique for improving the performance of interpreted programs is just-in-time

compilation. Here the virtual machine monitors which sequences of bytecode are

frequently executed and translates them to machine code for direct execution on the

hardware.

History

A selection of textbooks that teach programming, in languages both popular and

obscure. These are only a few of the thousands of programming languages and

dialects that have been designed in history.

Early developments

The first programming languages predate the modern computer. The 19th century

had "programmable" looms and player piano scrolls which implemented what are

today recognized as examples of domain-specific programming languages. By the

beginning of the twentieth century, punch cards encoded data and directed

mechanical processing. In the 1930s and 1940s, the formalisms of Alonzo Church's

lambda calculus and Alan Turing's Turing machines provided mathematical

abstractions for expressing algorithms; the lambda calculus remains influential in

language design.

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In the 1940s, the first electrically powered digital computers were created. The

computers of the early 1950s, notably the UNIVAC I and the IBM 701 used machine

language programs. First generation machine language programming was quickly

superseded by a second generation of programming languages known as Assembly

languages. Later in the 1950s, assembly language programming, which had evolved

to include the use of macro instructions, was followed by the development of three

higher-level programming languages: FORTRAN, LISP, and COBOL. Updated versions

of all of these are still in general use, and importantly, each has strongly influenced

the development of later languages. At the end of the 1950s, the language

formalized as Algol 60 was introduced, and most later programming languages are,

in many respects, descendants of Algol. The format and use of the early

programming languages was heavily influenced by the constraints of the interface.

Refinement

The period from the 1960s to the late 1970s brought the development of the major

language paradigms now in use, though many aspects were refinements of ideas in

the very first Third-generation programming languages:

APL introduced array programming and influenced functional programming.

PL/I (NPL) was designed in the early 1960s to incorporate the best ideas from

FORTRAN and COBOL.

In the 1960s, Simula was the first language designed to support object-

oriented programming; in the mid-1970s, Smalltalk followed with the first

"purely" object-oriented language.

C was developed between 1969 and 1973 as a systems programming

language, and remains popular.

Prolog, designed in 1972, was the first logic programming language.

In 1978, ML built a polymorphic type system on top of Lisp, pioneering

statically typed functional programming languages.

Each of these languages spawned an entire family of descendants, and most modern

languages count at least one of them in their ancestry.

The 1960s and 1970s also saw considerable debate over the merits of structured

programming, and whether programming languages should be designed to support

it. Edsger Dijkstra, in a famous 1968 letter published in the Communications of the

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ACM, argued that GOTO statements should be eliminated from all "higher level"

programming languages.

The 1960s and 1970s also saw expansion of techniques that reduced the footprint of

a program as well as improved productivity of the programmer and user. The card

deck for an early 4GL was a lot smaller for the same functionality expressed in a 3GL

deck.

Consolidation and growth

The 1980s were years of relative consolidation. C++ combined object-oriented and

systems programming. The United States government standardized Ada, a systems

programming language intended for use by defense contractors. In Japan and

elsewhere, vast sums were spent investigating so-called "fifth generation" languages

that incorporated logic programming constructs. The functional languages

community moved to standardize ML and Lisp. Rather than inventing new

paradigms, all of these movements elaborated upon the ideas invented in the

previous decade.

One important trend in language design during the 1980s was an increased focus on

programming for large-scale systems through the use of modules, or large-scale

organizational units of code. Modula-2, Ada, and ML all developed notable module

systems in the 1980s, although other languages, such as PL/I, already had extensive

support for modular programming. Module systems were often wedded to generic

programming constructs.

The rapid growth of the Internet in the mid-1990's created opportunities for new

languages. Perl, originally a Unix scripting tool first released in 1987, became

common in dynamic Web sites. Java came to be used for server-side programming.

These developments were not fundamentally novel, rather they were refinements to

existing languages and paradigms, and largely based on the C family of

programming languages.

Programming language evolution continues, in both industry and research. Current

directions include security and reliability verification, new kinds of modularity

(mixins, delegates, aspects), and database integration.

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The 4GLs are examples of languages which are domain-specific, such as SQL, which

manipulates and returns sets of data rather than the scalar values which are

canonical to most programming languages. Perl, for example, with its 'here

document' can hold multiple 4GL programs, as well as multiple JavaScript programs,

in part of its own perl code and use variable interpolation in the 'here document' to

support multi-language programming.

Measuring language usage

It is difficult to determine which programming languages are most widely used, and

what usage means varies by context. One language may occupy the greater number

of programmer hours, a different one have more lines of code, and a third utilize the

most CPU time. Some languages are very popular for particular kinds of applications.

For example, COBOL is still strong in the corporate data center, often on large

mainframes; FORTRAN in engineering applications; C in embedded applications and

operating systems; and other languages are regularly used to write many different

kinds of applications.

Various methods of measuring language popularity, each subject to a different bias

over what is measured, have been proposed:

counting the number of job advertisements that mention the language.

the number of books sold that teach or describe the language

estimates of the number of existing lines of code written in the language—

which may underestimate languages not often found in public searches

counts of language references found using a web search engine.

Taxonomies

There is no overarching classification scheme for programming languages. A given

programming language does not usually have a single ancestor language. Languages

commonly arise by combining the elements of several predecessor languages with

new ideas in circulation at the time. Ideas that originate in one language will diffuse

throughout a family of related languages, and then leap suddenly across familial

gaps to appear in an entirely different family.

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The task is further complicated by the fact that languages can be classified along

multiple axes. For example, Java is both an object-oriented language (because it

encourages object-oriented organization) and a concurrent language (because it

contains built-in constructs for running multiple threads in parallel). Python is an

object-oriented scripting language.

In broad strokes, programming languages divide into programming paradigms and a

classification by intended domain of use. Paradigms include procedural programming,

object-oriented programming, functional programming, and logic programming;

some languages are hybrids of paradigms or multi-paradigmatic. An assembly

language is not so much a paradigm as a direct model of an underlying machine

architecture. By purpose, programming languages might be considered general

purpose, system programming languages, scripting languages, domain-specific

languages, or concurrent/distributed languages (or a combination of these). Some

general purpose languages were designed largely with educational goals.

A programming language may also be classified by factors unrelated to programming

paradigm. For instance, most programming languages use English language

keywords, while a minority do not. Other languages may be classified as being

esoteric or not.

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Windows XP

Windows XP is a line of operating systems developed by Microsoft for use on

general-purpose computer systems, including home and business desktops,

notebook computers, and media centers. The letters "XP" stand for eXPerience. It

was codenamed "Whistler", after Whistler, British Columbia, as many Microsoft

employees skied at the Whistler-Blackcomb ski resort during its development.

Windows XP is the successor to both Windows 2000 Professional and Windows Me,

and is the first consumer-oriented operating system produced by Microsoft to be built

on the Windows NT kernel and architecture. Windows XP was first released on

October 25, 2001, and over 400 million copies were in use in January 2006,

according to an estimate in that month by an IDC analyst. It is succeeded by

Windows Vista, which was released to volume license customers on November 8,

2006, and worldwide to the general public on January 30, 2007.

The most common editions of the operating system are Windows XP Home Edition,

which is targeted at home users, and Windows XP Professional, which has additional

features such as support for Windows Server domains and two physical processors,

and is targeted at power users and business clients. Windows XP Media Center

Edition has additional multimedia features enhancing the ability to record and watch

TV shows, view DVD movies, and listen to music. Windows XP Tablet PC Edition is

designed to run the ink-aware Tablet PC platform. Two separate 64-bit versions of

Windows XP were also released, Windows XP 64-bit Edition for IA-64 (Itanium)

processors and Windows XP Professional x64 Edition for x86-64.

Windows XP is known for its improved stability and efficiency over the 9x versions of

Microsoft Windows. It presents a significantly redesigned graphical user interface, a

change Microsoft promoted as more user-friendly than previous versions of Windows.

New software management capabilities were introduced to avoid the "DLL hell" that

plagued older consumer-oriented 9x versions of Windows. It is also the first version

of Windows to use product activation to combat software piracy, a restriction that did

not sit well with some users and privacy advocates. Windows XP has also been

criticized by some users for security vulnerabilities, tight integration of applications

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such as Internet Explorer and Windows Media Player, and for aspects of its default

user interface.

Windows XP had been in development since early 1999, when Microsoft started

working on Windows Neptune, an operating system intended to be the "Home

Edition" equivalent to Windows 2000 Professional. It was eventually merged into the

Whistler project, which later became Windows XP.

Editions

The two major editions are Windows XP Home Edition, designed for home users, and

Windows XP Professional, designed for business and power-users. Other builds of

Windows XP include those built for specialized hardware and limited-feature versions

sold in Europe and select developing economies.

Windows XP Professional offers a number of features unavailable in the Home

Edition, including:

The ability to become part of a Windows Server domain, a group of computers

that are remotely managed by one or more central servers.

A sophisticated access control scheme that allows specific permissions on files

to be granted to specific users under normal circumstances. However, users

can use tools other than Windows Explorer (like cacls or File Manager), or

restart to Safe Mode to modify access control lists.

Remote Desktop server, which allows a PC to be operated by another

Windows XP user over a local area network or the Internet.

Offline Files and Folders, which allow the PC to automatically store a copy of

files from another networked computer and work with them while

disconnected from the network.

Encrypting File System, which encrypts files stored on the computer's hard

drive so they cannot be read by another user, even with physical access to

the storage medium.

Centralized administration features, including Group Policies, Automatic

Software Installation and Maintenance, Roaming User Profiles, and Remote

Installation Service (RIS).

Internet Information Services (IIS), Microsoft's HTTP and FTP Server.

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Support for two physical central processing units (CPU). (Because the number

of CPU cores and Hyper-threading capabilities on modern CPUs are considered

to be part of a single physical processor, multicore CPUs are supported using

XP Home Edition.)

Windows Management Instrumentation Console (WMIC): WMIC is a

command-line tool designed to ease WMI information retrieval about a

system by using simple keywords (aliases).

Windows XP for specialized hardware

Microsoft has also customized Windows XP to suit different markets. Six different

versions of Windows XP for specific hardware were designed, two of them specifically

for 64-bit processors.

Windows XP 64-bit Edition

This edition was designed specifically for Itanium-based workstations. This

edition was discontinued in early 2005, after HP, the last distributor of

Itanium-based workstations, stopped selling Itanium systems marketed as

'workstations'. However, Itanium support continues in the server editions of

Windows.

Windows XP Professional x64 Edition

Not to be confused with the aforementioned Itanium edition of Windows XP,

this edition is derived from Windows Server 2003 and supports the x86-64

extension of the Intel IA-32 architecture. x86-64 is implemented by AMD as

"AMD64", found in AMD's Opteron and Athlon 64 chips, and implemented by

Intel as "Intel 64" (formerly known as IA-32e and EM64T), found in Intel's

Pentium 4 and later chips.

Microsoft had previously supported other microprocessors with earlier

versions of the Windows NT operating system line (including two 64-bit lines,

the DEC Alpha and the MIPS R4000, although Windows NT used them as 32-

bit processors). The files necessary for all of the architectures were included

on the same installation CD and did not require the purchase of separate

versions.

Windows XP Media Center Edition

This edition is designed for media center PCs. Originally, it was only available

bundled with one of these computers, and could not be purchased separately.

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In 2003 the Media Center Edition was updated as "Windows XP Media Center

Edition 2003", which added additional features such as FM radio tuning.

Another update was released in 2004, and again in 2005, which was the first

edition available for System Builders. Many of the features of Windows XP

Media Center Edition 2005 (including screen dancers, auto playlist DJ, and

high end visual screen savers) were taken from the Windows XP Plus!

packages. These were originally shipped as add ons to Window XP to increase

the users experience of their Windows XP machine.

Windows XP Tablet PC Edition

Intended for specially designed notebook/laptop computers called tablet PCs,

the Tablet PC Edition is compatible with a pen-sensitive screen, supporting

handwritten notes and portrait-oriented screens. It cannot be purchased

separately from a Tablet PC without an MSDN (Microsoft Developers Network)

subscription.

Windows XP Embedded

An edition for specific consumer electronics, set-top boxes, kiosks/ATMs,

medical devices, arcade video games, point-of-sale terminals, and Voice over

Internet Protocol (VoIP) components.

Windows Fundamentals for Legacy PCs

In July 2006, Microsoft introduced a "thin-client" version of Windows XP called

Windows Fundamentals for Legacy PCs, which targets older machines (as

early as the original Pentium). It is only available to Software Assurance

customers. It is intended for those who would like to upgrade to Windows XP

to take advantage of its security and management capabilities, but can't

afford to purchase new hardware.

Windows XP Starter Edition

Windows XP Starter Edition is a lower-cost version of Windows XP available in

Thailand, Turkey, Malaysia, Indonesia, Russia, India, Colombia, Brazil, Argentina,

Peru, Bolivia, Chile, Mexico, Ecuador, Uruguay and Venezuela. It is similar to

Windows XP Home, but is limited to low-end hardware, can only run 3 programs at a

time, and has some other features either removed or disabled by default.

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According to a Microsoft press release, Windows XP Starter Edition is "a low-cost

introduction to the Microsoft Windows XP operating system designed for first-time

desktop PC users in developing countries."

Specializations

The Starter Edition includes some special features for certain markets where

consumers may not be computer literate. Not found in the Home Edition, these

include localized help features for those who may not speak English, a country-

specific computer wallpaper and screensavers, and other default settings designed

for easier use than typical Windows XP installations.

In addition, the Starter Edition also has some unique limitations to prevent it from

displacing more expensive versions of Windows XP. Only three applications can be

run at once on the Starter Edition, and each application may open a maximum of

three windows. The maximum screen resolution is 1024×768, and there is no

support for workgroup networking or domains. In addition, the Starter Edition is

licensed only for low-end processors like Intel's Celeron or AMD's Duron and

Sempron. There is also a 512 MB limit on main memory and a 120 GB disk size limit

(Microsoft has not made it clear, however, if this is for total disk space, per partition,

or per disk). There are also fewer options for customizing the themes, desktop, and

taskbar.

Market adoption

On October 9, 2006, Microsoft announced that they reached a milestone of

1,000,000 units of Windows XP Starter Edition sold. In the mass market, however,

the Starter Edition has not had much success. Many markets where it is available

have seen the uptake of illegally cracked or pirated versions of the software instead.

Windows XP Edition N

In March 2004, the European Commission fined Microsoft €497 million (US$603

million) and ordered the company to provide a version of Windows without Windows

Media Player. The Commission concluded that Microsoft "broke European Union

competition law by leveraging its near monopoly in the market for PC operating

systems onto the markets for work group server operating systems and for media

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players". Microsoft is currently appealing the ruling. In the meantime, a court-

compliant version has been released. This version does not include the company's

Windows Media Player but instead encourages users to pick and download their own

media player. Microsoft wanted to call this version Reduced Media Edition, but EU

regulators objected and suggested the Edition N name, with the N signifying "not

with Media Player" for both Home and Professional editions of Windows XP. Due to

the fact that it is sold at the same price as the version with Windows Media Player

included, Dell, Hewlett-Packard, Lenovo and Fujitsu Siemens have chosen not to

stock the product. However, Dell did offer the operating system for a short time.

Consumer interest has been low, with roughly 1,500 units shipped to OEMs, and no

reported sales to consumers.

Languages

Windows XP is available in many languages. In addition, add-ons translating the user

interface are also available for certain languages.

New and updated features

Windows XP introduced several new features to the Windows line, including:

Faster start-up and hibernation sequences

The ability to discard a newer device driver in favour of the previous one

(known as driver rollback), should a driver upgrade not produce desirable

results

A new, arguably more user-friendly interface, including the framework for

developing themes for the desktop environment

Fast user switching, which allows a user to save the current state and open

applications of their desktop and allow another user to log on without losing

that information

The ClearType font rendering mechanism, which is designed to improve text

readability on Liquid Crystal Display (LCD) and similar monitors

Remote Desktop functionality, which allows users to connect to a computer

running Windows XP from across a network or the Internet and access their

applications, files, printers, and devices

Support for most DSL modems and wireless network connections, as well as

networking over FireWire, and Bluetooth.

