In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language. An interpreter may be a program that either

1. executes the source code directly
2. translates source code into some efficient intermediate representation (code) and immediately executes this
3. explicitly executes stored precompiled code^[1] made by a compiler which is part of the interpreter system

Early versions of the Lisp programming language and Dartmouth BASIC would be examples of type 1. Perl, Python, MATLAB, and Ruby are examples of type 2, while UCSD Pascal is type 3: Source programs are compiled ahead of time and stored as machine independent code, which is then linked at run-time and executed by an interpreter and/or compiler (for JIT systems). Some systems, such as Smalltalk, contemporary versions of BASIC, Java and others, may also combine 2 and 3.

While interpreting and compiling are the two main means by which programming languages are implemented, these are not fully mutually exclusive categories, one of the reasons being that most interpreting systems also perform some translation work, just like compilers. The terms "interpreted language" or "compiled language" merely mean that the canonical implementation of that language is an interpreter or a compiler; a high level language is basically an abstraction which is (ideally) independent of particular implementations.

History[link]

The first interpreted high-level language was probably Lisp. Lisp was first implemented by Steve Russell on an IBM 704 computer. Russell had read McCarthy's paper, and realized (to McCarthy's surprise) that the Lisp eval function could be implemented in machine code.^[2] The result was a working Lisp interpreter which could be used to run Lisp programs, or more properly, 'evaluate Lisp expressions.'

Advantages and disadvantages of using interpreters[link]

Programmers usually write programs in high level code, which the CPU cannot execute; so this source code has to be converted into machine code. This conversion is done by a compiler or an interpreter. A compiler makes the conversion just once, while an interpreter typically converts it every time a program is executed (or in some languages like early versions of BASIC, every time a single instruction is executed).

Development cycle[link]

During program development, the programmer makes frequent changes to source code. With a compiler, each time the programmer makes a change to the source code, s/he must wait for the compiler to translate the altered source files, and link all of the binary code files together before the program can be executed. The larger the program, the longer the programmer must wait to see the results of the change. By contrast, a programmer using an interpreter does a lot less waiting, as the interpreter usually just needs to translate the code being worked on to an intermediate representation (or not translate it at all), thus requiring much less time before the changes can be tested.

Distribution[link]

A compiler converts source code into binary instruction for a specific processor's architecture, thus making it less portable. This conversion is made just once, on the developer's environment, and after that the same binary can be distributed to the user's machines where it can be executed without further translation. A cross compiler can generate binary code for the user machine even if it has a different processor than the machine where the code is compiled.

An interpreted program can be distributed as source code. It needs to be translated in each final machine, which takes more time but makes the program distribution independent of the machine's architecture. However, the portability of interpreted source code is dependent on the target machine actually having a suitable interpreter. If the interpreter needs to be supplied along with the source, the overall installation process is more complex than delivery of a monolithic executable.

The fact that interpreted code can easily be read and copied by humans can be of concern from the point of view of copyright. However, various systems of encryption and obfuscation exist. Delivery of intermediate code, such as bytecode, has a similar effect to obfuscation.

Efficiency[link]

The main disadvantage of interpreters is that when a program is interpreted, it typically runs more slowly than if it had been compiled. The difference in speeds could be tiny or great; often an order of magnitude and sometimes more. It generally takes longer to run a program under an interpreter than to run the compiled code but it can take less time to interpret it than the total time required to compile and run it. This is especially important when prototyping and testing code when an edit-interpret-debug cycle can often be much shorter than an edit-compile-run-debug cycle.

Interpreting code is slower than running the compiled code because the interpreter must analyze each statement in the program each time it is executed and then perform the desired action, whereas the compiled code just performs the action within a fixed context determined by the compilation. This run-time analysis is known as "interpretive overhead". Access to variables is also slower in an interpreter because the mapping of identifiers to storage locations must be done repeatedly at run-time rather than at compile time.

