Other portable computing devices

There are several categories of portable computing devices that can run on batteries but are not usually classified as laptops: portable computers, keyboardless tablet PCs, Internet tablets, PDAs, handheld computers (UMPCs) and smartphones.

The Compaq Portable

A keyboard-less tablet PC

A Palm TX PDA

A Nokia N800 Internet tablet

An OQO handheld computer

An Apple iPhone smartphone

A Portable computer is a general-purpose computer that can be easily moved from place to place, but cannot be used while in transit, usually because it requires some "setting-up" and an AC power source. The most famous example is the Osborne 1. Also called a "transportable" or a "luggable" PC.

A Tablet PC that lacks a keyboard (also known as a non-convertible Tablet PC) is shaped like slate or a paper notebook, features a touchscreen with a stylus and handwriting recognition software. Tablets may not be best suited for applications requiring a physical keyboard for typing, but are otherwise capable of carrying out most tasks that an ordinary laptop would be able to perform.

An Internet tablet is an Internet appliance in tablet form. Unlike a Tablet PC, an Internet tablet does not have much computing power and its applications suite is limited - it can not replace a general purpose computer. Internet tablets typically feature an MP3 and video player, a web browser, a chat application and a picture viewer.

A Personal digital assistant (PDA) is a small, usually pocket-sized, computer with limited functionality. It is intended to supplement and to synchronize with a desktop computer, giving access to contacts, address book, notes, e-mail and other features.

A Handheld computer, also known as an Ultra Mobile PC (UMPC) is a full-featured, PDA-sized computer running a general-purpose operating system.

A Smart phone is a PDA with an integrated cellphone functionality. Current smartphones have a wide range of features and installable applications.

Boundaries that separate these categories are blurry at times. For example, the OQO UMPC is also a PDA-sized tablet PC; the Apple eMate had the clamshell form factor of a laptop, but ran PDA software. The HP Omnibook line of laptops included some devices small enough to be called handheld computers. The hardware of the Nokia 770 internet tablet is essentially the same as that of a PDA such as the Zaurus 6000; the only reason it's not called a PDA is that it doesn't have PIM software. On the other hand, both the 770 and the Zaurus can run some desktop Linux software, usually with modifications.

[edit] Major brands and manufacturers

Main article: List of laptop brands and manufacturers

There is a multitude of laptop brands and manufacturers; several major brands, offering notebooks in various classes, are listed in the box to the right. The major brands usually offer good service and support, including well-executed documentation and driver downloads that will remain available for many years after a particular laptop model is no longer produced. Capitalizing on service, support and brand image, laptops from major brands are more expensive than laptops by smaller brands and ODMs. Some brands are specializing in a particular class of laptops, such as gaming laptops (Alienware), netbooks (EeePC) and laptops for children (OLPC). Many brands, including the major ones, do not design and do not manufacture their laptops. Instead, a small number of Original Design Manufacturers (ODMs) design new models of laptops, and the brands choose the models to be included in their lineup. In 2006, 7 major ODMs manufactured 7 of every 10 laptops in the world, with the largest one (Quanta Computer) having 30% world market share.[41] Therefore, there often are identical models available both from a major label and from a low-profile ODM in-house brand.

Major laptop brands Acer - TravelMate, Extensa, Ferrari and Aspire Apple - MacBook, MacBook Air and MacBook Pro ASUS - Asus Eee, Lamborghini Compaq - Evo, Armada, LTE, and Presario Dell - Inspiron, Latitude, Precision,Studio, Vostro and XPS Gateway Hewlett-Packard - HP Pavilion, HP Omnibook, HP Compaq Notebooks Lenovo - ThinkPad, IdeaPad, and 3000 series Panasonic - Toughbook, Satellite, Let's Note (Japan only) [42] Samsung - SENS: M, P, Q, R and X series Sony - VAIO: FJ Series, UX, TZ, NR, SZ, CR, FZ, and AR series Toshiba - Dynabook, Equium, Portege, Tecra, Satellite, Qosmio, Libretto

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Infection strategies

In order to replicate itself, a virus must be permitted to execute code and write to memory. For this reason, many viruses attach themselves to executable files that may be part of legitimate programs. If a user tries to start an infected program, the virus' code may be executed first. Viruses can be divided into two types, on the basis of their behavior when they are executed. Nonresident viruses immediately search for other hosts that can be infected, infect these targets, and finally transfer control to the application program they infected. Resident viruses do not search for hosts when they are started. Instead, a resident virus loads itself into memory on execution and transfers control to the host program. The virus stays active in the background and infects new hosts when those files are accessed by other programs or the operating system itself.