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User interface

The new "task grouping" feature introduced in Windows XP

Windows XP features a new task-based graphical user interface. The Start menu and

search capability were redesigned and many visual effects were added, including:

A translucent blue selection rectangle in Explorer

Drop shadows for icon labels on the desktop

Task-based sidebars in Explorer windows ("common tasks")

The ability to group the taskbar buttons of the windows of one application into

one button

The ability to lock the taskbar and other toolbars to prevent accidental

changes

The highlighting of recently added programs on the Start menu

Shadows under menus (Windows 2000 had shadows under mouse pointers,

but not menus)

Windows XP analyses the performance impact of visual effects and uses this to

determine whether to enable them, so as to prevent the new functionality from

consuming excessive additional processing overhead. Users can further customize

these settings. Some effects, such as alpha blending (transparency and fading), are

handled entirely by many newer video cards. However, if the video card is not

capable of hardware alpha blending, performance can be substantially hurt and

Microsoft recommends the feature should be turned off manually. Windows XP adds

the ability for Windows to use "Visual Styles" to change the user interface. However,

visual styles must be cryptographically signed by Microsoft to run. Luna is the name

of the new visual style that ships with Windows XP, and is enabled by default for

machines with more than 64 MiB of RAM. Luna refers only to one particular visual

style, not to all of the new user interface features of Windows XP as a whole. In

order to use unsigned visual styles, many users turn to software such as TGTSoft's

StyleXP or Stardock's WindowBlinds. Some users "patch" the uxtheme.dll file that

restricts the ability to use visual styles, created by the general public or the user, on

Windows XP.

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The default wallpaper, Bliss, is a BMP photograph of a landscape in the Napa Valley

outside Napa, California, with rolling green hills and a blue sky with stratocumulus

and cirrus clouds.

The Windows 2000 "classic" interface can be used instead if preferred. Several third

party utilities exist that provide hundreds of different visual styles. In addition,

another Microsoft-created theme, called "Royale", was included with Windows XP

Media Center Edition, and was also released for other versions of Windows XP.

System requirements

System requirements for Windows XP Home and Professional editions as follows:

Minimum Recommended

Processor 233 MHz 300 MHz or higher

Memory

64 MB RAM (may limit

performance and some

features)

128 MB RAM or higher

Video adapter and monitor Super VGA (800 x 600) Super VGA (800 x 600)

or higher resolution

Hard drive disk free space 1.5 GB 1.5 GB or higher

Drives CD-ROM CD-ROM or better

Devices Keyboard and mouse Keyboard and mouse

Others Sound card, speakers, and

headphones

Sound card, speakers,

and headphones

In addition to the Windows XP system requirements, Service Pack 2 requires an

additional 1.8 GB of free hard disk space during installation.

Service packs

Microsoft occasionally releases service packs for its Windows operating systems to fix

problems and add features. Each service pack is a superset of all previous service

packs and patches so that only the latest service pack needs to be installed, and also

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includes new revisions. Older patches need not be removed before application of the

most recent one.

Service Pack 1

Service Pack 1 (SP1) for Windows XP was released on September 9, 2002. It

contains post-RTM security fixes and hot-fixes, compatibility updates, optional .NET

Framework support, enabling technologies for new devices such as Tablet PCs, and a

new Windows Messenger 4.7 version. The most notable new features were USB 2.0

support, and a Set Program Access and Defaults utility that aimed at hiding various

middleware products. Users can control the default application for activities such as

web browsing and instant messaging, as well as hide access to some of Microsoft's

bundled programs. This utility was first brought into the older Windows 2000

operating system with its Service Pack 3.

On February 3, 2003, Microsoft released Service Pack 1 (SP1) again as Service Pack

1a (SP1a). This release removed Microsoft's Java virtual machine as a result of a

lawsuit with Sun Microsystems.

Service Pack 2

Service Pack 2 (SP2) (codenamed "Springboard") was released on August 6, 2004

after several delays, with a special emphasis on security. Unlike the previous service

packs, SP2 adds new functionality to Windows XP, including an enhanced firewall,

improved Wi-Fi support, such as WPA encryption compatibility, with a wizard utility,

a pop-up ad blocker for Internet Explorer, and Bluetooth support. Security

enhancements include a major revision to the included firewall which was renamed

to Windows Firewall and is enabled by default, advanced memory protection that

takes advantage of the NX bit that is incorporated into newer processors to stop

some forms of buffer overflow attacks, and removal of raw socket support (which

supposedly limits the damage done by zombie machines). Additionally, security-

related improvements were made to e-mail and web browsing. Windows XP Service

Pack 2 includes the Windows Security Center, which provides a general overview of

security on the system, including the state of anti-virus software, Windows Update,

and the new Windows Firewall. Third-party anti-virus and firewall applications can

interface with the new Security Center.

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On August 10, 2007, Microsoft announced a minor update to Service Pack 2, called

Service Pack 2c (SP2c). The update fixes the issue of the lowering number of

available product keys for Windows XP. This update will be only available to system

builders from their distributors in Windows XP Professional and Windows XP

Professional N operating systems. SP2c was released in September 2007.

Service Pack 3

Windows XP Service Pack 3 (SP3) is currently in development. As of November 2007,

Microsoft's web site indicates a "preliminary" release date to be in the first half of

2008. A feature set overview has been posted by Microsoft and details new features

available separately as standalone updates to Windows XP, as well as features

backported from Windows Vista, such as black hole router detection, Network Access

Protection and Windows Imaging Component.

Microsoft has begun a beta test of Service Pack 3. According to a file released with

the official beta, and relayed onto the internet, there are a total of 1073 fixes in SP3.

This update to Windows allows it to be installed without a product key, and be run

until the end of the mandatory 30-day activation period without one. The latest

testing build of Service Pack 3 is the Release Candidate build, 3264.

On December 4, 2007, Microsoft released a release candidate of Service Pack 3 to

both TechNet and MSDN Subscribers. On December 18, 2007, this version was made

publicly available via Microsoft Download Center.

Support lifecycle

Support for Windows XP without a service pack (RTM) ended on September 30, 2004

and support for Windows XP Service Pack 1 and 1a ended on October 10, 2006.

Mainstream support for Windows XP Service Pack 2 will end on April 14, 2009, four

years after its general availability. As per Microsoft's posted timetable, the company

will stop licensing Windows XP to OEMs and terminate retail sales of the operating

system June 30, 2008, 17 months after the release of Windows Vista.

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On April 14, 2009, Windows XP will began its "Extended Support" period that will last

for 5 years until April 8, 2014.

Common criticisms

Security issues

Windows XP has been criticized for its susceptibility to malware, viruses, trojan

horses, and worms. Security issues are compounded by the fact that users of the

Home edition, by default, receive an administrator account that provides unrestricted

access to the underpinnings of the system. If the administrator's account is broken

into, there is no limit to the control that can be asserted over the compromised PC.

Windows, with its large market share, has historically been a tempting target for

virus creators. Security holes are often invisible until they are exploited, making

preemptive action difficult. Microsoft has stated that the release of patches to fix

security holes is often what causes the spread of exploits against those very same

holes, as crackers figured out what problems the patches fixed, and then launch

attacks against unpatched systems. Microsoft recommends that all systems have

automatic updates turned on to prevent a system from being attacked by an

unpatched bug, but some business IT departments need to test updates before

deployment across systems to predict compatibility issues with custom software and

infrastructure. This deployment turn-around time also lengthens the time that

systems are left unsecure in the event of a released software exploit.

User interface and performance

Critics have claimed that the default Windows XP user interface (Luna) adds visual

clutter and wastes screen space while offering no new functionality and running more

slowly – with some even calling it 'the Fisher-Price interface'. Users who do not like

the new interface can easily switch back to the Windows Classic theme.

Integration of operating system features

In light of the United States v. Microsoft case which resulted in Microsoft being

convicted for abusing its operating system monopoly to overwhelm competition in

other markets, Windows XP has drawn fire for integrating user applications such as

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Windows Media Player and Windows Messenger into the operating system, as well as

for its close ties to the Windows Live ID service.

Backward compatibility

Some users switching from Windows 9x to XP disliked its lack of DOS support.

Although XP comes with the ability to run DOS programs in a virtual DOS machine

along with the COMMAND.COM program from MS-DOS, XP still has trouble running

many old DOS programs. This is largely due to the fact that it is NT-based and does

not use DOS as a base OS.

Windows Vista

Windows Vista (pronounced /�vɪstə/) is a line of operating systems developed by

Microsoft for use on personal computers, including home and business desktops,

laptops, Tablet PCs, and media centers. Prior to its announcement on July 22, 2005,

Windows Vista was known by its codename "Longhorn".. Development was

completed on November 8, 2006; over the following three months it was released in

stages to computer hardware and software manufacturers, business customers, and

retail channels. On January 30, 2007, it was released worldwide to the general

public,. and was made available for purchase and downloading from Microsoft's web

site.. The release of Windows Vista comes more than five years after the introduction

of its predecessor, Windows XP, making it the longest time span between two

releases of Microsoft Windows.

Windows Vista contains hundreds of new and reworked features; some of the most

significant include an updated graphical user interface and visual style dubbed

Windows Aero, improved searching features, new multimedia creation tools such as

Windows DVD Maker, and completely redesigned networking, audio, print, and

display sub-systems. Vista also aims to increase the level of communication between

machines on a home network using peer-to-peer technology, making it easier to

share files and digital media between computers and devices. Windows Vista includes

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version 3.0 of the .NET Framework, which aims to make it significantly easier for

developers to write applications than with the traditional Windows API.

Microsoft's primary stated objective with Windows Vista, however, has been to

improve the state of security in the Windows operating system.. One common

criticism of Windows XP and its predecessors has been their commonly exploited

security vulnerabilities and overall susceptibility to malware, viruses and buffer

overflows. In light of this, Microsoft chairman Bill Gates announced in early 2002 a

company-wide "Trustworthy Computing initiative" which aims to incorporate security

work into every aspect of software development at the company. Microsoft stated

that it prioritized improving the security of Windows XP and Windows Server 2003

above finishing Windows Vista, thus delaying its completion..

Windows Vista has received a number of negative assessments. PC World listed it #1

of "the 15 biggest tech disappointments of 2007," saying that "many users are

clinging to XP like shipwrecked sailors to a life raft, while others who made the

upgrade are switching back.". Criticism of Windows Vista include protracted

development time, more restrictive licensing terms, the inclusion of a number of new

Digital Rights Management technologies aimed at restricting the copying of protected

digital media, lack of device drivers for some hardware, and the usability of other

new features such as User Account Control.

Development

Microsoft started work on their plans for Windows Vista ("Longhorn") in 2001,. prior

to the release of Windows XP. It was originally expected to ship sometime late in

2003 as a minor step between Windows XP (codenamed "Whistler") and "Blackcomb"

(now known as Windows 7). Gradually, "Longhorn" assimilated many of the

important new features and technologies slated for "Blackcomb", resulting in the

release date being pushed back several times. Many of Microsoft's developers were

also re-tasked with improving the security of Windows XP.. Faced with ongoing

delays and concerns about feature creep, Microsoft announced on August 27, 2004

that it was making changes. The original "Longhorn", based on the Windows XP

source code, was scrapped, and Vista development started anew, building on the

Windows Server 2003 codebase, and re-incorporating only the features that would

be intended for an actual operating system release. Some previously announced

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features such as WinFS were dropped or postponed, and a new software

development methodology called the "Security Development Lifecycle" was

incorporated in an effort to address concerns with the security of the Windows

codebase.

After "Longhorn" was named Windows Vista, an unprecedented beta-test program

was started, involving hundreds of thousands of volunteers and companies. In

September 2005, Microsoft started releasing regular Community Technology

Previews (CTP) to beta testers. The first of these was distributed at the 2005

Microsoft Professional Developers Conference, and was subsequently released to

beta testers and Microsoft Developer Network subscribers. The builds that followed

incorporated most of the planned features for the final product, as well as a number

of changes to the user interface, based largely on feedback from beta testers.

Windows Vista was deemed feature-complete with the release of the "February CTP",

released on February 22, 2006, and much of the remainder of work between that

build and the final release of the product focused on stability, performance,

application and driver compatibility, and documentation. Beta 2, released in late

May, was the first build to be made available to the general public through

Microsoft's Customer Preview Program. It was downloaded by over five million

people. Two release candidates followed in September and October, both of which

were made available to a large number of users..

While Microsoft had originally hoped to have the operating system available

worldwide in time for Christmas 2006, it was announced in March 2006 that the

release date would be pushed back to January 2007, in order to give the company –

and the hardware and software companies which Microsoft depends on for providing

device drivers – additional time to prepare. Through much of 2006, analysts and

bloggers had speculated that Windows Vista would be delayed further, owing to anti-

trust concerns raised by the European Commission and South Korea, and due to a

perceived lack of progress with the beta releases. However, with the November 8,

2006 announcement of the completion of Windows Vista, Microsoft's lengthiest

operating system development project came to an end..

New or improved features

End-user features

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The appearance of Windows Explorer has changed since Windows XP.

Windows Aero: The new hardware-based graphical user interface, named

Windows Aero – an acronym for Authentic, Energetic, Reflective, and Open.

The new interface is intended to be cleaner and more aesthetically pleasing

than those of previous Windows, including new transparencies, live

thumbnails, live icons, animations, and eye candy.

Windows Shell: The new Windows shell is significantly different from Windows

XP, offering a new range of organization, navigation, and search capabilities.

Windows Explorer's task panel has been removed, integrating the relevant

task options into the toolbar. A "Favorite links" panel has been added,

enabling one-click access to common directories. The address bar has been

replaced with a breadcrumb navigation system. The preview panel allows

users to see thumbnails of various files and view the contents of documents.

The details panel shows information such as file size and type, and allows

viewing and editing of embedded tags in supported file formats. The Start

menu has changed as well; it no longer uses ever-expanding boxes when

navigating through Programs. The word "Start" itself has been removed in

favor of a blue Windows Orb (also called "Pearl").

Instant Search (also known as search as you type) : Windows Vista features a

new way of searching called Instant Search, which is significantly faster and

more in-depth (content-based) than the search features found in any of the

previous versions of Windows..

Windows Sidebar: A transparent panel anchored to the side of the screen

where a user can place Desktop Gadgets, which are small applets designed

for a specialized purpose (such as displaying the weather or sports scores).

Gadgets can also be placed on other parts of the desktop.

Windows Internet Explorer 7: New user interface, tabbed browsing, RSS, a

search box, improved printing,. Page Zoom, Quick Tabs (thumbnails of all

open tabs) , Anti-Phishing filter, a number of new security protection

features, Internationalized Domain Name support (IDN) , and improved web

standards support. IE7 in Windows Vista runs in isolation from other

applications in the operating system (protected mode) ; exploits and

malicious software are restricted from writing to any location beyond

Temporary Internet Files without explicit user consent.

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Windows Media Player 11

Windows Media Player 11, a major revamp of Microsoft's program for playing

and organizing music and video. New features in this version include word

wheeling (or "search as you type") , a new GUI for the media library, photo

display and organization, the ability to share music libraries over a network

with other Vista machines, Xbox 360 integration, and support for other Media

Center Extenders.

Backup and Restore Center: Includes a backup and restore application that

gives users the ability to schedule periodic backups of files on their computer,

as well as recovery from previous backups. Backups are incremental, storing

only the changes each time, minimizing the disk usage. It also features

Complete PC Backup (available only in Ultimate, Business, and Enterprise

versions) which backs up an entire computer as an image onto a hard disk or

DVD. Complete PC Backup can automatically recreate a machine setup onto

new hardware or hard disk in case of any hardware failures. Complete PC

Restore can be initiated from within Windows Vista, or from the Windows

Vista installation CD in the event the PC is so corrupt that it cannot start up

normally from the hard disk.