There are various compromises between the development speed when using an interpreter and the execution speed when using a compiler. Some systems (such as some Lisps) allow interpreted and compiled code to call each other and to share variables. This means that once a routine has been tested and debugged under the interpreter it can be compiled and thus benefit from faster execution while other routines are being developed. Many interpreters do not execute the source code as it stands but convert it into some more compact internal form. Many BASIC interpreters replace keywords with single byte tokens which can be used to find the instruction in a jump table. Others achieve even higher levels of program compaction by using a bit-oriented rather than a byte-oriented program memory structure, where commands tokens can occupy perhaps 5 bits of a 16 bit word, leaving 11 bits for their label or address operands.

An interpreter might well use the same lexical analyzer and parser as the compiler and then interpret the resulting abstract syntax tree.

Regress[link]

Interpretation cannot be used as the sole method of execution: even though an interpreter can itself be interpreted and so on, a directly executed program is needed somewhere at the bottom of the stack.

Variations[link]

Bytecode interpreters[link]

There is a spectrum of possibilities between interpreting and compiling, depending on the amount of analysis performed before the program is executed. For example, Emacs Lisp is compiled to bytecode, which is a highly compressed and optimized representation of the Lisp source, but is not machine code (and therefore not tied to any particular hardware). This "compiled" code is then interpreted by a bytecode interpreter (itself written in C). The compiled code in this case is machine code for a virtual machine, which is implemented not in hardware, but in the bytecode interpreter. The same approach is used with the Forth code used in Open Firmware systems: the source language is compiled into "F code" (a bytecode), which is then interpreted by a virtual machine.

Control tables - that do not necessarily ever need to pass through a compiling phase - dictate appropriate algorithmic control flow via customized interpreters in similar fashion to bytecode interpreters.

Abstract Syntax Tree interpreters[link]

In the spectrum between interpreting and compiling, another approach is transforming the source code into an optimized Abstract Syntax Tree (AST) then executing the program following this tree structure, or using it to generate native code Just-In-Time.^[3] In this approach, each sentence needs to be parsed just once. As an advantage over bytecode, the AST keeps the global program structure and relations between statements (which is lost in a bytecode representation), and when compressed provides a more compact representation.^[4] Thus, using AST has been proposed as a better intermediate format for Just-in-time compilers than bytecode. Also, it allows to perform better analysis during runtime.

However, for interpreters, an AST causes more overhead than a bytecode interpreter, because of nodes related to syntax performing no useful work, of a less sequential representation (requiring traversal of more pointers) and of overhead visiting the tree.^[5]

Just-in-time compilation[link]

Further blurring the distinction between interpreters, byte-code interpreters and compilation is just-in-time compilation (or JIT), a technique in which the intermediate representation is compiled to native machine code at runtime. This confers the efficiency of running native code, at the cost of startup time and increased memory use when the bytecode or AST is first compiled. Adaptive optimization is a complementary technique in which the interpreter profiles the running program and compiles its most frequently-executed parts into native code. Both techniques are a few decades old, appearing in languages such as Smalltalk in the 1980s.^[6]

Just-in-time compilation has gained mainstream attention amongst language implementers in recent years, with Java, the .NET Framework and most modern JavaScript implementations now including JITs.

Applications[link]

• Interpreters are frequently used to execute command languages, and glue languages since each operator executed in command language is usually an invocation of a complex routine such as an editor or compiler.
• Self-modifying code can easily be implemented in an interpreted language. This relates to the origins of interpretation in Lisp and artificial intelligence research.
• Virtualisation. Machine code intended for one hardware architecture can be run on another using a virtual machine, which is essentially an interpreter.
• Sandboxing: An interpreter or virtual machine is not compelled to actually execute all the instructions the source code it is processing. In particular, it can refuse to execute code that violates any security constraints it is operating under.

