cheap one way airline tickets life insurance for seniors online website design cd interest rates domain name registrations print business cards affordable web design sex toy home parties mortgage life insurance stop identity theft hyundai dealers computer hardware repair hard money loans wireless home security web design company sympathy flowers circus city commercial real estate investing cheapest term life insurance legal credit repair office computer furniture life insurance broker anxiety depression bussiness loans web design services real estate auctions bankruptcy lawyers discount promotional products kitchen bar stools compare cd rates make money online business chrysler dealers bad credit second mortgage ordering checks bankruptcy consultant blue cross blue shield health insurance dsl provider hewlett packard desktop experian credit reports mold inspection t mobile plans best air purifiers screen printed signs notary public supplies stop smoking cigarettes dedicated server hosting home security alarm systems hoodia supplement high speed internet providers sierra club credit history repair office space for rent personal loan for bad credit all in one laser printers short term loan web site design services event security online advertising agencies term life insurance companies business opportunity seekers individual health care insurance king koil mattress web hosting reviews auto loan rates computer memory chips low cost individual health insurance quick payday loans dsl internet service mortgage calculator monthly criminal defense lawyer cheap all in one printers bankruptcy help interest only home loans security camera system repair your credit ram memory sending flowers cheap domain web hosting bankruptcy service what is a reverse mortgage scottrade com gmc vehicles affordable hosting mini cooper usa franchise for sale home air purifiers auto insurance quote match date instant loans memory upgrade home security alarm system online advertising company auto glass specialist home personal trainer advertising magnets top cd rates good website design bodybuilding com dance supply telephone services corporate christmas cards new homes guide lung cancer treatment mortgage for bad credit bad credit mortgage companies sealy posturepedic mattress color business cards mini cooper dealerships donald trump university high speed internet provider new business opportunity elliptical exercise equipment personal loans for poor credit treadmill equipment mini cooper dealer health-insurance www hoodia ford hybrid cars custom website design first time tax accounting gmc canyon phoenix home loan mortgage calculator bad credit mortgage loans multi domain web hosting bare serious skin care infra red sauna web design firms surgery for weight loss buying laptops web design service real estate investment properties home security service homebased business opportunity computer memory upgrade secured personal loan visco elastic foam mattress cheap exercise equipment investing in real estate tractors for sale advertising banners hoodia weight loss get a mortgage with bad credit eva the rabbit dildo carnival cruise ships toshiba pcs auto loan rate phishing auto glass quote hoodia patch park city utah lodging small business website design credit repair companies chevrolet dealerships chapter 13 bankruptcy computer ram memory herbalife business satellite internet services custom garage doors does hoodia work car insurance companies how to make money fast domain name registrar radio city christmas spectacular california reverse mortgage wrought iron lighting prostate cancer cure dating match cheap domain hosting public relations firms discount las vegas vacation las vegas discount vacation www zappos com cheaptickets com magic bullet express blender 2007 ford mustang remove spyware notary public seals home security monitoring christy towel declaring bankruptcy free website design identity theft insurance hoodia facts high speed internet connections commercial real estate mortgage breast cancer information best cheap web hosting magazine subscription services public relations agencies blue cross blue shield plans employment background check advertising on internet dsl connections identity theft help rolling stones tickets memory foam mattress pads personal loans online www overstock chevrolet dealer hp store direcway satellite internet radio city tickets spread blue cross quotes swivel bar stools business cards printers san diego sea world mortgage quote high speed internet access local online advertising ergonomic computer desk manhattan beach dsl internet service providers website design company home security alarm free mortgage rate calculator radio city music hall christmas government loans ftv princess cruise deals personal loan calculator cheapest web hosting new home bar stools leather tax filing all in one printers how to quit smoking personal business cards computer repair service 2007 chevrolet agency credit repair digital security camera bahamas vacations bahamas vacation packages local telephone service massage therapy courses cheap business cards vegas hotel discount life insurance bi polar credit