Windows Mail: A replacement for Outlook Express that includes a new mail

store that improves stability, and features integrated Instant Search. It has

the Phishing Filter like IE7 and Junk mail filtering that is enhanced through

regular updates via Windows Update..

Windows Calendar is a new calendar and task application.

Windows Photo Gallery, a photo and movie library management application.

WPG can import from digital cameras, tag and rate individual items, adjust

colors and exposure, create and display slideshows (with pan and fade

effects), and burn slideshows to DVD.

Windows DVD Maker, a companion program to Windows Movie Maker that

provides the ability to create video DVDs based on a user's content. Users can

design a DVD with title, menu, video, soundtrack, pan and zoom motion

effects on pictures or slides.

Windows Media Center, which was previously exclusively bundled as a

separate version of Windows XP, known as Windows XP Media Center Edition,

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has been incorporated into the Home Premium and Ultimate editions of

Windows Vista.

Games and Games Explorer: Games included with Windows have been

modified to showcase Vista's graphics capabilities. New games are Chess

Titans, Mahjong Titans and Purble Place. A new Games Explorer special folder

holds shortcuts and information to all games on the user's computer.

Windows Mobility Center.

Windows Mobility Center is a control panel that centralizes the most relevant

information related to mobile computing (brightness, sound, battery level /

power scheme selection, wireless network, screen orientation, presentation

settings, etc.).

Windows Meeting Space replaces NetMeeting. Users can share applications (or

their entire desktop) with other users on the local network, or over the

Internet using peer-to-peer technology (higher versions than Starter and

Home Basic can take advantage of hosting capabilities, limiting previous to

"join" mode only)

Shadow Copy automatically creates daily backup copies of files and folders.

Users can also create "shadow copies" by setting a System Protection Point

using the System Protection tab in the System control panel. The user can be

presented multiple versions of a file throughout a limited history and be

allowed to restore, delete, or copy those versions. This feature is available

only in the Business, Enterprise, and Ultimate editions of Windows Vista and

is inherited from Windows Server 2003..

Windows Update with Windows Ultimate Extras

Windows Update: Software and security updates have been simplified,. now

operating solely via a control panel instead of as a web application. Windows

Mail's spam filter and Windows Defender's definitions are updated

automatically via Windows Update. Users that choose the recommended

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setting for Automatic Updates will have the latest drivers installed and

available when they add a new device.

Parental controls: Allows administrators to control which websites, programs,

and games each standard user can use and install. This feature is not

included in the Business or Enterprise editions of Vista.

Windows SideShow: Enables the auxiliary displays on newer laptops or on

supported Windows Mobile devices. It is meant to be used to display device

gadgets while the computer is on or off.

Speech recognition is integrated into Vista.. It features a redesigned user

interface and configurable command-and-control commands. Unlike the Office

2003 version, which works only in Office and WordPad, Speech Recognition in

Windows Vista works for any accessible application. In addition, it currently

supports several languages: British and American English, Spanish, French,

German, Chinese (Traditional and Simplified) , and Japanese.

New fonts, including several designed for screen reading, and improved

Chinese (Yahei, JhengHei) , Japanese (Meiryo) and Korean (Malgun) fonts.

See. ClearType has also been enhanced and enabled by default.

Problem Reports and Solutions, a control panel which allows users to view

previously sent problems and any solutions or additional information that is

available.

Improved audio controls allow the system-wide volume or volume of

individual audio devices and even individual applications to be controlled

separately. New audio functionalities such as Room Correction, Bass

Management, Speaker Fill and Headphone virtualization have also been

incorporated.

Windows System Assessment Tool is a tool used to benchmark system

performance. Software such as games can retrieve this rating and modify its

own behavior at runtime to improve performance. The benchmark tests CPU,

RAM, 2-D and 3-D graphics acceleration, Graphics Memory and Hard disk

space...

Windows Ultimate Extras: The Ultimate Edition of Windows Vista provides

access to extra games and tools, available through Windows Update. This

replaces the Microsoft Plus! software bundle that was sold alongside prior

versions of Windows.

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Disk Management: A utility to modify hard disk drive partitions, including

shrinking, creating and formatting new partitions.

Performance Diagnostic Console includes various tools for tuning and

monitoring system performance and resources activities of CPU, disks,

network, memory and other resources. It shows the operations on files, the

opened connections, etc.

Core technologies

Windows Vista is intended to be a technology-based release, to provide a solid base

to include advanced technologies, many of which are related to how the system

functions, and hence not readily visible to the user. An example of this is the

complete restructuring of the architecture of the audio, print, display, and

networking subsystems; while the results of this work will be visible to software

developers, end-users will only see what appear to be evolutionary changes in the

user interface.

Vista includes technologies such as ReadyBoost and ReadyDrive which employ fast

flash memory (located on USB drives and hybrid hard disk drives respectively) to

improve system performance by caching commonly-used programs and data. This

manifests itself in improved battery life on notebook computers as well, since a

hybrid drive can be spun down when not in use. Another new technology called

ReadyDrive utilizes machine learning techniques to analyze usage patterns in order

to allow Windows Vista to make intelligent decisions about what content should be

present in system memory at any given time.

As part of the redesign of the networking architecture, IPv6 has been fully

incorporated into the operating system, and a number of performance improvements

have been introduced, such as TCP window scaling. Prior versions of Windows

typically needed third-party wireless networking software to work properly; this is no

longer the case with Vista, as it includes more comprehensive wireless networking

support.

For graphics, Vista introduces a new Windows Display Driver Model, as well as a

major revision to Direct3D. The new driver model facilitates the new Desktop

Window Manager, which provides the tearing-free desktop and special effects that

are the cornerstones of Windows Aero. Direct3D 10, developed in conjunction with

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major display driver manufacturers, is a new architecture with more advanced

shader support, and allows the graphics processing unit to render more complex

scenes without assistance from the CPU. It features improved load balancing

between CPU and GPU and also optimizes data transfer between them..

At the core of the operating system, many improvements have been made to the

memory manager, process scheduler, heap manager, and I/O scheduler. A Kernel

Transaction Manager has been implemented that gives applications the ability to

work with the file system and registry using atomic transaction operations.

Security-related technologies

Improved security was a primary design goal for Vista.. Microsoft's Trustworthy

Computing initiative, which aims to improve public trust in its products, has had a

direct effect on its development. This effort has resulted in a number of new security

and safety features.

User Account Control is perhaps the most significant and visible of these changes.

User Account Control is a security technology that makes it possible for users to use

their computer with fewer privileges by default. This was often difficult in previous

versions of Windows, as the previous "limited" user accounts proved too restrictive

and incompatible with a large proportion of application software, and even prevented

some basic operations such as looking at the calendar from the notification tray. In

Windows Vista, when an action requiring administrative rights is requested, the user

is first prompted for an administrator name and password; in cases where the user is

already an administrator, the user is still prompted to confirm the pending privileged

action. User Account Control asks for credentials in a Secure Desktop mode, where

the entire screen is blacked out, temporarily disabled, and only the authorization

window is active and highlighted. The intent is to stop a malicious program 'spoofing'

the user interface, attempting to capture admin credentials.

Internet Explorer 7's new security and safety features include a phishing filter, IDN

with anti-spoofing capabilities, and integration with system-wide parental controls.

For added security, ActiveX controls are disabled by default. Also, Internet Explorer

operates in a "protected mode" which operates with lower permissions than the user

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and it runs in isolation from other applications in the operating system, preventing it

from accessing or modifying anything besides the Temporary Internet Files

directory.. Microsoft's anti-spyware product, Windows Defender, has been

incorporated into Windows, providing protection against malware and other threats.

Changes to various system configuration settings (such as new auto-starting

applications) are blocked unless the user gives consent.

Another significant new feature is BitLocker Drive Encryption, a data protection

technology included in the Enterprise and Ultimate editions of Vista that provides

encryption for the entire operating system volume. Bitlocker can work in conjunction

with a Trusted Platform Module chip (version 1.2) that is on a computer's

motherboard, or with a USB key..

A variety of other privilege-restriction techniques are also built into Vista. An

example is the concept of "integrity levels" in user processes, whereby a process

with a lower integrity level cannot interact with processes of a higher integrity level

and cannot perform DLL–injection to a processes of a higher integrity level. The

security restrictions of Windows services are more fine-grained, so that services

(especially those listening on the network) have no ability to interact with parts of

the operating system they do not need to. Obfuscation techniques such as address

space layout randomization are used to increase the amount of effort required of

malware before successful infiltration of a system. Code Integrity verifies that

system binaries haven’t been tampered with by malicious code.

As part of the redesign of the network stack, Windows Firewall has been upgraded,

with new support for filtering both incoming and outgoing traffic. Advanced packet

filter rules can be created which can grant or deny communications to specific

services.

Business technologies

While much of the focus of Vista's new capabilities has been on the new user

interface, security technologies, and improvements to the core operating system,

Microsoft is also adding new deployment and maintenance features.

The WIM image format (Windows IMage) is the cornerstone of Microsoft's new

deployment and packaging system. WIM files, which contain an image of

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Windows Vista, can be maintained and patched without having to rebuild new

images. Windows Images can be delivered via Systems Management Server

or Business Desktop Deployment technologies. Images can be customized

and configured with applications then deployed to corporate client personal

computers using little to no touch by a system administrator. ImageX is the

Microsoft tool used to create and customize images.

Windows Deployment Services replaces Remote Installation Services for

deploying Vista and prior versions of Windows.

Approximately 700 new Group Policy settings have been added, covering

most aspects of the new features in the operating system, as well as

significantly expanding the configurability of wireless networks, removable

storage devices, and user desktop experience. Vista also introduced an XML

based format (ADMX) to display registry-based policy settings, making it

easier to manage networks that span geographic locations and different

languages. .

Services for UNIX has been renamed "Subsystem for UNIX-based

Applications," and is included with the Enterprise and Ultimate editions of

Vista. Network File System (NFS) client support is also included.

Multilingual User Interface - Unlike previous version of Windows which

required language packs to be loaded to provide local language support,

Windows Vista Ultimate and Enterprise editions support the ability to

dynamically change languages based on the logged on user's preference.

Wireless Projector support

Business customers who are enrolled in the Microsoft Software Assurance program

are offered a set of additional tools and services collectively known as the "Desktop

Optimization Pack". This includes the Microsoft SoftGrid application virtualization

platform, an asset inventory service, and additional tools for maintaining Group

Policy settings in a fashion similar to a revision control system.

Developer technologies

Windows Vista includes a large number of new application programming interfaces.

Chief among them is the inclusion of version 3.0 of the .NET Framework, which

consists of a class library and Common Language Runtime. Version 3.0 includes four

new major components:.

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Windows Presentation Foundation is a user interface subsystem and

framework based vector graphics, which makes use of 3D computer graphics

hardware and Direct3D technologies. It provides the foundation for building

applications and blending together application UI, documents, and media

content. It is the successor to Windows Forms.

Windows Communication Foundation is a service-oriented messaging

subsystem which enables applications and systems to interoperate locally or

remotely using Web services.

Windows Workflow Foundation provides task automation and integrated

transactions using workflows. It is the programming model, engine and tools

for building workflow-enabled applications on Windows.

Windows CardSpace is a component which securely stores digital identities of

a person, and provides a unified interface for choosing the identity for a

particular transaction, such as logging into a website.

These technologies are also available for Windows XP and Windows Server 2003 to

facilitate their introduction to and usage by developers and end users.

There are also significant new development APIs in the core of the operating system,

notably the completely re-architected audio, networking, print, and video interfaces,

major changes to the security infrastructure, improvements to the deployment and

installation of applications ("ClickOnce" and Windows Installer 4.0) , new device

driver development model ("Windows Driver Foundation") , Transactional NTFS,

mobile computing API advancements (power management, Tablet PC Ink support,

SideShow) and major updates to (or complete replacements of) many core

subsystems such as Winlogon and CAPI.

There are some issues for software developers using some of the graphics APIs in

Vista. Games or programs which are built solely on Vista's version of DirectX, 10,

cannot work on prior versions of Windows, as DirectX 10 is not backwards-

compatible with these versions.. According to a Microsoft blog, there are three

choices for OpenGL implementation on Vista. An application can use the default

implementation, which translates OpenGL calls into the Direct3D API and is frozen at

OpenGL version 1.4, or an application can use an Installable Client Driver (ICD) ,

which comes in two flavors: legacy and Vista-compatible. A legacy ICD, the kind

already provided by independent hardware vendors targeting Windows XP, disable

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the Desktop Window Manager. A Vista-compatible ICD takes advantage of a new

API, and is fully compatible with the Desktop Window Manager.. At least two primary

vendors, ATI and NVIDIA, are expected to provide full Vista-compatible ICDs in the

near future.. However, hardware overlay is not supported, because it is considered

as an obsolete feature in Vista. ATI and NVIDIA strongly recommend using

compositing desktop/FBOs for same functionality..

Deprecated features

Some notable Windows XP features and components have been replaced or removed

in Windows Vista, including Windows Messenger, the network Messenger Service,

HyperTerminal, MSN Explorer, Active Desktop, and the replacement of NetMeeting

with Windows Meeting Space. Windows Vista also does not include the Windows XP

"Luna" visual theme, or most of the classic color schemes which have been part of

Windows since the Windows 3.x era. The "Hardware profiles" startup feature has

been removed as well, along with support for older motherboard technologies like

the EISA bus, APM and Game port support. There is a way to enable Game port

support on Vista by applying an older driver.. IP over FireWire (TCP/IP over IEEE

1394) has been removed as well..

Some traditional features of Windows have been either eliminated or severely

crippled. For example, Wordpad will no longer open or save files in .doc format, and

Windows Sound Recorder has had much of its functionality removed. WinHlp32.exe,

used to display 32-bit .hlp files (help pages), is no longer included in Windows Vista

as Microsoft considers it obsolete,. though it is available as a separate download.

Microsoft prohibits software manufacturers from re-introducing the .hlp help system

with their products. Finally, Telnet.exe is no longer installed by default, but is still

included as an installable feature..

Editions

Windows Vista ships in six editions.. These editions are roughly divided into two

target markets, consumer and business, with editions varying to cater for specific

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sub-markets. For consumers, there are four editions, with three available for

developed countries. Windows Vista Starter edition is limited to emerging markets.

Windows Vista Home Basic is intended for budget users with low needs. Windows

Vista Home Premium covers the majority of the consumer market. Windows Vista

Ultimate contains the complete feature-set and is aimed at enthusiasts. For

businesses, there are two versions. Windows Vista Business is specifically designed

for small business., while Windows Vista Enterprise, the premium business edition. is

only available to customers participating in Microsoft's Software Assurance program.

All editions except Windows Vista Starter support both 32-bit (x86) and 64-bit (x64)

processor architectures.

In the European Union, Home Basic N and Business N versions are also available.

These versions come without Windows Media Player, due to EU sanctions brought

against Microsoft for violating anti-trust laws. Similar sanctions exist in South Korea.

Visual styles

Windows Vista has four distinct visual styles.

Windows Aero

Windows Vista's premier visual style is built on a new desktop composition

engine called Desktop Window Manager. Windows Aero introduces support for

3D graphics (Windows Flip 3D) , translucency effects (Glass) , live

thumbnails, window animations, and other visual effects, and is intended for

mainstream and high-end graphics cards. To enable these features, the

contents of every open window is stored in video memory to facilitate tearing-

free movement of windows. As such, Windows Aero has significantly higher

hardware requirements than its predecessors. 128 MB of graphics memory is

the minimum requirement, depending on resolution used.. Windows Aero

(including Windows Flip 3D) is not included in the Starter and Home Basic

editions.

Windows Vista Standard

This mode is a variation of Windows Aero without the glass effects, window

animations, and other advanced graphical effects such as Windows Flip 3D.