Punched card interpreter[link]

The term "interpreter" often referred to a piece of unit record equipment that could read punched cards and print the characters in human-readable form on the card. The IBM 550 Numeric Interpreter and IBM 557 Alphabetic Interpreter are typical examples from 1930 and 1954, respectively.

See also[link]

• Command-line interpreter
• Interpreted language
• Compiled language
• Dynamic compilation
• Partial evaluation
• Meta-circular evaluator

Notes and references[link]

1. ^ In this sense, the CPU is also an interpreter, of machine instructions.
2. ^ According to what reported by Paul Graham in Hackers & Painters, p. 185, McCarthy said: "Steve Russell said, look, why don't I program this eval..., and I said to him, ho, ho, you're confusing theory with practice, this eval is intended for reading, not for computing. But he went ahead and did it. That is, he compiled the eval in my paper into IBM 704 machine code, fixing bug, and then advertised this as a Lisp interpreter, which it certainly was. So at that point Lisp had essentially the form that it has today..."
3. ^ AST intermediate representations, Lambda the Ultimate forum
4. ^ A Tree-Based Alternative to Java Byte-Codes, Thomas Kistler, Michael Franz
5. ^ http://webkit.org/blog/189/announcing-squirrelfish/
6. ^ L. Deutsch, A. Schiffman, Efficient implementation of the Smalltalk-80 system, Proceedings of 11th POPL symposium, 1984.

External links[link]

• IBM Card Interpreters page at Columbia University
• Theoretical Foundations For Practical 'Totally Functional Programming' (Chapter 7 especially) Doctoral dissertation tackling the problem of formalising what is an interpreter

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. (November 2009)

Language interpretation is the facilitating of oral or sign-language communication, either simultaneously or consecutively, between users of different languages. The process is described by both the words interpreting and interpretation. Translation studies deal with the systematic study of the theory, the description and the application of language interpretation and translation.^[1]

In professional parlance, interpreting denotes the facilitating of communication from one language form into its equivalent, or approximate equivalent, in another language form; while interpretation denotes the actual product of this work, that is, the message thus rendered into speech, sign language, writing, non-manual signals, or other language form. This important distinction is observed in order to avoid confusion.

An interpreter is a person who converts a thought or expression in a source language into an expression with a comparable meaning in a target language in "real time". The interpreter's function is to convey every semantic element (tone and register) and every intention and feeling of the message that the source-language speaker is directing to target-language recipients.

Comparison to translation[link]

Despite being used in a non-technical sense as interchangeable, interpretation and translation are not synonymous. Interpreting takes a message from a source language and renders that message into a different target language(ex: English into French). In interpreting, the interpreter will take in a complex concept from one language, choose the most appropriate vocabulary in the target language to faithfully render the message in a linguistically, emotionally, tonally, and culturally equivalent message. Translation is the transference of meaning from text to text (written or recorded), with the translator having time and access to resources (dictionaries, glossaries, etc.) to produce an accurate document or verbal artifact. Lesser known is "transliteration," used within sign language interpreting, takes one form of a language and transfer those same words into another form ^[2](ex: spoken English into a signed form of English, Signed Exact English, not ASL).

In court interpretation, it is not acceptable to omit anything from the source, no matter how fast the source speaks, since not only is accuracy a principal canon for interpreters, but mandatory. The alteration of even a single word in a material can totally mislead the triers of fact. The most important factor for this level of accuracy is the use of a team of two or more interpreters during a lengthy process, with one actively interpreting and the second monitoring for greater accuracy.

Translators have time to consider and revise each word and sentence before delivering their product to the client. While live interpretation's goal is to achieve total accuracy at all times, details of the original (source) speech can be missed and interpreters can ask for clarification from the speaker. In any language, including signed languages, when a word is used for which there is no exact match, expansion may be necessary in order to fully interpret the intended meaning of the word (ex: the English word "hospitable" may require several words or phrases to encompass its complex meaning). Another unique situation is when an interpreted message appears much shorter or longer than the original message. The message may appear shorter at times because of unique efficiencies within a certain language. English to Spanish is a prime example: Spanish uses gender specific nouns, not used in English, which convey information in a more condensed package thus requiring more words and time in an English interpretation to provide the same plethora of information. Because of situations like these, interpreting often requires a "lag" or "processing" time. This time allows the interpreter to take in subjects and verbs in order to rearrange grammar appropriately while picking accurate vocabulary before starting the message. While working with interpreters, it is important to remember lag time in order to avoid accidentally interrupting one another and to receive the entire message.