report repair personalized baby shower invitations colon cleansing kits bad credit mortgage rates reverse mortgage lenders mobile ring tones elliptical fitness equipment office space for lease elliptical exercise equipment my credit report colon cleansing kits credit file report cheap web site hosting qoutes south lake tahoe visco foam mattress display banners dedicated web hosting distance learning college good website design notebook pc telephone companies individual health insurance coverage hewlett packard computer no credit personal loans computer office furniture reverse mortgage companies uk business cards file for bankruptcy trading stocks laptop sales cable tv providers mortgage rate calculator commercial office furniture pontiac dealers colon cleansing product radio city christmas spectacular menopause symptom cheap banners 2006 pontiac short term life insurance commercial mortgage house mortgage calculator bath and body works prevent identity theft life insurance agent credit repair service cheap world of warcraft gold experian credit services toshiba notebook computer stop smoking aid world of warcraft gold sales credit repair companies commercial mortgage loans bussiness loans blue cross blue shield insurance high speed satellite internet bodybuilding com no credit loans cheap stock photos universal life insurance world of warcraft gold sale low cost term life insurance magic bullet as seen on tv wool area rugs experian credit bureau how to quit smoking blue cross ppo advertising promotional products cheese gift baskets gmc auto best air purifiers satellite internet providers golf promotional products www cheaptickets com home exercise equipment cheap business cards pontiac g6 auto corporate promotional products best home equity loans cd interest rates usa people search free internet advertising wireless home security system princess cruise deals jumbo cd rates chrysler cars blue cross blue shield health insurance best mortgage tax accounting acura dealer print business cards credit score repair las vegas hotel deals real hoodia computer repair new york magic bullet mixer high speed internet provider cheap hosting toshiba pcs hoodia gordonii cactus information on bankruptcy www ftd com computer repair company auto glass dallas humana medicare sell mary kay radio city music hall tickets ftd gifts custom business cards personalized christmas cards repair bad credit website and design outdoor banners sell stock photos lake las vegas credit report online vitamins online 2007 chevrolet local telephone service preventing identity theft swivel bar stools individual health insurance quote atk prepaid cell phone service free credit repair online personal training personal unsecured loans bank cd rates auto glass shops blue cross quotes anti aging skin care life insurance on line felony lawyer real estate loans credit bureau report bulk domain name registration memory foam mattress sale chrysler dealer wireless internet access cheap truck rental linux web hosting color business cards best hoodia breast cancer treatment notary public supplies pontiac cars low cost individual health insurance ftd com foam mattress topper home garage doors term life insurance policy cheapest web hosting valentines flowers auto glass replacements repair my credit hecm reverse mortgage www overstock rolling stones concert tickets senior life insurance rolling stones concerts minka hyundai cars new chevrolet cars high speed internet connection auto loan interest rates portable infrared sauna computer repair service mens skin care products order business cards discount vegas vacation satellite internet service provider website design rates chair massage bankruptcy attorneys plastic business cards computer printers online florists phoenix drunk driving accident buy laptops bankruptcy attorney bankruptcy advice telephone providers custom vinyl banners cash advance loans home base business opportunity web design services internet advertising company bahamas vacation deals discount treadmill free web site advertising business telephone service pontiac vehicles web hosting reviews ftv personalized promotional products outdoor christmas lights mlm business opportunity commercial exercise equipment domain name hosting dating on line hyundai deals construction management high speed wireless internet cheap all in one printers become a notary public sex toy home parties all in one printers auto loan rate bad credit mortgage rates business computer repair home security alarm systems magazine subscription services buy laptop satellite internet isp high speed internet connections www overstock com find dentist hoodia products toshiba notebook computers design your own business cards make money online business pontiac car quality business cards reverse mortgage info finance companies professional web site design audi dealership multiple domain web hosting 2006 chevrolet adelphia cable tv blue cross and blue shield mortgage quote permanent life insurance commercial mortgage rate saturn car company life insurance company auto glass prices quick payday loans computer memory upgrade home security service advertising agency