Like Windows Aero, it uses the Desktop Window Manager, and has generally

the same video hardware requirements as Windows Aero. This is the default

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mode for the Windows Vista Home Basic Edition. The Starter Edition does not

support this mode.

Windows Vista Basic

This mode has aspects that are similar to Windows XP's visual style with the

addition of subtle animations such as those found on progress bars. It does

not employ the Desktop Window Manager; as such, it does not feature

transparency or translucency, window animation, Windows Flip 3D or any of

the functions provided by the DWM. The Basic mode does not require the new

Windows Display Driver Model (WDDM) for display drivers, and has similar

graphics card requirements to Windows XP. For computers with graphics

cards that are not powerful enough to support Windows Aero, this is the

default graphics mode.

Windows Classic

Windows Classic has the look and feel of Windows 2000 and Windows Server

2003, does not use the Desktop Window Manager, and does not require a

WDDM driver. As with prior versions of Windows, this visual style supports

"color schemes," which are a collection of color settings. Windows Vista

includes six classic color schemes, comprised of four high-contrast color

schemes and the default color schemes from Windows 98 and Windows 2000.

Hardware requirements

Computers capable of running Windows Vista are classified as Vista Capable and

Vista Premium Ready.. A Vista Capable or equivalent PC is capable of running all

editions of Windows Vista although some of the special features and high end

graphics options may require additional or more advanced hardware. A Vista

Premium Ready PC can take advantage of Vista's "high-end" features..

Windows Vista's "Basic" and "Classic" interfaces work with virtually any graphics

hardware that supports Windows XP or 2000; accordingly, most discussion around

Vista's graphics requirements centers on those for the Windows Aero interface. As of

Windows Vista Beta 2, the NVIDIA GeForce 6 series and later, the ATI Radeon 9500

and later, Intel's GMA 950 integrated graphics, and a handful of VIA chipsets and S3

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Graphics discrete chips are supported. Although originally supported, the GeForce FX

5 series has been dropped from newer drivers from NVIDIA. The last driver from

NVIDIA to support the GeForce FX series on Vista was 96.85... Microsoft offers a tool

called the Windows Vista Upgrade Advisor. to assist XP and Vista users in

determining what versions of Windows their machine is capable of running. Although

the installation media included in retail packages is a 32-bit DVD, customers without

a DVD-ROM or customers who wish for a 64-bit install media are able to acquire this

media through the Windows Vista Alternate Media program..

Windows Vista system requirements

Vista Capable. Vista Premium Ready.

Processor 800 MHz 1.0 GHz

Memory 512 MB RAM 1 GB RAM

Graphics card DirectX 9 capable

DirectX 9 capable GPU with Hardware

Pixel Shader v2.0 and WDDM 1.0

driver support

Graphics memory N/A

128 MB RAM supports up to 2,756,000

total pixels (e.g. 1920 × 1200) or 512

MB+ for greater resolutions such as

2560x1600.

HDD capacity 20 GB 40 GB

HDD free space 15 GB 15 GB

Other drives CD-ROM DVD-ROM

Service Pack 1

Microsoft occasionally releases service packs for its Windows operating systems to fix

problems and add features. Windows Vista Service Pack 1 (SP1) is currently in

development. Microsoft is planning to release SP1 alongside Windows Server 2008 in

the first quarter of 2008.... The first beta of Windows Vista Service Pack 1, build

16659, was released on September 24, 2007 and is currently being tested by

TechBeta participants in the Windows Vista SP1 Beta Program as well as TechNet and

MSDN subscribers..

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On December 12, 2007, Microsoft released Windows Vista Service Pack 1 (SP1)

Release Candidate as an open beta to the general public. The RC build is

documented to contain 489 patches, most of which are documented in the Microsoft

Knowledge Base but are unavailable for download..

A whitepaper published by Microsoft near the end of August 2007 outlined the scope

and intent of the service pack, identifying three major areas of improvement:

reliability and performance, administration experience, and support for newer

hardware and standards.

One area of particular note is performance. Areas of improvement include file copy

operations, hibernation, logging off on domain-joined machines, Javascript parsing in

Internet Explorer, network file share browsing. Windows Explorer ZIP file handling.

and Windows Disk Defragmenter. The ability to choose individual drives to

defragment is being reintroduced as well.

Service Pack 1 introduces support for some new hardware and software standards,

notably the exFAT file system, 802.11n wireless networking, IPv6 over VPN

connections, and the Secure Socket Tunneling Protocol. An updated version of

Windows Installer is included that provides support for multi-package transactions

and embedding the user interface of a child Windows Installer package inside a

parent installation session. Booting a system using Extensible Firmware Interface on

x64 systems is also being introduced; this feature had originally been slated for the

initial release of Vista but was delayed due to a lack of compatible hardware at the

time.

Two areas which have seen changes in Service Pack 1 that have come as the result

of concerns from software vendors. One of these is desktop search; users will be

able to change the default desktop search program to one provided by a third party

instead of the Microsoft desktop search program that comes with Windows Vista.

Third-party desktop search programs will be able to seamlessly tie in their services

into the operating system. These changes come in part due to complaints from

Google, whose Google Desktop Search application was hindered by the presence of

Vista's built-in desktop search. In June 2007, Google claimed that the changes being

introduced for Service Pack 1 "are a step in the right direction, but they should be

improved further to give consumers greater access to alternate desktop search

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providers ". The other area of note is a set of new security APIs being introduced for

the benefit of antivirus software that currently relies on the unsupported practice of

patching the kernel.

An update to Direct3D, 10.1, is planned for inclusion,. which is expected to make

mandatory several features which were previously optional in Direct3D 10 hardware.

The whitepaper also notes that Service Pack 1 will include a kernel that will be up-to-

date with the version to be shipped with Windows Server 2008.

Support for the Group Policy Management Console is being removed; a replacement

is planned for release the same time frame as the release of the service pack.

Criticism

Windows Vista has received a number of negative assessments. Criticisms of

Windows Vista include protracted development time, more restrictive licensing

terms, the inclusion of a number of technologies aimed at restricting the copying of

protected digital media, and the usability of the new User Account Control security

technology. Reviewers have also noted some similarities between Vista's Aero

interface and that of Apple's Aqua interface for the Mac OS X operating system.

Moreover, some concerns have been raised about many PCs meeting "Vista Premium

Ready" hardware requirements and Vista's pricing.

Hardware requirements

Whilst, according to Microsoft, "nearly all PCs on the market today [2005] will run

Windows Vista”, the higher requirements of some of the 'premium' features, such as

the Aero interface, have impacted many upgraders. According to The Times in May

2006, the full set of features “would be available to less than 5 percent of Britain’s

PC market”. This continuing lack of clarity eventually lead to a class action against

Microsoft as people found themselves with new computers that were unable to run

the new software despite assurances.

Slow file operations

When released, Vista performed file operations such as copying and deletion more

slowly than other operating systems. Large copies required when migrating from one

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computer to another seemed difficult or impossible without workarounds such as

using the command line. This inability to perform basic file operations efficiently

attracted strong criticism. After six months, Microsoft confirmed the existence of

these problems by releasing a special performance and reliability update, which was

later disseminated through Windows Update, and will be included in SP1.

Licensing and cost

The introduction of additional licensing restrictions has been criticized. Criticism of

upgrade licenses pertaining to Windows Vista Starter through Home Premium was

expressed by Ars Technica's Ken Fisher, who noted that the new requirement of

having a prior operating system already installed was going to cause irritation for

users who reinstall Windows on a regular basis. It has been revealed that an

Upgrade copy Windows Vista can be installed clean without first installing a previous

version of Windows. On the first install, Windows will refuse to activate. The user

must then reinstall that same copy of Vista. Vista will then activate on the reinstall,

thus allowing a user to install an Upgrade of Windows Vista without owning a

previous operating system. As with Windows XP, separate rules still apply to OEM

versions of Vista installed on new PCs; these are not legally transferable. The cost of

Windows Vista has also been a source of concern and commentary. A majority of

users in a poll said that the prices of various Windows Vista editions posted on the

Microsoft Canada website in August 2006 make the product too expensive. A BBC

News report on the day of Vista's release suggested that, "there may be a backlash

from consumers over its pricing plans - with the cost of Vista versions in the US

roughly half the price of equivalent versions in the UK".

Digital rights management

Another common criticism concerns the integration of new forms of digital rights

management into the operating system, specifically the introduction of the Protected

Video Path. This architecture is designed such that "premium content" from HD DVD

or Blu-ray discs may mandate that the connections between PC components be

encrypted. Devices such as graphic cards must be approved by Microsoft. Depending

on what the content demands, the devices may not pass premium content over non-

encrypted outputs, or they must artificially degrade the quality of the signal on such

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outputs or not display it all. There is also a revocation mechanism that allows

Microsoft to disable drivers of compromised devices in end-user PCs over the

Internet. Peter Gutmann, security researcher and author of the open source cryptlib

library, claims that these mechanisms violate fundamental rights of the user (such as

fair use), unnecessarily increase the cost of hardware, and make systems less

reliable, (the "tilt bit" is a particular worry; if triggered, the entire graphic subsystem

performs a reset) and vulnerable to denial-of-service attacks. Proponents have

claimed that Microsoft had no choice but to follow the demands of the movie studios,

and that the technology will not actually be enabled until after 2010; Microsoft also

noted that content protection mechanisms have existed in Windows as far back as

Windows Me, and that the new protections will not apply to any existing content

(only future content).

User Account Control

Concerns have been raised about the new User Account Control (UAC) security

technology. While Yankee Group analyst Andrew Jaquith believes that critical security

vulnerabilities may be "reduced by as much as 80%," he also noted that "while the

new security system shows promise, it is far too chatty and annoying". This

statement was made over six months before Vista was actually released. When

Windows Vista was released in November 2006, Microsoft had reduced the number of

operating system tasks that triggered UAC prompts, and added file and registry

virtualization to reduce the number of legacy applications that trigger UAC prompts.

Despite reductions in UAC prompts they still are triggered by many programs,

particularly programs not designed for Windows Vista.

Software Protection Platform

Vista includes an enhanced set of anti-piracy technologies, based on Windows XP's

WGA, called Software Protection Platform (SPP). A major component of this is a new

reduced functionality mode, which Vista enters when it detects that the user has

"failed product activation or of that copy being identified as counterfeit or non-

genuine", which is described in a Microsoft white paper as follows: "The default Web

browser will be started and the user will be presented with an option to purchase a

new product key. There is no start menu, no desktop icons, and the desktop

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background is changed to black. [...] After one hour, the system will log the user out

without warning". This has been criticised for being overly draconian, especially

given reports of "false positives" by SPP's predecessor, and at least one temporary

validation server outage. Microsoft will be removing the reduced functionality mode

in Service Pack 1 in favor of prominent notices on systems not found to be genuine.

Public reception and sales

Due to the large growth of the PC market since the release of Windows XP, initial

sales of the operating system set a new high. Within its first month, 20 million copies

of Vista were sold, double the amount of XP sales within its first month in October

2001, five years earlier. That said, as a factor of the new market, Vista sales were

not high. For example, rival operating system Mac OS X Leopard's first month's sales

also doubled over the number of sales from the release of Mac OS X Jaguar five

years earlier in August 2002. However, in the case of Jaguar-to-Leopard sales, as

opposed to XP-to-Vista sales, users had less pressure to upgrade due to the

intermediate releases of both Mac OS X Panther and Tiger.

Even with these high sales, Windows Vista has received worse-than-expected public

reception due to the various criticisms and concerns about the operating system,

forcing Microsoft to extend Windows XP support and allow continued sales of the

older OS, as well as a general reduction in the percentage of PC sales.

Windows Vista has received relatively poor reception among consumers, with some

people having installed XP onto computers that were preloaded with Vista or

reverting their own upgrades, and many computer manufacturers are even including

XP restore disks with new computers. There is also a relative market decrease in

consumers intending to purchase PCs: according to studies conducted by

ChangeWave in November 2007, among potential buyers within the next three

months, only 71 percent of consumers (not counting businesses) intend tend to buy

PCs (with the remaining 29 percent instead favoring Macs), a number down from

approximately 87 percent in October 2005. 83 percent of those intending to

purchase Macs said that they "are choosing Macs because of Leopard and their

distaste for Vista". The same study also showed an increase in businesses intending

to buy Macs instead of Windows-based PCs, up from 4 to 7 percent between

November 2005 and November 2007, partly due to Vista's reception, as well.

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Business adoption of Vista has been slower than anticipated, with the vast majority

still favoring Windows XP and even waiting for Windows 7, Microsoft's next version of

Windows scheduled for release in 2010. According to InformationWeek, in December

2006, 6 percent of business enterprises were expected to employ Vista within the

first year, yet as of December 2007, only about 1 percent of enterprises are actually

using Vista. Furthermore, while many businesses have bought licenses to run

Windows Vista, these companies are holding off deployment.

The usage share, as measured through web browser user agent strings, shows Vista

to have gained approximately 6.3% of the desktop OS market as of Q3 2007.

Though starting off slowly, speed appears to have increased in the last quarter. Even

so, as of December 2007, Amazon.com's bestselling operating systems list indicates

that Vista sales are still trailing the combined sales of Mac OS X Leopard and

Windows XP.

On Sunday,14 October 2007 The Dutch Consumers Association called for a boycott of

Windows Vista, after the software giant refused to offer free copies of Windows XP to

users who had problems with Vista. The 'Consumentenbond' said that the product

has many teething problems, and "is just not ready". The association claims it

received over 5000 complaints about Vista. Many printers and other hardware failed

to work, the association says, computers crash frequently and peripherals are very

slow.

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Algorithm

In mathematics, computing, linguistics, and related disciplines, an algorithm is a

definite list of well-defined instructions for completing a task; that given an initial

state, will proceed through a well-defined series of successive states, eventually

terminating in an end-state. The transition from one state to the next is not

necessarily deterministic; some algorithms, known as probabilistic algorithms,

incorporate randomness.

The concept of an algorithm originated as a means of recording procedures for

solving mathematical problems such as finding the common divisor of two numbers

or multiplying two numbers. A partial formalization of the concept began with

attempts to solve the Entscheidungsproblem (the "decision problem") that David

Hilbert posed in 1928. Subsequent formalizations were framed as attempts to define

"effective calculability" or "effective method”; those formalizations included the

Gödel-Herbrand-Kleene recursive functions of 1930, 1934 and 1935, Alonzo Church's

lambda calculus of 1936, Emil Post's "Formulation I" of 1936, and Alan Turing's

Turing machines of 1936-7 and 1939.

Etymology

Al-Khwārizmī, Persian astronomer and mathematician, wrote a treatise in Arabic in

825 AD, On Calculation with Hindu Numerals. It was translated into Latin in the 12th

century as Algoritmi de numero Indorum, which title was likely intended to mean

"Algoritmus on the numbers of the Indians", where "Algoritmi" was the translator's

rendition of the author's name in the genitive case; but people misunderstanding the

title treated Algoritmi as a Latin plural and this led to the word "algorithm" (Latin

algorismus) coming to mean "calculation method". The intrusive "th" is most likely

due to a false cognate with the Greek αριθμος (arithmos) meaning "number".

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Flowcharts are often used to graphically represent algorithms.

Why algorithms are necessary: an informal definition

No generally accepted formal definition of "algorithm" exists yet. We can, however,

derive clues to the issues involved and an informal meaning of the word from the

following quotation from Boolos and Jeffrey:

"No human being can write fast enough, or long enough, or small enough to

list all members of an enumerably infinite set by writing out their names, one

after another, in some notation. But humans can do something equally useful,

in the case of certain enumerably infinite sets: They can give explicit

instructions for determining the nth member of the set, for arbitrary

finite n. Such instructions are to be given quite explicitly, in a form in which

they could be followed by a computing machine, or by a human who is

capable of carrying out only very elementary operations on symbols" .