Modes[link]

Simultaneous[link]

Interpreter place at the European Court of Justice

In (extempore) simultaneous interpretation (SI), the interpreter renders the message in the target-language as quickly as he or she can formulate it from the source language, while the source-language speaker continuously speaks; a spoken language SI interpreter, sitting in a sound-proof booth, speaks into a microphone, while clearly seeing and hearing the source-language speaker via earphones. The simultaneous interpretation is rendered to the target-language listeners via their earphones. Moreover, SI is the common mode used by sign language interpreters, although the person using the source language, the interpreter and the target language recipient (since either the hearing person or the deaf person may be delivering the message) must necessarily be in close proximity.

The first introduction and employment of extempore simultaneous interpretation was the Nuremberg Trials, with four official working languages.

Consecutive[link]

Interpreters facilitate 3-way conversation among Vladimir Putin (Russian), Muammar Gaddafi (Arabic) and Mireille Mathieu (French) (2008).

In consecutive interpreting (CI), the interpreter speaks after the source-language speaker has finished speaking. The speech is divided into segments, and the interpreter sits or stands beside the source-language speaker, listening and taking notes as the speaker progresses through the message. When the speaker pauses or finishes speaking, the interpreter then renders a portion of the message or the entire message in the target language.

Consecutive interpretation is rendered as "short CI" or "long CI". In short CI, the interpreter relies on memory, each message segment being brief enough to memorize. In long CI, the interpreter takes notes of the message to aid rendering long passages. These informal divisions are established with the client before the interpretation is effected, depending upon the subject, its complexity, and the purpose of the interpretation.

On occasion, document sight translation is required of the interpreter during consecutive interpretation work. Sight translation combines interpretation and translation; the interpreter must render the source-language document to the target-language as if it were written in the target language. Sight translation occurs usually, but not exclusively, in judicial and medical work.

The CI interpreter Patricia Stöcklin renders Klaus Bednarz's speech to Garry Kasparov.

The CI interpreter Patricia Stöcklin takes notes on Garry Kasparov's speech.

The CI interpreter Patricia Stöcklin renders Garry Kasparov's speech to the audience.

Consecutively-interpreted speeches, or segments of them, tend to be short. Fifty years ago, the CI interpreter would render speeches of 20 or 30 minutes; today, 10 or 15 minutes is considered too long, particularly since audiences usually prefer not to sit through 20 minutes of speech they cannot understand.

Often, if not previously advised, the source-language speaker is unaware that he or she may speak more than a single sentence before the CI interpretation is rendered and might stop after each sentence to await its target-language rendering. Sometimes, however, depending upon the setting or subject matter, and upon the interpreter's capacity to memorize, the interpreter may ask the speaker to pause after each sentence or after each clause. Sentence-by-sentence interpreting requires less memorization and therefore lower likelihood for omissions, yet its disadvantage is in the interpreter's not having heard the entire speech or its gist, and the overall message is sometimes harder to render both because of lack of context and because of interrupted delivery (for example, imagine a joke told in bits and pieces, with breaks for translation in between). This method is often used in rendering speeches, depositions, recorded statements, court witness testimony, and medical and job interviews, but it is usually best to complete a whole idea before it is interpreted.

Full (i.e., unbroken) consecutive interpreting of whole thoughts allows for the full meaning of the source-language message to be understood before the interpreter renders it in the target language. This affords a truer, more accurate, and more accessible interpretation than does simultaneous interpretation.