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Security engineering

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Security engineering is a specialized field of engineering that deals with the development of detailed engineering plans and designs for security features, controls and systems. It is similar to other systems engineering activities in that its primary motivation is to support the delivery of engineering solutions that satisfy pre-defined functional and user requirements, but with the added dimension of preventing misuse and malicious behavior. These constraints and restrictions are often asserted as a security policy.

In one form or another, Security Engineering has existed as an informal field of study for several centuries. For example, the fields of locksmithing and security printing have been around for many years.

Due to recent catastrophic events, most notably 9/11, Security Engineering has quickly become a rapidly growing field. In fact, in a recent report completed in 2006, it was estimated that the global security industry was valued at US$150 billion.^[1]

Security engineering involves aspects of social science, psychology (such as designing a system to 'fail well' instead of trying to eliminate all sources of error) and economics, as well as physics, chemistry, mathematics, architecture and landscaping.[1] Some of the techniques used, such as fault tree analysis, are derived from safety engineering.

Other techniques such as cryptography were previously restricted to military applications. One of the pioneers of security engineering as a formal field of study is Ross Anderson.

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Theoretical computer science

The broader field of theoretical computer science encompasses both the classical theory of computation, and a wide range of other topics that focus on the more abstract, logical and mathematical aspects of computing.

P \rightarrow Q \, \Gamma\vdash x : Int
Mathematical logic Automata theory Number theory Graph theory Type theory Category theory Computational geometry Quantum computing theory

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Computing

RAM (Random Access Memory)

Computing is usually defined as the activity of using and developing computer technology, computer hardware and software. It is the computer-specific part of information technology. Computer science (or computing science) is the study and the science of the theoretical foundations of information and computation and their implementation and application in computer systems.

Computing Curricula 2005[1] defined computing:

In a general way, we can define computing to mean any goal-oriented activity requiring, benefiting from, or creating computers. Thus, computing includes designing and building hardware and software systems for a wide range of purposes; processing, structuring, and managing various kinds of information; doing scientific studies using computers; making computer systems behave intelligently; creating and using communications and entertainment media; finding and gathering information relevant to any particular purpose, and so on. The list is virtually endless, and the possibilities are vast.

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Computer scientist

From Wikipedia, the free encyclopedia

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A computer scientist is a person who has acquired knowledge of computer science, the study of the theoretical foundations of information and computation and their application in computer systems.

Computer scientists typically work on the design of the software side of computer systems, versus the hardware side which computer engineers mainly focus on, although there is overlap. Computer scientists can work on, and research in, areas such as algorithm development and design, software engineering, information theory, database theory, computational complexity theory, human-computer interaction, computer programming, programming language theory, computer graphics, and computer vision.

Their specific jobs notwithstanding, the term computer scientist should not be used interchangeably with the previous terms. Overall, computer scientists study the theoretical foundations of computing from which the other fields (software engineering, information theory, database theory, computational complexity theory, human-computer interaction, computer programming, programming language theory, computer graphics, and computer vision) derive. As its name implies, computer science is a pure science, not an applied science or applied business field. As an analogy to the medical field, a computer scientist is like the cancer researcher who might study molecular biology or biochemistry in-depth, while an information technology specialist is like the physician who studies those fields at a higher level and focuses on their application to patient care.

Computer scientists can follow more practical applications of their knowledge, doing things such as software development, web development and database programming. Computer scientists can also be found in the field of information technology consulting.

Computer scientists normally get their degree in computer science at an accredited university or institution.

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The programmer subculture of hackers

Main article: Hacker (programmer subculture)

A Hacker Emblem proposed by Eric S. Raymond.

The computer security use is contrasted by the different understanding of hacker as a person who follows a spirit of playful cleverness and loves programming. It is found in an originally academic movement unrelated to computer security and most visibly associated with free software and open source. It also has a hacker ethic, based on the idea that writing software and sharing the result on a voluntary basis is a good idea, and that information should be free, but that it's not up to the hacker to make it free by breaking into private computer systems. Academic hackers disassociate from the mass media's pejorative use of the word 'hacker' referring to computer security, and usually prefer the term 'cracker' for that meaning.