The words "enumerably infinite" mean "countable using integers perhaps extending

to infinity". Thus Boolos and Jeffrey are saying that an algorithm implies instructions

for a process that "creates" output integers from an arbitrary "input" integer or

integers that, in theory, can be chosen from 0 to infinity. Thus we might expect an

algorithm to be an algebraic equation such as y = m + n — two arbitrary "input

variables" m and n that produce an output y. As we see in Algorithm

characterizations — the word algorithm implies much more than this, something on

the order of (for our addition example):

Precise instructions (in language understood by "the computer") for a "fast,

efficient, good" process that specifies the "moves" of "the computer"

(machine or human, equipped with the necessary internally-contained

information and capabilities) to find, decode, and then munch arbitrary input

integers/symbols m and n, symbols + and = ... and (reliably, correctly,

"effectively") produce, in a "reasonable" time, output-integer y at a specified

place and in a specified format.

The concept of algorithm is also used to define the notion of decidability (logic). That

notion is central for explaining how formal systems come into being starting from a

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small set of axioms and rules. In logic, the time that an algorithm requires to

complete cannot be measured, as it is not apparently related with our customary

physical dimension. From such uncertainties, that characterize ongoing work, stems

the unavailability of a definition of algorithm that suits both concrete (in some sense)

and abstract usage of the term.

For a detailed presentation of the various points of view around the definition

of "algorithm" see Algorithm characterizations. For examples of simple

addition algorithms specified in the detailed manner described in Algorithm

characterizations, see Algorithm examples.

Formalization of algorithms

Algorithms are essential to the way computers process information, because a

computer program is essentially an algorithm that tells the computer what specific

steps to perform (in what specific order) in order to carry out a specified task, such

as calculating employees’ paychecks or printing students’ report cards. Thus, an

algorithm can be considered to be any sequence of operations that can be performed

by a Turing-complete system. Authors who assert this thesis include Savage and

Gurevich:

"...Turing's informal argument in favor of his thesis justifies a stronger thesis:

every algorithm can be simulated by a Turing machine" ...according to

Savage, "an algorithm is a computational process defined by a Turing

machine."

Typically, when an algorithm is associated with processing information, data are read

from an input source or device, written to an output sink or device, and/or stored for

further processing. Stored data are regarded as part of the internal state of the

entity performing the algorithm. In practice, the state is stored in a data structure,

but an algorithm requires the internal data only for specific operation sets called

abstract data types.

For any such computational process, the algorithm must be rigorously defined:

specified in the way it applies in all possible circumstances that could arise. That is,

any conditional steps must be systematically dealt with, case-by-case; the criteria

for each case must be clear (and computable).

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Because an algorithm is a precise list of precise steps, the order of computation will

almost always be critical to the functioning of the algorithm. Instructions are usually

assumed to be listed explicitly, and are described as starting 'from the top' and going

'down to the bottom', an idea that is described more formally by flow of control.

So far, this discussion of the formalization of an algorithm has assumed the premises

of imperative programming. This is the most common conception, and it attempts to

describe a task in discrete, 'mechanical' means. Unique to this conception of

formalized algorithms is the assignment operation, setting the value of a variable. It

derives from the intuition of 'memory' as a scratchpad. There is an example below of

such an assignment.

For some alternate conceptions of what constitutes an algorithm see functional

programming and logic programming .

Termination

Some writers restrict the definition of algorithm to procedures that eventually finish.

In such a category Kleene places the "decision procedure or decision method or

algorithm for the question". Others, including Kleene, include procedures that could

run forever without stopping; such a procedure has been called a "computational

method" or "calculation procedure or algorithm"; however, Kleene notes that such a

method must eventually exhibit "some object".

Minsky makes the pertinent observation, in regards to determining whether an

algorithm will eventually terminate (from a particular starting state):

"But if the length of the process is not known in advance, then 'trying' it may

not be decisive, because if the process does go on forever — then at no time

will we ever be sure of the answer"

As it happens, no other method can do any better, as was shown by Alan Turing with

his celebrated result on the undecidability of the so-called halting problem. There is

no algorithmic procedure for determining of arbitrary algorithms whether or not they

terminate from given starting states. The analysis of algorithms for their likelihood of

termination is called Termination analysis.

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In the case of non-halting computation method (calculation procedure) success can

no longer be defined in terms of halting with a meaningful output. Instead, terms of

success that allow for unbounded output sequences must be defined. For example,

an algorithm that verifies if there are more zeros than ones in an infinite random

binary sequence must run forever to be effective. If it is implemented correctly,

however, the algorithm's output will be useful: for as long as it examines the

sequence, the algorithm will give a positive response while the number of examined

zeros outnumber the ones, and a negative response otherwise. Success for this

algorithm could then be defined as eventually outputting only positive responses if

there are actually more zeros than ones in the sequence, and in any other case

outputting any mixture of positive and negative responses.

See the examples of (im-)"proper" subtraction at partial function for more about

what can happen when an algorithm fails for certain of its input numbers — e.g. (i)

non-termination, (ii) production of "junk" (output in the wrong format to be

considered a number) or no number(s) at all (halt ends the computation with no

output), (iii) wrong number(s), or (iv) a combination of these. Kleene proposed that

the production of "junk" or failure to produce a number is solved by having the

algorithm detect these instances and produce e.g. an error message (he suggested

"0"), or preferably, force the algorithm into an endless loop. Davis does this to his

subtraction algorithm — he fixes his algorithm in a second example so that it is

proper subtraction. Along with the logical outcomes "true" and "false" Kleene also

proposes the use of a third logical symbol "u" — undecided — thus an algorithm will

always produce something when confronted with a "proposition". The problem of

wrong answers must be solved with an independent "proof" of the algorithm e.g.

using induction:

"We normally require auxiliary evidence for this (that the algorithm correctly

defines a mu recursive function), e.g. in the form of an inductive proof that,

for each argument value, the computation terminates with a unique value"

Expressing algorithms

Algorithms can be expressed in many kinds of notation, including natural languages,

pseudocode, flowcharts, and programming languages. Natural language expressions

of algorithms tend to be verbose and ambiguous, and are rarely used for complex or

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technical algorithms. Pseudocode and flowcharts are structured ways to express

algorithms that avoid many of the ambiguities common in natural language

statements, while remaining independent of a particular implementation language.

Programming languages are primarily intended for expressing algorithms in a form

that can be executed by a computer, but are often used as a way to define or

document algorithms.

There is a wide variety of representations possible and one can express a given

Turing machine program as a sequence of machine tables (see more at finite state

machine and state transition table), as flowcharts (see more at state diagram), or as

a form of rudimentary machine code or assembly code called "sets of quadruples"

(see more at Turing machine).

Sometimes it is helpful in the description of an algorithm to supplement small "flow

charts" (state diagrams) with natural-language and/or arithmetic expressions written

inside "block diagrams" to summarize what the "flow charts" are accomplishing.

Representations of algorithms are generally classed into three accepted levels of

Turing machine description:

1 High-level description:

"...prose to describe an algorithm, ignoring the implementation details. At this

level we do not need to mention how the machine manages its tape or head"

2 Implementation description:

"...prose used to define the way the Turing machine uses its head and the

way that it stores data on its tape. At this level we do not give details of

states or transition function"

3 Formal description:

Most detailed, "lowest level", gives the Turing machine's "state table".

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For an example of the simple algorithm "Add m+n" described in all three

levels

Implementation

Most algorithms are intended to be implemented as computer programs. However,

algorithms are also implemented by other means, such as in a biological neural

network (for example, the human brain implementing arithmetic or an insect looking

for food), in an electrical circuit, or in a mechanical device.

Example

One of the simplest algorithms is to find the largest number in an (unsorted) list of

numbers. The solution necessarily requires looking at every number in the list, but

only once at each. From this follows a simple algorithm, which can be stated in a

high-level description English prose, as:

High-level description:

1. Assume the first item is largest.

2. Look at each of the remaining items in the list and if it is larger than the

largest item so far, make a note of it.

3. The last noted item is the largest in the list when the process is complete.

(Quasi-) Formal description: Written in prose but much closer to the high-level

language of a computer program, the following is the more formal coding of the

algorithm in pseudocode or pidgin code:

Algorithm LargestNumber

Input: A non-empty list of numbers L.

Output: The largest number in the list L.

largest ← L0

for each item in the list L≥1, do

if the item > largest, then

largest ← the item

return largest

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"←" is a loose shorthand for "changes to". For instance, "largest ← item"

means that the value of largest changes to the value of item.

"return" terminates the algorithm and outputs the value that follows.

For a more complex example of an algorithm, see Euclid's algorithm for the greatest

common divisor, one of the earliest algorithms known.

Algorithm analysis

As it happens, it is important to know how much of a particular resource (such as

time or storage) is required for a given algorithm. Methods have been developed for

the analysis of algorithms to obtain such quantitative answers; for example, the

algorithm above has a time requirement of O(n), using the big O notation with n as

the length of the list. At all times the algorithm only needs to remember two values:

the largest number found so far, and its current position in the input list. Therefore it

is said to have a space requirement of O(1). (Note that the size of the inputs is not

counted as space used by the algorithm.)

Different algorithms may complete the same task with a different set of instructions

in less or more time, space, or effort than others. For example, given two different

recipes for making potato salad, one may have peel the potato before boil the potato

while the other presents the steps in the reverse order, yet they both call for these

steps to be repeated for all potatoes and end when the potato salad is ready to be

eaten.

The analysis and study of algorithms is a discipline of computer science, and is often

practiced abstractly without the use of a specific programming language or

implementation. In this sense, algorithm analysis resembles other mathematical

disciplines in that it focuses on the underlying properties of the algorithm and not on

the specifics of any particular implementation. Usually pseudocode is used for

analysis as it is the simplest and most general representation.

Classes

There are various ways to classify algorithms, each with its own merits.

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Classification by implementation

One way to classify algorithms is by implementation means.

Recursion or iteration: A recursive algorithm is one that invokes (makes

reference to) itself repeatedly until a certain condition matches, which is a

method common to functional programming. Iterative algorithms use

repetitive constructs like loops and sometimes additional data structures like

stacks to solve the given problems. Some problems are naturally suited for

one implementation or the other. For example, towers of hanoi is well

understood in recursive implementation. Every recursive version has an

equivalent (but possibly more or less complex) iterative version, and vice

versa.

Logical: An algorithm may be viewed as controlled logical deduction. This

notion may be expressed as:

Algorithm = logic + control.

The logic component expresses the axioms that may be used in the

computation and the control component determines the way in which

deduction is applied to the axioms. This is the basis for the logic programming

paradigm. In pure logic programming languages the control component is

fixed and algorithms are specified by supplying only the logic component. The

appeal of this approach is the elegant semantics: a change in the axioms has

a well defined change in the algorithm.

Serial or parallel or distributed: Algorithms are usually discussed with the

assumption that computers execute one instruction of an algorithm at a time.

Those computers are sometimes called serial computers. An algorithm

designed for such an environment is called a serial algorithm, as opposed to

parallel algorithms or distributed algorithms. Parallel algorithms take

advantage of computer architectures where several processors can work on a

problem at the same time, whereas distributed algorithms utilise multiple

machines connected with a network. Parallel or distributed algorithms divide

the problem into more symmetrical or asymmetrical subproblems and collect

the results back together. The resource consumption in such algorithms is not

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only processor cycles on each processor but also the communication overhead

between the processors. Sorting algorithms can be parallelized efficiently, but

their communication overhead is expensive. Iterative algorithms are generally

parallelizable. Some problems have no parallel algorithms, and are called

inherently serial problems.

Deterministic or non-deterministic: Deterministic algorithms solve the

problem with exact decision at every step of the algorithm whereas non-

deterministic algorithm solve problems via guessing although typical guesses

are made more accurate through the use of heuristics.

Exact or approximate: While many algorithms reach an exact solution,

approximation algorithms seek an approximation that is close to the true

solution. Approximation may use either a deterministic or a random strategy.

Such algorithms have practical value for many hard problems.

Classification by design paradigm

Another way of classifying algorithms is by their design methodology or paradigm.

There is a certain number of paradigms, each different from the other. Furthermore,

each of these categories will include many different types of algorithms. Some

commonly found paradigms include:

Divide and conquer. A divide and conquer algorithm repeatedly reduces an

instance of a problem to one or more smaller instances of the same problem

(usually recursively), until the instances are small enough to solve easily. One

such example of divide and conquer is merge sorting. Sorting can be done on

each segment of data after dividing data into segments and sorting of entire

data can be obtained in conquer phase by merging them. A simpler variant of

divide and conquer is called decrease and conquer algorithm, that solves

an identical subproblem and uses the solution of this subproblem to solve the

bigger problem. Divide and conquer divides the problem into multiple

subproblems and so conquer stage will be more complex than decrease and

conquer algorithms. An example of decrease and conquer algorithm is binary

search algorithm.

Dynamic programming. When a problem shows optimal substructure,

meaning the optimal solution to a problem can be constructed from optimal

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solutions to subproblems, and overlapping subproblems, meaning the same

subproblems are used to solve many different problem instances, a quicker

approach called dynamic programming avoids recomputing solutions that

have already been computed. For example, the shortest path to a goal from a

vertex in a weighted graph can be found by using the shortest path to the

goal from all adjacent vertices. Dynamic programming and memoization go

together. The main difference between dynamic programming and divide and

conquer is that subproblems are more or less independent in divide and

conquer, whereas subproblems overlap in dynamic programming. The

difference between dynamic programming and straightforward recursion is in

caching or memoization of recursive calls. When subproblems are

independent and there is no repetition, memoization does not help; hence

dynamic programming is not a solution for all complex problems. By using

memoization or maintaining a table of subproblems already solved, dynamic

programming reduces the exponential nature of many problems to polynomial

complexity.

The greedy method. A greedy algorithm is similar to a dynamic

programming algorithm, but the difference is that solutions to the

subproblems do not have to be known at each stage; instead a "greedy"

choice can be made of what looks best for the moment. The greedy method

extends the solution with the best possible decision (not all feasible decisions)

at an algorithmic stage based on the current local optimum and the best

decision (not all possible decisions) made in previous stage. It is not

exhaustive, and does not give accurate answer to many problems. But when

it works, it will be the fastest method. The most popular greedy algorithm is

finding the minimal spanning tree as given by Kruskal.

Linear programming. When solving a problem using linear programming,

specific inequalities involving the inputs are found and then an attempt is

made to maximize (or minimize) some linear function of the inputs. Many

problems (such as the maximum flow for directed graphs) can be stated in a

linear programming way, and then be solved by a 'generic' algorithm such as

the simplex algorithm. A more complex variant of linear programming is

called integer programming, where the solution space is restricted to the

integers.

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Reduction. This technique involves solving a difficult problem by transforming

it into a better known problem for which we have (hopefully) asymptotically

optimal algorithms. The goal is to find a reducing algorithm whose complexity

is not dominated by the resulting reduced algorithm's. For example, one

selection algorithm for finding the median in an unsorted list involves first

sorting the list (the expensive portion) and then pulling out the middle

element in the sorted list (the cheap portion). This technique is also known as

transform and conquer.

Search and enumeration. Many problems (such as playing chess) can be

modeled as problems on graphs. A graph exploration algorithm specifies rules

for moving around a graph and is useful for such problems. This category also

includes search algorithms, branch and bound enumeration and backtracking.

The probabilistic and heuristic paradigm. Algorithms belonging to this

class fit the definition of an algorithm more loosely.

1. Probabilistic algorithms are those that make some choices randomly (or

pseudo-randomly); for some problems, it can in fact be proven that the

fastest solutions must involve some randomness.

2. Genetic algorithms attempt to find solutions to problems by mimicking

biological evolutionary processes, with a cycle of random mutations yielding

successive generations of "solutions". Thus, they emulate reproduction and

"survival of the fittest". In genetic programming, this approach is extended to

algorithms, by regarding the algorithm itself as a "solution" to a problem.