Whispered[link]

In whispered interpreting (chuchotage, in French), the interpreter sits or stands next to the small target-language audience whilst whispering a simultaneous interpretation of the matter to hand; this method requires no equipment, but may be done via a microphone and headphones if the participants prefer. Chuchotage is used in circumstances where the majority of a group speaks the source language, and a minority (ideally no more than three people) do not speak it.

Relay[link]

Relay interpreting is usually used when there are several target languages. A source-language interpreter interprets the text to a language common to every interpreter, who then render the message to their respective target languages. For example, a Japanese source message first is rendered to English to a group of interpreters, who listen to the English and render the message into Arabic, French, and Russian, the other target languages. In heavily multilingual meetings, there may be more than one "intermediate" language, i.e. a Greek source language could be interpreted into English and then from English to other languages, and, at the same time, it may also be directly interpreted into French, and from French into yet more languages. This solution is most often used in the multilingual meetings of the EU institutions.

Liaison[link]

Liaison interpreting involves relaying what is spoken to one, between two, or among many people. This can be done after a short speech, or consecutively, sentence-by-sentence, or as chuchotage (whispering); aside from notes taken at the time, no equipment is used.

Types[link]

Conference[link]

Conference interpreting is the interpretation of a conference, either simultaneously or consecutively, although the advent of multi-lingual meetings has consequently reduced the consecutive interpretation in the last 20 years.

Conference interpretation is divided between two markets: the institutional and private. International institutions (EU, UN, EPO, et cetera), holding multi-lingual meetings, often favour interpreting several foreign languages to the interpreters' mother tongues. Local private markets tend to bi-lingual meetings (the local language plus another) and the interpreters work both into and out of their mother tongues; the markets are not mutually exclusive. The International Association of Conference Interpreters (AIIC) is the only worldwide association of conference interpreters. Founded in 1953, it assembles more than 2,800 professional conference interpreters in more than 90 countries.

Judicial[link]

See also Legal translation. Judicial, Legal, or Court Interpreting, occurs in courts of justice, administrative tribunals, and wherever a legal proceeding is held (i.e. a police station for an interrogation, a conference room for a deposition or the locale for taking a sworn statement). Legal interpreting can be the consecutive interpretation of witnesses' testimony for example, or the simultaneous interpretation of entire proceedings, by electronic means, for one person, or all of the people attending.

The right to a competent interpreter for anyone who does not understand the language of the court (especially for the accused in a criminal trial) is usually considered a fundamental rule of justice. Therefore, this right is often guaranteed in national constitutions, declarations of rights, fundamental laws establishing the justice system or by precedents set by the highest courts. However, it is not a constitutionally required procedure (in the United States) that a certified interpreter be present at police interrogation.^[3]

Depending upon the regulations and standards adhered to per state and venue, court interpreters usually work alone when interpreting consecutively, or as a team, when interpreting simultaneously. In addition to practical mastery of the source and target languages, thorough knowledge of law and legal and court procedures is required of court interpreters. They often are required to have formal authorisation from the State to work in the Courts — and then are called certified court interpreters.^[4] In many jurisdictions, the interpretation is considered an essential part of the evidence. Incompetent interpretation, or simply failure to swear in the interpreter, can lead to a mistrial.

Escort[link]

In escort interpreting, an interpreter accompanies a person or a delegation on a tour, on a visit, or to a meeting or interview. An interpreter in this role is called an escort interpreter or an escorting interpreter. This is liaison interpreting.

Public sector[link]

Also known as community interpreting, is the type of interpreting occurring in fields such as legal, health, and local government, social, housing, environmental health, education, and welfare services. In community interpreting, factors exist which determine and affect language and communication production, such as speech's emotional content, hostile or polarized social surroundings, its created stress, the power relationships among participants, and the interpreter's degree of responsibility — in many cases more than extreme; in some cases, even the life of the other person depends upon the interpreter's work.