In this hacker culture, a computer hacker is a person who enjoys designing software and building programs with a sense for aesthetics and playful cleverness. The term hack in this sense can be traced back to "describe the elaborate college pranks that...students would regularly devise" (Levy, 1984 p.10). To be considered a 'hack' was an honour among like-minded peers as "to qualify as a hack, the feat must be imbued with innovation, style and technical virtuosity" (levy, 1984 p.10)

According to Eric S. Raymond,^[21] the Open source and Free Software hacker subculture developed in the 1960s among ‘academic hackers’^[22] working on early minicomputers in computer science environments in the United States. After 1969 it fused with the technical culture of the pioneers of the Arpanet. The PDP-10 machine AI at MIT, which was running the ITS operating system and was connected to the Arpanet, provided an early hacker meeting point. After 1980 the subculture coalesced with the culture of Unix, and after 1987 with elements of the early microcomputer hobbyists that themselves had connections to radio amateurs in the 1920s. Since the mid-1990s, it has been largely coincident with what is now called the free software and open source movement.

Many programmers have been labeled "great hackers,"^[23] but the specifics of who that label applies to is a matter of opinion. Certainly major contributors to computer science such as Edsger Dijkstra and Donald Knuth, as well as the inventors of popular software such as Linus Torvalds (Linux), and Dennis Ritchie and Ken Thompson (the C programming language) are likely to be included in any such list; see also List of programmers. People primarily known for their contributions to the consciousness of the academic hacker culture include Richard Stallman, the founder of the free software movement and the GNU project, president of the Free Software Foundation and author of the famous Emacs text editor as well as the GNU Compiler Collection (GCC), and Eric S. Raymond, one of the founders of the Open Source Initiative and writer of the famous text The Cathedral and the Bazaar and many other essays, maintainer of the Jargon File (which was previously maintained by Guy L. Steele, Jr.).

Within the academic hacker culture, the term hacker is also used for a programmer who reaches a goal by employing a series of modifications to extend existing code or resources. In this sense, it can have a negative connotation of using kludges to accomplish programming tasks that are ugly, inelegant, and inefficient. This derogatory form of the noun "hack" is even used among users of the positive sense of "hacker" (some argue that it should not be, due to this negative meaning; others argue that some kludges can, for all their ugliness and imperfection, still have "hack value"). In a very universal sense, hacker also means someone who makes things work beyond perceived limits in a clever way in general.^[4] That is, people who apply the creative attitude of software hackers in fields other than computing. This includes even activities that predate computer hacking, for example reality hackers.^[24] More recent examples of this usage are wetware hackers and media hackers. According to the Jargon File the word hacker was used in a similar meaning among radio amateurs already in the 1950s.^[25]

The culture sometimes uses jargon which is "incomprehensible to outsiders".^[26] Examples are 'losing' "when a piece of equipment is not working"^[26] and 'munged' "when a piece of equipment is ruined".^[26]

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Hacker definition controversy

The terms hacker and hack are marked by contrasting positive and negative connotations. Computer programmers often use the words hacking and hacker to express admiration for the work of a skilled software developer, but may also use them in a negative sense to describe the production of inelegant kludges. Some frown upon using hacking as a synonym for security cracking -- in distinct contrast to the larger world, in which the word hacker is typically used to describe someone who "hacks into" a system by evading or disabling security measures.

[edit] Controversy and ambiguity

While "hack" was originally more used as a verb for "messing about" with (e.g. "I hack around with computers"), the meaning of the term has shifted over the decades since it first came into use in a computer context. As usage has spread more widely, the primary meaning of newer users of the word has shifted to one which conflicts with the original primary emphasis.

Currently, "hacker" is used in two main ways, one pejorative and one complimentary. In popular usage and in the media, it most often refers to computer intruders or criminals, with associated pejorative connotations. (For example, "An Internet 'hacker' broke through state government security systems in March.") In the computing community, the primary meaning is a complimentary description for a particularly brilliant programmer or technical expert. (For example, "Linus Torvalds, the creator of Linux, is considered by some to be a genius hacker.") A large segment of the technical community insist the latter is the "correct" usage of the word (see the Jargon File definition below).

The mainstream media's current usage of the term may be traced back to the early 1980s (see History). When the term was introduced to wider society by the mainstream media in 1983, even those in the computer community referred to computer intrusion as "hacking", although not as the exclusive use of that word. In reaction to the increasing media use of the term exclusively with the criminal connotation, the computer community began to differentiate their terminology. Several alternative terms such as "black hat" and "cracker" were coined in an effort to distinguish between those performing criminal activities, and those whose activities were the legal ones referred to more frequently in the historical use of the term "hack". Analogous terms such as "white hats" and "gray hats" developed as a result. However, since network news use of the term pertained primarily to the criminal activities despite this attempt by the technical community to preserve and distinguish the original meaning, the mainstream media and general public continue to describe computer criminals with all levels of technical sophistication as "hackers" and does not generally make use of the word in any of its non-criminal connotations.