3. Heuristic algorithms, whose general purpose is not to find an optimal solution,

but an approximate solution where the time or resources are limited. They

are not practical to find perfect solutions. An example of this would be local

search, tabu search, or simulated annealing algorithms, a class of heuristic

probabilistic algorithms that vary the solution of a problem by a random

amount. The name "simulated annealing" alludes to the metallurgic term

meaning the heating and cooling of metal to achieve freedom from defects.

The purpose of the random variance is to find close to globally optimal

solutions rather than simply locally optimal ones, the idea being that the

random element will be decreased as the algorithm settles down to a solution.

Classification by field of study

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Every field of science has its own problems and needs efficient algorithms. Related

problems in one field are often studied together. Some example classes are search

algorithms, sorting algorithms, merge algorithms, numerical algorithms, graph

algorithms, string algorithms, computational geometric algorithms, combinatorial

algorithms, machine learning, cryptography, data compression algorithms and

parsing techniques.

Fields tend to overlap with each other, and algorithm advances in one field may

improve those of other, sometimes completely unrelated, fields. For example,

dynamic programming was originally invented for optimization of resource

consumption in industry, but is now used in solving a broad range of problems in

many fields.

Classification by complexity

Algorithms can be classified by the amount of time they need to complete compared

to their input size. There is a wide variety: some algorithms complete in linear time

relative to input size, some do so in an exponential amount of time or even worse,

and some never halt. Additionally, some problems may have multiple algorithms of

differing complexity, while other problems might have no algorithms or no known

efficient algorithms. There are also mappings from some problems to other

problems. Owing to this, it was found to be more suitable to classify the problems

themselves instead of the algorithms into equivalence classes based on the

complexity of the best possible algorithms for them.

Legal issues

Algorithms, by themselves, are not usually patentable. In the United States, a claim

consisting solely of simple manipulations of abstract concepts, numbers, or signals

do not constitute "processes" and hence algorithms are not patentable. However,

practical applications of algorithms are sometimes patentable. For example, in

Diamond v. Diehr, the application of a simple feedback algorithm to aid in the curing

of synthetic rubber was deemed patentable. The patenting of software is highly

controversial, and there are highly criticized patents involving algorithms, especially

data compression algorithms, such as Unisys' LZW patent.

History: Development of the notion of "algorithm"

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Origin of the word

The word algorithm comes from the name of the 9th century Persian mathematician

Abu Abdullah Muhammad ibn Musa al-Khwarizmi whose works introduced Indian

numerals and algebraic concepts. He worked in Baghdad at the time when it was the

centre of scientific studies and trade. The word algorism originally referred only to

the rules of performing arithmetic using Arabic numerals but evolved via European

Latin translation of al-Khwarizmi's name into algorithm by the 18th century. The

word evolved to include all definite procedures for solving problems or performing

tasks.

Discrete and distinguishable symbols

Tally-marks: To keep track of their flocks, their sacks of grain and their money the

ancients used tallying: accumulating stones or marks scratched on sticks, or making

discrete symbols in clay. Through the Babylonian and Egyptian use of marks and

symbols, eventually Roman numerals and the abacus evolved. Tally marks appear

prominently in unary numeral system arithmetic used in Turing machine and Post-

Turing machine computations.

Manipulation of symbols as "place holders" for numbers: algebra

The work of the ancient Greek geometers, Persian mathematician Al-Khwarizmi —

often considered as the "father of algebra", and Western European mathematicians

culminated in Leibniz's notion of the calculus ratiocinator:

"A good century and a half ahead of his time, Leibniz proposed an algebra of

logic, an algebra that would specify the rules for manipulating logical concepts

in the manner that ordinary algebra specifies the rules for manipulating

numbers".

Mechanical contrivances with discrete states

The clock: Bolter credits the invention of the weight-driven clock as “The key

invention [of Europe in the Middle Ages]", in particular the verge escapement that

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provides us with the tick and tock of a mechanical clock. “The accurate automatic

machine” led immediately to "mechanical automata" beginning in the thirteenth

century and finally to “computational machines" – the difference engine and

analytical engines of Charles Babbage and Countess Ada Lovelace.

Jacquard loom, Hollerith punch cards, telegraphy and telephony — the

electromechanical relay: Bell and Newell indicate that the Jacquard loom,

precursor to Hollerith cards, and “telephone switching technologies” were the roots

of a tree leading to the development of the first computers. By the mid-1800s the

telegraph, the precursor of the telephone, was in use throughout the world, its

discrete and distinguishable encoding of letters as “dots and dashes” a common

sound. By the late 1800s the ticker tape was in use, as was the use of Hollerith cards

in the 1890 U.S. census. Then came the Teletype with its punched-paper use of

Baudot code on tape.

Telephone-switching networks of electromechanical relays (invented 1835) was

behind the work of George Stibitz, the inventor of the digital adding device. As he

worked in Bell Laboratories, he observed the “burdensome’ use of mechanical

calculators with gears. "He went home one evening in 1937 intending to test his

idea.... When the tinkering was over, Stibitz had constructed a binary adding

device".

Davis observes the particular importance of the electromechanical relay (with its two

"binary states" open and closed):

It was only with the development, beginning in the 1930s, of

electromechanical calculators using electrical relays, that machines were built

having the scope Babbage had envisioned."

Mathematics during the 1800s up to the mid-1900s

Symbols and rules: In rapid succession the mathematics of George Boole, Gottlob

Frege, and Giuseppe Peano reduced arithmetic to a sequence of symbols

manipulated by rules. Peano's The principles of arithmetic, presented by a new

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method was "the first attempt at an axiomatization of mathematics in a symbolic

language".

But Heijenoort gives Frege this kudos: Frege’s is "perhaps the most important single

work ever written in logic. ... in which we see a " 'formula language', that is a lingua

characterica, a language written with special symbols, "for pure thought", that is,

free from rhetorical embellishments ... constructed from specific symbols that are

manipulated according to definite rules". The work of Frege was further simplified

and amplified by Alfred North Whitehead and Bertrand Russell in their Principia

Mathematica.

The paradoxes: At the same time a number of disturbing paradoxes appeared in

the literature, in particular the Burali-Forti paradox, the Russell paradox, and the

Richard Paradox. The resultant considerations led to Kurt Gödel’s paper — he

specifically cites the paradox of the liar — that completely reduces rules of recursion

to numbers.

Effective calculability: In an effort to solve the Entscheidungsproblem defined

precisely by Hilbert in 1928, mathematicians first set about to define what was

meant by an "effective method" or "effective calculation" or "effective calculability"

(i.e. a calculation that would succeed). In rapid succession the following appeared:

Alonzo Church, Stephen Kleene and J.B. Rosser's λ-calculus, a finely-honed definition

of "general recursion" from the work of Gödel acting on suggestions of Jacques

Herbrand and subsequent simplifications by Kleene, Church's proof that the

Entscheidungsproblem was unsolvable, Emil Post's definition of effective calculability

as a worker mindlessly following a list of instructions to move left or right through a

sequence of rooms and while there either mark or erase a paper or observe the

paper and make a yes-no decision about the next instruction, Alan Turing's proof of

that the Entscheidungsproblem was unsolvable by use of his "a- [automatic-]

machine" -- in effect almost identical to Post's "formulation", J. Barkley Rosser's

definition of "effective method" in terms of "a machine", S. C. Kleene's proposal of a

precursor to "Church thesis" that he called "Thesis I", and a few years later Kleene's

renaming his Thesis "Church's Thesis" and proposing "Turing's Thesis".

Emil Post and Alan Turing

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Here is a remarkable coincidence of two men not knowing each other but describing

a process of men-as-computers working on computations — and they yield virtually

identical definitions.

Emil Post described the actions of a "computer" (human being) as follows:

"...two concepts are involved: that of a symbol space in which the work

leading from problem to answer is to be carried out, and a fixed unalterable

set of directions.

His symbol space would be

"a two way infinite sequence of spaces or boxes... The problem solver or

worker is to move and work in this symbol space, being capable of being in,

and operating in but one box at a time.... a box is to admit of but two

possible conditions, i.e. being empty or unmarked, and having a single mark

in it, say a vertical stroke.

"One box is to be singled out and called the starting point. ...a specific

problem is to be given in symbolic form by a finite number of boxes [i.e.

INPUT] being marked with a stroke. Likewise the answer [i.e. OUTPUT] is to

be given in symbolic form by such a configuration of marked boxes....

"A set of directions applicable to a general problem sets up a deterministic

process when applied to each specific problem. This process will terminate

only when it comes to the direction of type (C ) [i.e. STOP]."

Alan Turing’s work preceded that of Stibitz; it is unknown if Stibitz knew of the work

of Turing. Turing’s biographer believed that Turing’s use of a typewriter-like model

derived from a youthful interest: “ Alan had dreamt of inventing typewriters as a

boy; Mrs. Turing had a typewriter; and he could well have begun by asking himself

what was meant by calling a typewriter 'mechanical' ”. Given the prevalence of Morse

code and telegraphy, ticker tape machines, and Teletypes we might conjecture that

all were influences.

Turing — his model of computation is now called a Turing machine — begins, as did

Post, with an analysis of a human computer that he whittles down to a simple set of

basic motions and "states of mind". But he continues a step further and creates a

machine as a model of computation of numbers:

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"Computing is normally done by writing certain symbols on paper. We may

suppose this paper is divided into squares like a child's arithmetic book....I

assume then that the computation is carried out on one-dimensional paper,

i.e. on a tape divided into squares. I shall also suppose that the number of

symbols which may be printed is finite....

"The behavior of the computer at any moment is determined by the symbols

which he is observing, and his "state of mind" at that moment. We may

suppose that there is a bound B to the number of symbols or squares which

the computer can observe at one moment. If he wishes to observe more, he

must use successive observations. We will also suppose that the number of

states of mind which need be taken into account is finite...

"Let us imagine that the operations performed by the computer to be split up

into 'simple operations' which are so elementary that it is not easy to imagine

them further divided".

Turing's reduction yields the following:

"The simple operations must therefore include:

"(a) Changes of the symbol on one of the observed squares

"(b) Changes of one of the squares observed to another square within L

squares of one of the previously observed squares.

"It may be that some of these change necessarily invoke a change of state of mind.

The most general single operation must therefore be taken to be one of the

following:

"(A) A possible change (a) of symbol together with a possible change of state

of mind.

"(B) A possible change (b) of observed squares, together with a possible

change of state of mind"

"We may now construct a machine to do the work of this computer."

A few years later, Turing expanded his analysis (thesis, definition) with this forceful

expression of it:

"A function is said to be "effectivey calculable" if its values can be found by

some purely mechanical process. Although it is fairly easy to get an intuitive

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grasp of this idea, it is neverthessless desirable to have some more definite,

mathematical expressible definition . . . [he discusses the history of the

definition pretty much as presented above with respect to Gödel, Herbrand,

Kleene, Church, Turing and Post] . . . We may take this statement literally,

understanding by a purely mechanical process one which could be carried out

by a machine. It is possible to give a mathematical description, in a certain

normal form, of the structures of these machines. The development of these

ideas leads to the author's definition of a computable function, and to an

identification of computability † with effective calculability . . . .

"† We shall use the expression "computable function" to mean a function

calculable by a machine, and we let "effectively calculabile" refer to the

intuitive idea without particular identification with any one of these

definitions".

J. B. Rosser and S. C. Kleene

J. Barkley Rosser boldly defined an ‘effective [mathematical] method’ in the following

manner (boldface added):

"'Effective method' is used here in the rather special sense of a method each

step of which is precisely determined and which is certain to produce the

answer in a finite number of steps. With this special meaning, three different

precise definitions have been given to date. The simplest of these to state

(due to Post and Turing) says essentially that an effective method of

solving certain sets of problems exists if one can build a machine

which will then solve any problem of the set with no human

intervention beyond inserting the question and (later) reading the

answer. All three definitions are equivalent, so it doesn't matter which one is

used. Moreover, the fact that all three are equivalent is a very strong

argument for the correctness of any one."

Rosser's footnote #5 references the work of (1) Church and Kleene and their

definition of λ-definability, in particular Church's use of it in his An Unsolvable

Problem of Elementary Number Theory; (2) Herbrand and Gödel and their use of

recursion in particular Gödel's use in his famous paper On Formally Undecidable

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Propositions of Principia Mathematica and Related Systems I; and (3) Post and

Turing in their mechanism-models of computation.

Stephen C. Kleene defined as his now-famous "Thesis I" known as "the Church-

Turing Thesis". But he did this in the following context (boldface in original):

"12. Algorithmic theories... In setting up a complete algorithmic theory,

what we do is to describe a procedure, performable for each set of values of

the independent variables, which procedure necessarily terminates and in

such manner that from the outcome we can read a definite answer, "yes" or

"no," to the question, "is the predicate value true?”"

History after 1950

A number of efforts have been directed toward further refinement of the definition of

"algorithm", and activity is on-going because of issues surrounding, in particular,

foundations of mathematics (especially the Church-Turing Thesis) and philosophy of

mind (especially arguments around artificial intelligence). For more, see Algorithm

characterizations.

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Compiler

A diagram of the operation of a typical multi-language, multi-target compiler.

A compiler is a computer program (or set of programs) that translates text written

in a computer language (the source language) into another computer language (the

target language). The original sequence is usually called the source code and the

output called object code. Commonly the output has a form suitable for processing

by other programs (e.g., a linker), but it may be a human-readable text file.

The most common reason for wanting to translate source code is to create an

executable program. The name "compiler" is primarily used for programs that

translate source code from a high-level programming language to a lower level

language (e.g., assembly language or machine language). A program that translates

from a low level language to a higher level one is a decompiler. A program that

translates between high-level languages is usually called a language translator,

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source to source translator, or language converter. A language rewriter is usually a

program that translates the form of expressions without a change of language.

A compiler is likely to perform many or all of the following operations: lexical

analysis, preprocessing, parsing, semantic analysis, code generation, and code

optimization.

History

Software for early computers was exclusively written in assembly language for many

years. Higher level programming languages were not invented until the benefits of

being able to reuse software on different kinds of CPUs started to become

significantly greater than the cost of writing a compiler. The very limited memory

capacity of early computers also created many technical problems when

implementing a compiler.

Towards the end of the 1950s, machine-independent programming languages were

first proposed. Subsequently, several experimental compilers were developed. The

first compiler was written by Grace Hopper, in 1952, for the A-0 programming

language. The FORTRAN team led by John Backus at IBM is generally credited as

having introduced the first complete compiler, in 1957. COBOL was an early

language to be compiled on multiple architectures, in 1960.

In many application domains the idea of using a higher level language quickly caught

on. Because of the expanding functionality supported by newer programming

languages and the increasing complexity of computer architectures, compilers have

become more and more complex.

Early compilers were written in assembly language. The first self-hosting compiler —

capable of compiling its own source code in a high-level language — was created for

Lisp by Hart and Levin at MIT in 1962. Since the 1970s it has become common

practice to implement a compiler in the language it compiles, although both Pascal

and C have been popular choices for implementation language. Building a self-

hosting compiler is a bootstrapping problem -- the first such compiler for a language

must be compiled either by a compiler written in a different language, or (as in Hart

and Levin's Lisp compiler) compiled by running the compiler in an interpreter.

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Compilers in education

Compiler construction and compiler optimization are taught at universities as part of

the computer science curriculum. Such courses are usually supplemented with the

implementation of a compiler for an educational programming language. A well-

documented example is Niklaus Wirth's PL/0 compiler, which Wirth used to teach

compiler construction in the 1970s. In spite of its simplicity, the PL/0 compiler

introduced several influential concepts to the field:

1. Program development by stepwise refinement

2. The use of a recursive descent parser

3. The use of EBNF to specify the syntax of a language

4. A code generator producing portable P-code

5. The use of T-diagrams in the formal description of the bootstrapping problem

Compiler output

One method used to classify compilers is by the platform on which the generated

code they produce executes. This is known as the target platform.