Medical[link]

Medical interpreting is a subset of public service interpreting, consisting of communication among medical personnel and the patient and his or her family or among medical personnels speaking different languages, facilitated by an interpreter, usually formally educated and qualified to provide such interpretation services. In some situations medical employees who are multilingual may participate part-time as members of internal language banks.^[5] The medical interpreter must have a strong knowledge of medicine, common medical procedures, the patient interview, the medical examination processes, ethics, and the daily workings of the hospital or clinic where he or she works, in order to effectively serve both the patient and the medical personnel. Moreover, and very important, medical interpreters often are cultural liaisons for people (regardless of language) who are unfamiliar with or uncomfortable in hospital, clinical, or medical settings. Situation in China: There's no mandatory certificate for medical interpreters in China as of 2012. Most interpretation in hospitals in China is done by doctors, who are proficient in both Chinese and English(mostly) in his/her specialty.They interpret more in academic setings than for communications between doctors and patients. When a patient needs English service in a Chinese hospital, more often than not he/she will be directed to a staff member in the hospital, who are recognized by his/her colleagues as proficient in English. The actual quality of such service for patients or medical translation for communications between doctors speaking different languages is unknown by the interpreting community as interpreters who lack medical background rarely receive reputation for medical translation in the medical community.

Sign language[link]

Two Sign Language interpreters working for a school.

When a hearing person speaks, an interpreter will render the speaker's meaning into the sign language used by the deaf party. When a deaf person signs, an interpreter will render the meaning expressed in the signs into the spoken language for the hearing party, which is sometimes referred to as voice interpreting or voicing. This may be performed either as simultaneous or consecutive interpreting. Skilled sign language interpreters will position themselves in a room or space that allows them both to be seen by deaf participants and heard by hearing participants clearly and to see and hear participants clearly. In some circumstances, an interpreter may interpret from one sign language into an alternate sign language.

Deaf people also work as interpreters. They team with hearing counterparts to provide interpretation for deaf individuals who may not share the standard sign language used in that country, who have minimal language skills, are developmentally delayed or have other mental and/or physical disabilities which make communication a unique challenge. In other cases the hearing interpreter may interpret in one language then the Deaf interpreter might interpret it into another form of that language (pidgins). They also relay information from one medium of language into another — for example, when a person is signing visually, the deaf interpreter could be hired to copy those signs into a deaf-blind person's hand and add visual information.

In the United States, Sign Language Interpreters have National and State level associations. The Registry of Interpreters for the Deaf (RID), a non-profit national certifying body. In addition to training requirements and stringent certification testing, the RID members must abide by a Code of Professional Conduct, Grievance Process and Continuing Education Requirement.

In Europe each country has their own national association of sign language interpreters. Some countries have more than one national association due to regional or language differences. The European Forum of Sign Language Interpreters (efsli) is the umbrella organization of sign language interpreters in Europe.

Sign Language Interpreters can be found in all types of interpreting situations, as listed in this article. Most interpreters have had formal training, in an Interpreter Training Program (ITP). ITP lengths vary, being available as a two-year or four-year degree or certificate. There are graduate programs available as well.

Media[link]

By its very nature, media interpreting has to be conducted in the simultaneous mode. It is provided particularly for live television coverages such as press conferences, live or taped interviews with political figures, musicians, artists, sportsmen or people from the business circle. In this type of interpreting, the interpreter has to sit in a sound-proof booth where ideally he/she can see the speakers on a monitor and the set. All equipment should be checked before recording begins. In particular, satellite connections have to be double-checked to ensure that the interpreter's voice is not sent back and the interpreter gets to hear only one channel at a time. In the case of interviews recorded outside the studio and some current affairs programme, the interpreter interprets what he or she hears on a TV monitor. Background noise can be a serious problem. The interpreter working for the media has to sound as slick and confident as a television presenter.