Saturday, July 7, 2007

Computer intro...

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.

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 that point.

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

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.[3]

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

1+2+3+...+n = {{n(n+1)} \over 2}

and arrive at the correct answer (500,500) with little work. [4] 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.

[edit] 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.
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 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. [5]

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,[6] 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. This means that 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.[7]

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.[8] 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.

[edit] Example

A traffic light showing red.
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
  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 number 2
  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.[9]

[edit] 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 comprised of many separate components but since the mid-1970s CPUs have typically been constructed on a single integrated circuit called a microprocessor.


[edit] Control unit

Main articles: CPU design and 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.[10] 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.[11]

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

  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.

[edit] Arithmetic/logic unit (ALU)

Main article: Arithmetic logic unit

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.

[edit] Memory

Main article: Computer storage
Magnetic core memory was popular main memory for computers through the 1960s until it was completely replaced by semiconductor memory.
Magnetic core memory was popular main memory for computers through the 1960s until it was completely replaced by semiconductor memory.

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 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. [12]

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.

[edit] Input/output (I/O)

Main article: Input/output
Hard disks are common I/O devices used with computers.
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 disks, floppy disks and optical discs serve as both input and output devices. Computer networking is another form of I/O.

Practically any device that can be made to interface digitally may be used as I/O. The computer in the Engine Control Unit of a modern automobile might read the position of the pedals and steering wheel, the output of the oxygen sensor and devices that monitor the speed of each wheel. The output devices include the various lights and gauges that the driver sees as well as the engine controls such as the spark ignition circuits and fuel injection systems. In a digital wristwatch, the computer reads the buttons and causes numbers and symbols to be shown on the liquid crystal display.

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.

[edit] Multitasking

Main article: Computer 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.

[edit] Multiprocessing

Main article: Multiprocessing
Cray designed many supercomputers that used multiprocessing heavily.
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.[13] 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.

[edit] Networking and the Internet

Main articles: Computer networking and Internet
Visualization of a portion of the routes on 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 US 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 US 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 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.

[edit] Further topics

[edit] Hardware

Main article: Computer 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
First Generation (Mechanical/Electromechanical) Calculators Antikythera mechanism, Difference Engine, Norden bombsight
Programmable Devices Jacquard loom, Analytical Engine, Harvard Mark I, Z3
Second Generation (Vacuum Tubes) Calculators Atanasoff-Berry Computer
Programmable Devices ENIAC, EDSAC, EDVAC, UNIVAC I
Third Generation (Discrete transistors and SSI, MSI, LSI Integrated circuits) Mainframes System/360, BUNCH
Minicomputer PDP-8, PDP-11, System/32, System/36
Fourth Generation (VLSI integrated circuits) 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 [14] x86-64, PowerPC, MIPS, SPARC
Embedded computer 8048, 8051
Personal computer Desktop computer, Home computer, Laptop computer, Personal digital assistant (PDA), Portable computer, Tablet computer, Wearable computer
Server class computer
Theoretical/experimental Quantum computer
Chemical computer
DNA computing
Optical computer
Other Hardware Topics
Peripheral device (Input/output) Input Mouse, Keyboard, Joystick, Image scanner
Output Monitor, Printer
Both Floppy disk drive, Hard disk, Optical disc drive, Teleprinter
Computer busses Short range RS-232, SCSI, PCI, USB
Long range (Computer networking) Ethernet, ATM, FDDI

[edit] Software

Main article: Computer 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
Operating system Unix/BSD UNIX System V, AIX, HP-UX, Solaris (SunOS), FreeBSD, NetBSD, IRIX
GNU/Linux List of Linux distributions, Comparison of Linux distributions
Microsoft Windows Windows 9x, Windows NT, Windows CE
DOS 86-DOS (QDOS), PC-DOS, MS-DOS, FreeDOS
Mac OS Mac OS classic, Mac OS X
Embedded and real-time List of embedded operating systems
Experimental Amoeba, Oberon/Bluebottle, Plan 9 from Bell Labs
Library Multimedia DirectX, OpenGL, OpenAL
Programming library C standard library, Standard template library
Data Protocol TCP/IP, Kermit, FTP, HTTP, SMTP
File format HTML, XML, JPEG, MPEG, PNG
User interface Graphical user interface (WIMP) Microsoft Windows, GNOME, QNX Photon, CDE, GEM
Text user interface Command line interface, shells
Other
Application 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
Misc Artificial intelligence, Antivirus software, Malware scanner, Installer/Package management systems, File manager

[edit] 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 JavaScript, Python, Ruby, PHP, Perl, CGI, ASP, JSP

[edit] 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, Sound recording and reproduction

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

History...

History of computing

Main article: History of computing
The Jacquard loom was one of the first programmable devices.
The Jacquard loom was one of the first programmable devices.

It is difficult to define any one device as the earliest computer. The very definition of a computer has changed and it is therefore impossible to identify the first computer. Many devices once called "computers" would no longer qualify as such by today's standards.

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 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".[2] 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 US 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.

Defining characteristics of five first operative digital computers
Computer Shown working Binary Electronic Programmable Turing complete
Zuse Z3 May 1941 Yes No By punched film stock Yes (1998)
Atanasoff-Berry Computer Summer 1941 Yes Yes No No
Colossus December 1943 / January 1944 Yes Yes Partially, by rewiring No
Harvard Mark I - IBM ASCC 1944 No No By punched paper tape Yes (1998)
ENIAC 1944 No Yes Partially, by rewiring Yes
1948 No 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:

EDSAC was one of the first computers to implement the stored program (von Neumann) architecture.
EDSAC was one of the first computers to implement the stored program (von Neumann) architecture.

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 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.
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.