A native or hosted compiler is one whose output is intended to directly run on the

same type of computer and operating system as the compiler itself runs on. The

output of a cross compiler is designed to run on a different platform. Cross compilers

are often used when developing software for embedded systems that are not

intended to support a software development environment.

The output of a compiler that produces code for a virtual machine (VM) may or may

not be executed on the same platform as the compiler that produced it. For this

reason such compilers are not usually classified as native or cross compilers.

Compiled versus interpreted languages

Higher-level programming languages are generally divided for convenience into

compiled languages and interpreted languages. However, there is rarely anything

about a language that requires it to be exclusively compiled, or exclusively

interpreted. The categorization usually reflects the most popular or widespread

implementations of a language — for instance, BASIC is thought of as an interpreted

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language, and C a compiled one, despite the existence of BASIC compilers and C

interpreters.

In a sense, all languages are interpreted, with "execution" being merely a special

case of interpretation performed by transistors switching on a CPU. Modern trends

toward just-in-time compilation and bytecode interpretation also blur the traditional

categorizations.

There are exceptions. Some language specifications spell out that implementations

must include a compilation facility; for example, Common Lisp. Other languages

have features that are very easy to implement in an interpreter, but make writing a

compiler much harder; for example, APL, SNOBOL4, and many scripting languages

allow programs to construct arbitrary source code at runtime with regular string

operations, and then execute that code by passing it to a special evaluation function.

To implement these features in a compiled language, programs must usually be

shipped with a runtime library that includes a version of the compiler itself.

Hardware compilation

The output of some compilers may target hardware at a very low level. For example

a Field Programmable Gate Array (FPGA) or structured Application-specific integrated

circuit (ASIC). Such compilers are said to be hardware compilers or synthesis tools

because the programs they compile effectively control the final configuration of the

hardware and how it operates; there are no instructions that are executed in

sequence - only an interconnection of transistors or lookup tables. For example, XST

is the Xilinx Synthesis Tool used for configuring FPGAs. Similar tools are available

from Altera, Synplicity, Synopsys and other vendors.

Compiler design

The approach taken to compiler design is affected by the complexity of the

processing that needs to be done, the experience of the person(s) designing it, and

the resources (e.g., people and tools) available.

A compiler for a relatively simple language written by one person might be a single,

monolithic piece of software. When the source language is large and complex, and

high quality output is required the design may be split into a number of relatively

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independent phases, or passes. Having separate phases means development can be

parceled up into small parts and given to different people. It also becomes much

easier to replace a single phase by an improved one, or to insert new phases later

(e.g., additional optimizations).

The division of the compilation processes in phases (or passes) was championed by

the Production Quality Compiler-Compiler Project (PQCC) at Carnegie Mellon

University. This project introduced the terms front end, middle end (rarely heard

today), and back end.

All but the smallest of compilers have more than two phases. However, these phases

are usually regarded as being part of the front end or the back end. The point at

where these two ends meet is always open to debate. The front end is generally

considered to be where syntactic and semantic processing takes place, along with

translation to a lower level of representation (than source code).

The middle end is usually designed to perform optimizations on a form other than

the source code or machine code. This source code/machine code independence is

intended to enable generic optimizations to be shared between versions of the

compiler supporting different languages and target processors.

The back end takes the output from the middle. It may perform more analysis,

transformations and optimizations that are for a particular computer. Then, it

generates code for a particular processor and OS.

This front-end/middle/back-end approach makes it possible to combine front ends

for different languages with back ends for different CPUs. Practical examples of this

approach are the GNU Compiler Collection, LLVM, and the Amsterdam Compiler Kit,

which have multiple front-ends, shared analysis and multiple back-ends.

One-pass versus multi-pass compilers

Classifying compilers by number of passes has its background in the hardware

resource limitations of computers. Compiling involves performing lots of work and

early computers did not have enough memory to contain one program that did all of

this work. So compilers were split up into smaller programs which each made a pass

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over the source (or some representation of it) performing some of the required

analysis and translations.

The ability to compile in a single pass is often seen as a benefit because it simplifies

the job of writing a compiler and one pass compilers are generally faster than multi-

pass compilers. Many languages were designed so that they could be compiled in a

single pass (e.g., Pascal).

In some cases the design of a language feature may require a compiler to perform

more than one pass over the source. For instance, consider a declaration appearing

on line 20 of the source which affects the translation of a statement appearing on

line 10. In this case, the first pass needs to gather information about declarations

appearing after statements that they affect, with the actual translation happening

during a subsequent pass.

The disadvantage of compiling in a single pass is that it is not possible to perform

many of the sophisticated optimizations needed to generate high quality code. It can

be difficult to count exactly how many passes an optimizing compiler makes. For

instance, different phases of optimization may analyze one expression many times

but only analyze another expression once.

Splitting a compiler up into small programs is a technique used by researchers

interested in producing provably correct compilers. Proving the correctness of a set

of small programs often requires less effort than proving the correctness of a larger,

single, equivalent program.

While the typical multi-pass compiler outputs machine code from its final pass, there

are several other types:

A "source-to-source compiler" is a type of compiler that takes a high level

language as its input and outputs a high level language. For example, an

automatic parallelizing compiler will frequently take in a high level language

program as an input and then transform the code and annotate it with parallel

code annotations (e.g. OpenMP) or language constructs (e.g. Fortran's DOALL

statements).

Stage compiler that compiles to assembly language of a theoretical machine,

like some Prolog implementations

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o This Prolog machine is also known as the Warren Abstract Machine (or

WAM). Bytecode compilers for Java, Python, and many more are also a

subtype of this.

Just-in-time compiler, used by Smalltalk and Java systems, and also by

Microsoft .Net's Common Intermediate Language (CIL)

o Applications are delivered in bytecode, which is compiled to native

machine code just prior to execution.

Front end

The front end analyzes the source code to build an internal representation of the

program, called the intermediate representation or IR. It also manages the symbol

table, a data structure mapping each symbol in the source code to associated

information such as location, type and scope. This is done over several phases,

which includes some of the following:

1. Line reconstruction. Languages which strop their keywords or allow

arbitrary spaces within identifiers require a phase before parsing, which

converts the input character sequence to a canonical form ready for the

parser. The top-down, recursive-descent, table-driven parsers used in the

1960s typically read the source one character at a time and did not require a

separate tokenizing phase. Atlas Autocode, and Imp (and some

implementations of Algol and Coral66) are examples of stropped languages

whose compilers would have a Line Reconstruction phase.

2. Lexical analysis breaks the source code text into small pieces called tokens.

Each token is a single atomic unit of the language, for instance a keyword,

identifier or symbol name. The token syntax is typically a regular language,

so a finite state automaton constructed from a regular expression can be used

to recognize it. This phase is also called lexing or scanning, and the software

doing lexical analysis is called a lexical analyzer or scanner.

3. Preprocessing. Some languages, e.g., C, require a preprocessing phase which

supports macro substitution and conditional compilation. Typically the

preprocessing phase occurs before syntactic or semantic analysis; e.g. in the

case of C, the preprocessor manipulates lexical tokens rather than syntactic

forms. However, some languages such as Scheme support macro

substitutions based on syntactic forms.

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4. Syntax analysis involves parsing the token sequence to identify the syntactic

structure of the program. This phase typically builds a parse tree, which

replaces the linear sequence of tokens with a tree structure built according to

the rules of a formal grammar which define the language's syntax. The parse

tree is often analyzed, augmented, and transformed by later phases in the

compiler.

5. Semantic analysis is the phase in which the compiler adds semantic

information to the parse tree and builds the symbol table. This phase

performs semantic checks such as type checking (checking for type errors),

or object binding (associating variable and function references with their

definitions), or definite assignment (requiring all local variables to be

initialized before use), rejecting incorrect programs or issuing warnings.

Semantic analysis usually requires a complete parse tree, meaning that this

phase logically follows the parsing phase, and logically precedes the code

generation phase, though it is often possible to fold multiple phases into one

pass over the code in a compiler implementation.

Back end

The term back end is sometimes confused with code generator because of the

overlapped functionality of generating assembly code. Some literature uses middle

end to distinguish the generic analysis and optimization phases in the back end from

the machine-dependent code generators.

The main phases of the back end include the following:

1. Analysis: This is the gathering of program information from the intermediate

representation derived from the input. Typical analyses are data flow analysis

to build use-define chains, dependence analysis, alias analysis, pointer

analysis, escape analysis etc. Accurate analysis is the basis for any compiler

optimization. The call graph and control flow graph are usually also built

during the analysis phase.

2. Optimization: the intermediate language representation is transformed into

functionally equivalent but faster (or smaller) forms. Popular optimizations

are inline expansion, dead code elimination, constant propagation, loop

transformation, register allocation or even automatic parallelization.

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3. Code generation: the transformed intermediate language is translated into

the output language, usually the native machine language of the system. This

involves resource and storage decisions, such as deciding which variables to

fit into registers and memory and the selection and scheduling of appropriate

machine instructions along with their associated addressing modes (see also

Sethi-Ullman algorithm).

Compiler analysis is the prerequisite for any compiler optimization, and they tightly

work together. For example, dependence analysis is crucial for loop transformation.

In addition, the scope of compiler analysis and optimizations vary greatly, from as

small as a basic block to the procedure/function level, or even over the whole

program (interprocedural optimization). Obviously, a compiler can potentially do a

better job using a broader view. But that broad view is not free: large scope analysis

and optimizations are very costly in terms of compilation time and memory space;

this is especially true for interprocedural analysis and optimizations.

The existence of interprocedural analysis and optimizations is common in modern

commercial compilers from HP, IBM, SGI, Intel, Microsoft, and Sun Microsystems.

The open source GCC was criticized for a long time for lacking powerful

interprocedural optimizations, but it is changing in this respect. Another good open

source compiler with full analysis and optimization infrastructure is Open64, which is

used by many organizations for research and commercial purposes.

Due to the extra time and space needed for compiler analysis and optimizations,

some compilers skip them by default. Users have to use compilation options to

explicitly tell the compiler which optimizations should be enabled.

Related techniques

Assembly language is not a high-level language and a program that compiles it is

more commonly known as an assembler, with the inverse program known as a

disassembler.

A program that translates from a low level language to a higher level one is a

decompiler.

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A program that translates between high-level languages is usually called a language

translator, source to source translator, language converter, or language rewriter. The

last term is usually applied to translations that do not involve a change of language.

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Network topology

Diagram of different network topologies.

Network topology is the study of the arrangement or mapping of the elements

(links, nodes, etc.) of a network, especially the physical (real) and logical (virtual)

interconnections between nodes.

A local area network (LAN) is one example of a network that exhibits both a physical

topology and a logical topology. Any given node in the LAN will have one or more

links to one or more other nodes in the network and the mapping of these links and

nodes onto a graph results in a geometrical shape that determines the physical

topology of the network. Likewise, the mapping of the flow of data between the

nodes in the network determines the logical topology of the network. It is important

to note that the physical and logical topologies might be identical in any particular

network but they also may be different.

Any particular network topology is determined only by the graphical mapping of the

configuration of physical and/or logical connections between nodes - Network

Topology is, therefore, technically a part of graph theory. Distances between nodes,

physical interconnections, transmission rates, and/or signal types may differ in two

networks and yet their topologies may be identical.

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Basic types of topologies

The arrangement or mapping of the elements of a network gives rise to certain basic

topologies which may then be combined to form more complex topologies (hybrid

topologies). The most common of these basic types of topologies are (refer to the

illustration at the top right of this page):

Bus (Linear, Linear Bus)

Star

Ring

Mesh

o partially connected mesh (or simply 'mesh')

o fully connected mesh

Tree

Hybrid

Classification of network topologies

There are also three basic categories of network topologies:

physical topologies

signal topologies

logical topologies

The terms signal topology and logical topology are often used interchangeably even

though there is a subtle difference between the two and the distinction is not often

made between the two.

Physical topologies

The mapping of the nodes of a network and the physical connections between them

– i.e., the layout of wiring, cables, the locations of nodes, and the interconnections

between the nodes and the cabling or wiring system.

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Classification of physical topologies

Point-to-point

The simplest topology is a permanent link between two endpoints. Switched point-to-

point topologies are the basic model of conventional telephony. The value of a

permanent point-to-point network is the value of guaranteed, or nearly so,

communications between the two endpoints. The value of an on-demand point-to-

point connection is proportional to the number of potential pairs of subscribers, and

has been expressed as Metcalfe's Law.

Permanent (dedicated)

Easiest to understand, of the variations of point-to-point topology, is a point-to-point

communications channel that appears, to the user, to be permanently associated

with the two endpoints. Children's "tin-can telephone" is one example, with a

microphone to a single public address speaker is another. These are examples of

physical dedicated channels. Within many switched telecommunications systems, it is

possible to establish a permanent circuit. One example might be a telephone in the

lobby of a public building, which is programmed to ring only the number of a

telephone dispatcher. "Nailing down" a switched connection saves the cost of running

a physical circuit between the two points. The resources in such a connection can be

released when no longer needed, as, for example, a television circuit from a parade

route back to the studio.

Switched

Using circuit-switching or packet-switching technologies, a point-to-point circuit can

be set up dynamically, and dropped when no longer needed. This is the basic mode

of conventional telephony.

Bus

Linear bus

The type of network topology in which all of the nodes of the network are

connected to a common transmission medium which has exactly two

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endpoints (this is the 'bus', which is also commonly referred to as the

backbone, or trunk) – all data that is transmitted between nodes in the

network is transmitted over this common transmission medium and is able to

be received by all nodes in the network virtually simultaneously (disregarding

propagation delays).

Note: The two endpoints of the common transmission medium are normally

terminated with a device called a terminator that exhibits the characteristic

impedance of the transmission medium and which dissipates or absorbs the

energy that remains in the signal to prevent the signal from being reflected or

propagated back onto the transmission medium in the opposite direction,

which would cause interference with and degradation of the signals on the

transmission medium (See Electrical termination).

Distributed bus

The type of network topology in which all of the nodes of the network are

connected to a common transmission medium which has more than two

endpoints that are created by adding branches to the main section of the

transmission medium – the physical distributed bus topology functions in

exactly the same fashion as the physical linear bus topology (i.e., all nodes

share a common transmission medium).

Notes:

1.) All of the endpoints of the common transmission medium are normally

terminated with a device called a 'terminator' (see the note under linear bus).

2.) The physical linear bus topology is sometimes considered to be a special

case of the physical distributed bus topology – i.e., a distributed bus with no

branching segments.

3.) The physical distributed bus topology is sometimes incorrectly referred to

as a physical tree topology – however, although the physical distributed bus

topology resembles the physical tree topology, it differs from the physical tree

topology in that there is no central node to which any other nodes are

connected, since this hierarchical functionality is replaced by the common

bus.

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Star

The type of network topology in which each of the nodes of the network is

connected to a central node with a point-to-point link in a 'hub' and 'spoke'

fashion, the central node being the 'hub' and the nodes that are attached to

the central node being the 'spokes' (e.g., a collection of point-to-point links

from the peripheral nodes that converge at a central node) – all data that is

transmitted between nodes in the network is transmitted to this central node,

which is usually some type of device that then retransmits the data to some

or all of the other nodes in the network, although the central node may also

be a simple common connection point (such as a 'punch-down' block) without

any active device to repeat the signals.

Notes:

1.) A point-to-point link (described above) is sometimes categorized as a

special instance of the physical star topology – therefore, the simplest type of

network that is based upon the physical star topology would consist of one

node with a single point-to-point link to a second node, the choice of which

node is the 'hub' and which node is the 'spoke' being arbitrary[1].

2.) After the special case of the point-to-point link, as in note 1.) above, the

next simplest type of network that is based upon the physical star topology

would consist of one central node – the 'hub' – with two separate point-to-

point links to two peripheral nodes – the 'spokes'.