Media interpreting has gained more visibility and presence especially after the Gulf War. Television channels have begun to hire staff simultaneous interpreters. The interpreter renders the press conferences, telephone beepers, interviews and similar live coverage for the viewers. It is more stressful than other types of interpreting as the interpreter has to deal with a wide range of technical problems coupled with the control room's hassle and wrangling during live coverage.

Modalities[link]

Interpreting services can be delivered in multiple modalities. The most common modality through which interpreting services are provided is on-site interpreting.

On-site[link]

Also called "in-person interpreting," this delivery method requires the interpreter to be physically present in order for the interpretation to take place. In on-site interpreting settings, all of the parties who wish to speak to one another are usually located in the same place. This is by far the most common modality used for most public and social service settings.

Telephone[link]

Also referred to as "over-the-phone interpreting," "telephonic interpreting," and "tele-interpreting," telephone interpreting enables the interpreter to deliver interpretation via telephone. The interpreter is added to a conference call. Telephone interpreting may be used in place of on-site interpreting in some cases, especially when no on-site interpreter is readily available at the location where services are needed. However, telephone interpreting is more commonly used for situations in which all parties who wish to communicate are already speaking to one another via telephone (e.g. applications for insurance or credit cards that are taken over the phone, inquiries from consumers to businesses that take place via telephone, etc.)

Video[link]

A Video Interpreter sign used at locations offering VRS or VRI services

Interpretation services via Video Remote Interpreting (VRI) or a Video Relay Service (VRS) are useful where one of the parties is deaf, hard-of-hearing or speech-impaired (mute). In such cases the interpretation flow is normally within the same principal language, such as French Sign Language (FSL) to spoken French, Spanish Sign Language (SSL) to spoken Spanish, British Sign Language (BSL) to spoken English, and American Sign Language (ASL) also to spoken English (since BSL and ASL are completely distinct), etc.... Multilingual sign language interpreters, who can also translate as well across principal languages (such as to and from SSL, to and from spoken English), are also available, albeit less frequently. Such activities involve considerable effort on the part of the translator, since sign languages are distinct natural languages with their own construction and syntax, different from the aural version of the same principal language.

With video interpreting, sign language interpreters work remotely with live video and audio feeds, so that the interpreter can see the deaf or mute party, converse with the hearing party and vice versa. Much like telephone interpreting, video interpreting can be used for situations in which no on-site interpreters are available. However, video interpreting cannot be used for situations in which all parties are speaking via telephone alone. VRI and VRS interpretation requires all parties to have the necessary equipment. Some advanced equipment enables interpreters to control the video camera, in order to zoom in and out, and to point the camera toward the party that is signing.

Venues[link]

The majority of professional full-time conference interpreters work for phone interpreting agencies, health care institutions, courts, school systems and international organisations like the United Nations, the European Union, or the African Union.

The world's largest employer of interpreters is currently the European Commission,^[6] which employs hundreds of staff and freelance interpreters working into the official languages of the European Union. The European Union's other institutions (the European Parliament and the European Court of Justice) have smaller interpreting services.

The United Nations employs interpreters at almost all its sites throughout the world. Because it has only six official languages, however, it is a smaller employer than the European Union.

Interpreters may also work as freelance operators in their local, regional and national communities, or may take on contract work under an interpreting business or service. They would typically take on work as described above.

The U.S. military in Iraq and Afghanistan employ hundreds of interpreters to assist with its communications with the local population.