As a result of this difference, the definition is the subject of heated controversy. The wider dominance of the pejorative connotation is resented by many who object to the term being taken from their cultural jargon and used negatively,^[8] including those who have historically preferred to self-identify as hackers. Many advocate using the more recent and nuanced alternate terms when describing criminals and others who negatively take advantage of security flaws in software and hardware. Others prefer to follow common popular usage, arguing that the positive form is confusing and unlikely to become widespread in the general public. A minority still stubbornly use the term in both original senses despite the controversy, leaving context to clarify (or leave ambiguous) which meaning is intended. It is noteworthy, however, that the positive definition of hacker was widely used as the predominant form for many years before the negative definition was popularized.

"Hacker" can therefore be seen as a shibboleth, identifying those who use the technically-oriented sense (as opposed to the exclusively intrusion-oriented sense) as members of the computing community.

A possible middle ground position has been suggested, based on the observation that "hacking" describes a collection of skills which are used by hackers of both descriptions for differing reasons. The analogy is made to locksmithing, specifically picking locks, which — aside from its being a skill with a fairly high tropism to 'classic' hacking — is a skill which can be used for good or evil. The primary weakness of this analogy is the inclusion of script kiddies in the popular usage of "hacker", despite the lack of an underlying skill and knowledge base.

History

See also: Timeline of computer security hacker history

A timeline of the noun "hack" and etymologically related terms as they evolved in historical English:

• c. 1700, originally, "person hired to do routine work," short for hackney "an ordinary horse" (c.1300) later, coach for hire, and taxicab driver (hackie).
• Early 20th century: hack is one of many slang terms in use by railroaders for a train's caboose.^[9] Subsequent spread of this usage from professional rail workers to model rail hobbyists is likely, but not proven.
• 1950s: amateur radio enthusiasts defined the term hacking as creatively tinkering to improve performance.
• 1959: hack is defined in MIT's Tech Model Railroad Club Dictionary as "1) an article or project without constructive end; 2) a project undertaken on bad self-advice; 3) an entropy booster; 4) to produce, or attempt to produce, a hack(3)." hacker is defined as "one who hacks, or makes them." Much of the TMRC's jargon is later imported into early computing culture.
• 1963: The first recorded reference to hackers in the computer sense is made in The Tech (MIT Student Magazine).^[10]
• 1972: Stewart Brand publishes "S P A C E W A R: Fanatic Life and Symbolic Death Among the Computer Bums" in Rolling Stone, an early piece describing computer culture. In it, Alan Kay is quoted as saying "A true hacker is not a group person. He's a person who loves to stay up all night, he and the machine in a love-hate relationship... They're kids who tended to be brilliant but not very interested in conventional goals[...] It's a term of derision and also the ultimate compliment."
• 1980: The August issue of Psychology Today prints (with commentary by Philip Zimbardo) "The Hacker Papers", an excerpt from a Stanford Bulletin Board discussion on the addictive nature of computer use.
• 1982: In the film TRON, Kevin Flynn (Jeff Bridges) describes his intentions to break into ENCOM's computer system, saying "I've been doing a little hacking here". CLU is the software he uses for this.
• 1983: The movie WarGames, featuring a computer intrusion into NORAD, is released. A gang of 6 teenagers is caught breaking into dozens of computer systems, including that of Los Alamos National Laboratory.^[11] Newsweek features the cover story "Beware: Hackers at play."^[12] First Usenet post on the use of the term hacker in the media (CBS News) to mean computer criminal.^[13] Pressured by media coverage of computer intrusions, Congress begins work on new laws for computer security.^[14]
• 1984: Steven Levy publishes Hackers: Heroes of the Computer Revolution. The book publicizes, and perhaps originates the phrase "Hacker Ethic" and gives a codification of its principles.
• 1988: Stalking the Wily Hacker, an article by Clifford Stoll appears in the May 1988 issue of the Communications of the ACM and uses the term hacker in the sense of a computer criminal. Later that year, the release by Robert Tappan Morris, Jr. of the so-called Morris worm provoked the popular media to spread this usage.
• 1989: The Cuckoo's Egg by Clifford Stoll is published, and its popularity further entrenches the term in the public's consciousness.