3.) Although most networks that are based upon the physical star topology

are commonly implemented using a special device such as a hub or switch as

the central node (i.e., the 'hub' of the star), it is also possible to implement a

network that is based upon the physical star topology using a computer or

even a simple common connection point as the 'hub' or central node –

however, since many illustrations of the physical star network topology depict

the central node as one of these special devices, some confusion is possible,

since this practice may lead to the misconception that a physical star network

requires the central node to be one of these special devices, which is not true

because a simple network consisting of three computers connected as in note

2.) above also has the topology of the physical star.

4.) Star networks may also be described as either broadcast multi-access or

nonbroadcast multi-access (NBMA), depending on whether the technology of

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the network either automatically propagates a signal at the hub to all spokes,

or only addresses individual spokes with each communication.

Extended star

A type of network topology in which a network that is based upon the physical

star topology has one or more repeaters between the central node (the 'hub'

of the star) and the peripheral or 'spoke' nodes, the repeaters being used to

extend the maximum transmission distance of the point-to-point links

between the central node and the peripheral nodes beyond that which is

supported by the transmitter power of the central node or beyond that which

is supported by the standard upon which the physical layer of the physical

star network is based.

Note: If the repeaters in a network that is based upon the physical extended

star topology are replaced with hubs or switches, then a hybrid network

topology is created that is referred to as a physical hierarchical star topology,

although some texts make no distinction between the two topologies.

Distributed Star

A type of network topology that is composed of individual networks that are

based upon the physical star topology connected together in a linear fashion –

i.e., 'daisy-chained' – with no central or top level connection point (e.g., two

or more 'stacked' hubs, along with their associated star connected nodes or

'spokes').

Ring

The type of network topology in which each of the nodes of the network is

connected to two other nodes in the network and with the first and last nodes

being connected to each other, forming a ring – all data that is transmitted

between nodes in the network travels from one node to the next node in a

circular manner and the data generally flows in a single direction only.

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Dual-ring

The type of network topology in which each of the nodes of the network is

connected to two other nodes in the network, with two connections to each of

these nodes, and with the first and last nodes being connected to each other

with two connections, forming a double ring – the data flows in opposite

directions around the two rings, although, generally, only one of the rings

carries data during normal operation, and the two rings are independent

unless there is a failure or break in one of the rings, at which time the two

rings are joined (by the stations on either side of the fault) to enable the flow

of data to continue using a segment of the second ring to bypass the fault in

the primary ring.

Mesh

The value of fully meshed networks is proportional to the exponent of the

number of subscribers, assuming that communicating groups of any two

endpoints, up to and including all the endpoints, is approximated by Reed's

Law.

Full

Fully connected

The type of network topology in which each of the nodes of the network is

connected to each of the other nodes in the network with a point-to-point link

– this makes it possible for data to be simultaneously transmitted from any

single node to all of the other nodes.

Note: The physical fully connected mesh topology is generally too costly and

complex for practical networks, although the topology is used when there are

only a small number of nodes to be interconnected.

Partial

Partially connected

The type of network topology in which some of the nodes of the network are

connected to more than one other node in the network with a point-to-point

link – this makes it possible to take advantage of some of the redundancy

that is provided by a physical fully connected mesh topology without the

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expense and complexity required for a connection between every node in the

network.

Note: In most practical networks that are based upon the physical partially

connected mesh topology, all of the data that is transmitted between nodes in

the network takes the shortest path (or an approximation of the shortest

path) between nodes, except in the case of a failure or break in one of the

links, in which case the data takes an alternate path to the destination. This

requires that the nodes of the network possess some type of logical 'routing'

algorithm to determine the correct path to use at any particular time.

Tree (also known as hierarchical):

The type of network topology in which a central 'root' node (the top level of

the hierarchy) is connected to one or more other nodes that are one level

lower in the hierarchy (i.e., the second level) with a point-to-point link

between each of the second level nodes and the top level central 'root' node,

while each of the second level nodes that are connected to the top level

central 'root' node will also have one or more other nodes that are one level

lower in the hierarchy (i.e., the third level) connected to it, also with a point-

to-point link, the top level central 'root' node being the only node that has no

other node above it in the hierarchy – the hierarchy of the tree is

symmetrical, each node in the network having a specific fixed number, f, of

nodes connected to it at the next lower level in the hierarchy, the number, f,

being referred to as the 'branching factor' of the hierarchical tree.

Notes:

1.) A network that is based upon the physical hierarchical topology must have

at least three levels in the hierarchy of the tree, since a network with a

central 'root' node and only one hierarchical level below it would exhibit the

physical topology of a star.

2.) A network that is based upon the physical hierarchical topology and with a

branching factor of 1 would be classified as a physical linear topology.

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3.) The branching factor, f, is independent of the total number of nodes in the

network and, therefore, if the nodes in the network require ports for

connection to other nodes the total number of ports per node may be kept

low even though the total number of nodes is large – this makes the effect of

the cost of adding ports to each node totally dependent upon the branching

factor and may therefore be kept as low as required without any effect upon

the total number of nodes that are possible.

4.) The total number of point-to-point links in a network that is based upon

the physical hierarchical topology will be one less than the total number of

nodes in the network.

5.) If the nodes in a network that is based upon the physical hierarchical

topology are required to perform any processing upon the data that is

transmitted between nodes in the network, the nodes that are at higher levels

in the hierarchy will be required to perform more processing operations on

behalf of other nodes than the nodes that are lower in the hierarchy.

Hybrid network topologies

The hybrid topology is a type of network topology that is composed of one or

more interconnections of two or more networks that are based upon different

physical topologies or a type of network topology that is composed of one or

more interconnections of two or more networks that are based upon the same

physical topology, but where the physical topology of the network resulting

from such an interconnection does not meet the definition of the original

physical topology of the interconnected networks (e.g., the physical topology

of a network that would result from an interconnection of two or more

networks that are based upon the physical star topology might create a

hybrid topology which resembles a mixture of the physical star and physical

bus topologies or a mixture of the physical star and the physical tree

topologies, depending upon how the individual networks are interconnected,

while the physical topology of a network that would result from an

interconnection of two or more networks that are based upon the physical

distributed bus network retains the topology of a physical distributed bus

network).

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Star-bus

A type of network topology in which the central nodes of one or more

individual networks that are based upon the physical star topology are

connected together using a common 'bus' network whose physical topology is

based upon the physical linear bus topology, the endpoints of the common

'bus' being terminated with the characteristic impedance of the transmission

medium where required – e.g., two or more hubs connected to a common

backbone with drop cables through the port on the hub that is provided for

that purpose (e.g., a properly configured 'uplink' port) would comprise the

physical bus portion of the physical star-bus topology, while each of the

individual hubs, combined with the individual nodes which are connected to

them, would comprise the physical star portion of the physical star-bus

topology.

Star-of-stars

Hierarchical star

A type of network topology that is composed of an interconnection of

individual networks that are based upon the physical star topology connected

together in a hierarchical fashion to form a more complex network – e.g., a

top level central node which is the 'hub' of the top level physical star topology

and to which other second level central nodes are attached as the 'spoke'

nodes, each of which, in turn, may also become the central nodes of a third

level physical star topology.

Notes:

1.) The physical hierarchical star topology is not a combination of the physical

linear bus and the physical star topologies, as cited in some texts, as there is

no common linear bus within the topology, although the top level 'hub' which

is the beginning of the physical hierarchical star topology may be connected

to the backbone of another network, such as a common carrier, which is,

topologically, not considered to be a part of the local network – if the top level

central node is connected to a backbone that is considered to be a part of the

local network, then the resulting network topology would be considered to be

a hybrid topology that is a mixture of the topology of the backbone network

and the physical hierarchical star topology.

2.) The physical hierarchical star topology is also sometimes incorrectly

referred to as a physical tree topology, since its physical topology is

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hierarchical, however, the physical hierarchical star topology does not have a

structure that is determined by a branching factor, as is the case with the

physical tree topology and, therefore, nodes may be added to, or removed

from, any node that is the 'hub' of one of the individual physical star topology

networks within a network that is based upon the physical hierarchical star

topology.

3.) The physical hierarchical star topology is commonly used in 'outside plant'

(OSP) cabling to connect various buildings to a central connection facility,

which may also house the 'demarcation point' for the connection to the data

transmission facilities of a common carrier, and in 'inside plant' (ISP) cabling

to connect multiple wiring closets within a building to a common wiring closet

within the same building, which is also generally where the main backbone or

trunk that connects to a larger network, if any, enters the building.

Star-wired ring

A type of hybrid physical network topology that is a combination of the

physical star topology and the physical ring topology, the physical star portion

of the topology consisting of a network in which each of the nodes of which

the network is composed are connected to a central node with a point-to-

point link in a 'hub' and 'spoke' fashion, the central node being the 'hub' and

the nodes that are attached to the central node being the 'spokes' (e.g., a

collection of point-to-point links from the peripheral nodes that converge at a

central node) in a fashion that is identical to the physical star topology, while

the physical ring portion of the topology consists of circuitry within the central

node which routes the signals on the network to each of the connected nodes

sequentially, in a circular fashion.

Note: In an 802.5 Token Ring network the central node is called a

Multistation Access Unit (MAU).

Hybrid mesh

A type of hybrid physical network topology that is a combination of the

physical partially connected topology and one or more other physical

topologies the mesh portion of the topology consisting of redundant or

alternate connections between some of the nodes in the network – the

physical hybrid mesh topology is commonly used in networks which require a

high degree of availability..

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Signal topology

The mapping of the actual connections between the nodes of a network, as

evidenced by the path that the signals take when propagating between the nodes.

Note: The term 'signal topology' is often used synonymously with the term

'logical topology', however, some confusion may result from this practice in

certain situations since, by definition, the term 'logical topology' refers to the

apparent path that the data takes between nodes in a network while the term

'signal topology' generally refers to the actual path that the signals (e.g.,

optical, electrical, electromagnetic, etc.) take when propagating between

nodes.

Example

In an 802.4 Token Bus network, the physical topology may be a physical bus,

a physical star, or a hybrid physical topology, while the signal topology is a

bus (i.e., the electrical signal propagates to all nodes simultaneously [ignoring

propagation delays and network latency] ), and the logical topology is a ring

(i.e., the data flows from one node to the next in a circular manner according

to the protocol).

Logical topology

The mapping of the apparent connections between the nodes of a network, as

evidenced by the path that data appears to take when traveling between the nodes.

Classification of logical topologies

The logical classification of network topologies generally follows the same

classifications as those in the physical classifications of network topologies, the path

that the data takes between nodes being used to determine the topology as opposed

to the actual physical connections being used to determine the topology.

Notes:

1.) Logical topologies are often closely associated with media access control

(MAC) methods and protocols.

2.) The logical topologies are generally determined by network protocols as

opposed to being determined by the physical layout of cables, wires, and

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network devices or by the flow of the electrical signals, although in many

cases the paths that the electrical signals take between nodes may closely

match the logical flow of data, hence the convention of using the terms

'logical topology' and 'signal topology' interchangeably.

3.) Logical topologies are able to be dynamically reconfigured by special types

of equipment such as routers and switches.

Daisy chains

Except for star-based networks, the easiest way to add more computers into a

network is by daisy-chaining, or connecting each computer in series to the next. If a

message is intended for a computer partway down the line, each system bounces it

along in sequence until it reaches the destination. A daisy-chained network can take

two basic forms: linear and ring.

A linear topology puts a two-way link between one computer and the next.

However, this was expensive in the early days of computing, since each

computer (except for the ones at each end) required two receivers and two

transmitters.

By connecting the computers at each end, a ring topology can be formed. An

advantage of the ring is that the number of transmitters and receivers can be

cut in half, since a message will eventually loop all of the way around. When a

node sends a message, the message is processed by each computer in the

ring. If a computer is not the destination node, it will pass the message to the

next node, until the message arrives at its destination. If the message is not

accepted by any node on the network, it will travel around the entire ring and

return to the sender. This potentially results in a doubling of travel time for

data, but since it is traveling at a fairly insignificant multiple of the speed of

light, the loss is usually negligible.

Centralization

The star topology reduces the probability of a network failure by connecting all of the

peripheral nodes (computers, etc.) to a central node. When the physical star

topology is applied to a logical bus network such as Ethernet, this central node

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(traditionally a hub) rebroadcasts all transmissions received from any peripheral

node to all peripheral nodes on the network, sometimes including the originating

node. All peripheral nodes may thus communicate with all others by transmitting to,

and receiving from, the central node only. The failure of a transmission line linking

any peripheral node to the central node will result in the isolation of that peripheral

node from all others, but the remaining peripheral nodes will be unaffected.

However, the disadvantage is that the failure of the central node will cause the

failure of all of the peripheral nodes also.

If the central node is passive, the originating node must be able to tolerate the

reception of an echo of its own transmission, delayed by the two-way round trip

transmission time (i.e. to and from the central node) plus any delay generated in the

central node. An active star network has an active central node that usually has the

means to prevent echo-related problems.

A tree topology (a.k.a. hierarchical topology) can be viewed as a collection of star

networks arranged in a hierarchy. This tree has individual peripheral nodes (e.g.

leaves) which are required to transmit to and receive from one other node only and

are not required to act as repeaters or regenerators. Unlike the star network, the

functionality of the central node may be distributed.

As in the conventional star network, individual nodes may thus still be isolated from

the network by a single-point failure of a transmission path to the node. If a link

connecting a leaf fails, that leaf is isolated; if a connection to a non-leaf node fails,

an entire section of the network becomes isolated from the rest.

In order to alleviate the amount of network traffic that comes from broadcasting all

signals to all nodes, more advanced central nodes were developed that are able to

keep track of the identities of the nodes that are connected to the network. These

network switches will "learn" the layout of the network by "listening" on each port

during normal data transmission, examining the data packets and recording the

address/identifier of each connected node and which port it's connected to in a

lookup table held in memory. This lookup table then allows future transmissions to

be forwarded to the intended destination only.

Decentralization

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In a mesh topology (i.e., a partially connected mesh topology), there are at least

two nodes with two or more paths between them to provide redundant paths to be

used in case the link providing one of the paths fails. This decentralization is often

used to advantage to compensate for the single-point-failure disadvantage that is

present when using a single device as a central node (e.g., in star and tree

networks). A special kind of mesh, limiting the number of hops between two nodes,

is a hypercube. The number of arbitrary forks in mesh networks makes them more

difficult to design and implement, but their decentralized nature makes them very

useful. This is similar in some ways to a grid network, where a linear or ring topology

is used to connect systems in multiple directions. A multi-dimensional ring has a

toroidal topology, for instance.

A fully connected network, complete topology or full mesh topology is a network

topology in which there is a direct link between all pairs of nodes. In a fully

connected network with n nodes, there are n(n-1)/2 direct links. Networks designed

with this topology are usually very expensive to set up, but provide a high degree of

reliability due to the multiple paths for data that are provided by the large number of

redundant links between nodes. This topology is mostly seen in military applications.

However, it can also be seen in the file sharing protocol BitTorrent in which users

connect to other users in the "swarm" by allowing each user sharing the file to

connect to other users also involved. Often in actual usage of BitTorrent any given

individual node is rarely connected to every single other node as in a true fully

connected network but the protocol does allow for the possibility for any one node to

connect to any other node when sharing files.

Hybrids

Hybrid networks use a combination of any two or more topologies in such a way that

the resulting network does not exhibit one of the standard topologies (e.g., bus, star,

ring, etc.). For example, a tree network connected to a tree network is still a tree

network, but two star networks connected together exhibit a hybrid network

topology. A hybrid topology is always produced when two different basic network

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topologies are connected. Two common examples for Hybrid network are: star ring

network and star bus network

A Star ring network consists of two or more star topologies connected using a

multistation access unit (MAU) as a centralized hub.

A Star Bus network consists of two or more star topologies connected using a

bus trunk (the bus trunk serves as the network's backbone).

While grid networks have found popularity in high-performance computing

applications, some systems have used genetic algorithms to design custom networks

that have the fewest possible hops in between different nodes. Some of the resulting

layouts are nearly incomprehensible, although they function quite well.