[edit] Contemporary use

The modern, computer-related use of the term is considered likely rooted in the goings on at the Massachusetts Institute of Technology (MIT) in the 1960s, long before computers became common; the word "hack" was local slang which had a large number of related meanings. One was a simple, but often inelegant, solution to a problem. It also meant any clever prank perpetrated by MIT students; logically the perpetrator was a hacker. To this day the terms hack and hacker are used in several ways at MIT, without necessarily referring to computers. When MIT students surreptitiously put a fake police car atop the dome on MIT's Building 10, that was a hack, and the students involved were therefore hackers. Another type of hacker — one who explores undocumented or unauthorized areas in buildings — is now called a reality hacker or urban spelunker.

The term was fused with computers when members of the Tech Model Railroad Club started working with a Digital Equipment Corporation PDP-1 computer and applied local model railroad slang to computers.

The earliest known use of the term in this manner is from the 20 November 1963 issue of The Tech, the student paper of MIT:

Many telephone services have been curtailed because of so-called hackers, according to Prof. Carlton Tucker, administrator of the Institute phone system. […] The hackers have accomplished such things as tying up all the tie-lines between Harvard and MIT, or making long-distance calls by charging them to a local radar installation. One method involved connecting the PDP-1 computer to the phone system to search the lines until a dial tone, indicating an outside line, was found. […] Because of the 'hacking', the majority of the MIT phones are 'trapped'.

Originally, the term "hack" was applied almost exclusively to programming or electrical engineering, but it has come to be used in some circles for almost any type of clever circumvention, in phrases such as "hack the media", "hack your brain" and "hack your reputation".

[edit] Negative usage in engineering

Another meaning of the term "hack", similar to kludge and distinct from both the positive and security-related meanings discussed above, derives from the everyday English sense "to cut or shape by or as if by crude or ruthless strokes" [Merriam-Webster]. In other words to "hack" at an original creation, as if with an axe, is to force-fit it into being usable for a task not intended by the original creator, and a "hacker" would be someone who does this habitually. (The original creator and the hacker may be the same person.)

This usage is common in both programming ^[15] and engineering. In programming, hacking in this sense appears to be tolerated and seen as a necessary compromise in many situations. In non-software engineering, the culture is less tolerant of unmaintainable solutions, even when intended to be temporary, and describing someone as a "hacker" might imply that they lack professionalism. In this sense, the term has no real positive connotations, except for the idea that the hacker is capable of doing modifications that allow a system to work in the short term, and so has some sort of marketable skills. There is always, however, the understanding that a more skillful, or technical, logician could have produced successful modifications that would not be considered a "hack-job".

The definition is similar to other, non-computer based, uses of the term "hack-job". For instance, a professional modification of a production sports car into a racing machine would not be considered a hack-job, but a cobbled together backyard mechanic's result could be. Even though the outcome of a race of the two machines could not be assumed, a quick inspection would instantly reveal the difference in the level of professionalism of the designers.

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Networking and Internet

Main articles: Computer networking and Internet

Visualization of a portion of the routes on the Internet.

Computers have been used to coordinate information between multiple locations since the 1950s. The U.S. military's SAGE system was 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 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.

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Multiprocessing

Main article: 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.^[19] 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 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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Input/output (I/O)

Main article: Input/output

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^{[citation needed]}. Modern desktop computers contain many smaller computers that assist the main CPU in performing I/O.

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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.

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.^[18]

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.

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How computers work

Main articles: Central processing unit and Microprocessor

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

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.^[16] 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.^[17]

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.

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How computers work

Main articles: Central processing unit and Microprocessor

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

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.^[16] 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.^[17]

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.

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Stored program architecture

Main articles: Computer program and Computer programming

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 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

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.^[9]

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.^[10] 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 may run from just a few instructions to many millions of instructions, as in a program for a word processor or a web browser. A typical modern computer can execute billions of instructions per second (gigahertz or GHz) and rarely make a mistake over many years of operation. Large computer programs comprising several million instructions may take teams of programmers years to write, thus the probability of the entire program having been written without error is highly unlikely.

Errors in computer programs are called "bugs". Bugs may be benign and not affect the usefulness of the program, or have only subtle effects. But in some cases they may cause the program to "hang" - become unresponsive to input such as mouse clicks or keystrokes, or to completely fail or "crash". Otherwise benign bugs may sometimes may be harnessed for malicious intent by an unscrupulous user writing an "exploit" - code designed to take advantage of a bug and disrupt a program's proper execution. 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.^[11]

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,^[12] 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.^[13]

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.^[14] 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. Producing software with an acceptably high reliability on a predictable schedule and budget has proved historically to be a great challenge; 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
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 to 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.^[15]

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