See also[link]

• Chartered Institute of Linguists
• Institute of Translation & Interpreting
• Inttranet
• Machine translation
• New Zealand Society of Translators and Interpreters
• Registry of Interpreters for the Deaf
• Telephone interpreting
• Translation studies
• United Nations Interpretation Service

Notes[link]

References[link]

• Bassnett, Susan (1990). Translation studies. London & New York: Routledge. ISBN 0-415-06528-3. 
• Bertone, Laura (2006) The Hidden Side of Babel: Unveiling Cognition, Intelligence and Sense, ISBN 987-21049-1-3 Evolucion, Organización intercultural
• Chuzhakin, Andrei (2007) "Applied Theory of Interpretation and Note-Taking", "Mir Perevoda 1 to 7", Ustny Perevod, Posledovatelny Perevod, Ace Perevoda Mir Perevoda.
• Gillies, Andrew (2005) Note-taking for Consecutive Interpreting, ISBN 1-900650-82-7.
• Jones, Roderick (1998) Conference Interpreting Explained, ISBN 1-900650-57-6.
• Rozan, Jean-François (1956) La Prise de Notes en Interprétation Consécutive, ISBN 2-8257-0053-3.
• Seleskovitch, Danica (1968) L'interprète dans les conférences internationales, Cahiers Champollion.
• Taylor-Bouladon, Valerie (2007) Conference Interpreting — Principles and Practice, 2nd Edition ISBN 1-4196-6069-1.

External links[link]

Wikimedia Commons has media related to: Interpretation

• The difference between consecutive and simultaneous interpreting
• What is conference interpreting?

Computer software, or just software, is a collection of computer programs and related data that provides the instructions for telling a computer what to do and how to do it. Software refers to one or more computer programs and data held in the storage of the computer for some purposes. In other words, software is a set of programs, procedures, algorithms and its documentation concerned with the operation of a data processing system. Program software performs the function of the program it implements, either by directly providing instructions to the computer hardware or by serving as input to another piece of software. The term was coined to contrast to the old term hardware (meaning physical devices). In contrast to hardware, software "cannot be touched".^[1] Software is also sometimes used in a more narrow sense, meaning application software only. Sometimes the term includes data that has not traditionally been associated with computers, such as film, tapes, and records.^[2]

Computer software is so called to distinguish it from computer hardware, which encompasses the physical interconnections and devices required to store and execute (or run) the software. At the lowest level, executable code consists of machine language instructions specific to an individual processor. A machine language consists of groups of binary values signifying processor instructions that change the state of the computer from its preceding state. Programs are an ordered sequence of instructions for changing the state of the computer in a particular sequence. It is usually written in high-level programming languages that are easier and more efficient for humans to use (closer to natural language) than machine language. High-level languages are compiled or interpreted into machine language object code. Software may also be written in an assembly language, essentially, a mnemonic representation of a machine language using a natural language alphabet. Assembly language must be assembled into object code via an assembler.

History[link]

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The first theory about software was proposed by Alan Turing in his 1935 essay Computable numbers with an application to the Entscheidungsproblem (Decision problem).^[3] The term "software" was first used in print by John W. Tukey in 1958.^[4] Colloquially, the term is often used to mean application software. In computer science and software engineering, software is all information processed by computer system, programs and data.^[4] The academic fields studying software are computer science and software engineering.

The history of computer software is most often traced back to the first software bug in 1946^{[citation needed]}. As more and more programs enter the realm of firmware, and the hardware itself becomes smaller, cheaper and faster as predicted by Moore's law, elements of computing first considered to be software, join the ranks of hardware. Most hardware companies today have more software programmers on the payroll than hardware designers^{[citation needed]}, since software tools have automated many tasks of Printed circuit board engineers. Just like the Auto industry, the Software industry has grown from a few visionaries operating out of their garage with prototypes. Steve Jobs and Bill Gates were the Henry Ford and Louis Chevrolet of their times^{[citation needed]}, who capitalized on ideas already commonly known before they started in the business. In the case of Software development, this moment is generally agreed to be the publication in the 1980s of the specifications for the IBM Personal Computer published by IBM employee Philip Don Estridge. Today his move would be seen as a type of crowd-sourcing.

Until that time, software was bundled with the hardware by Original equipment manufacturers (OEMs) such as Data General, Digital Equipment and IBM^{[citation needed]}. When a customer bought a minicomputer, at that time the smallest computer on the market, the computer did not come with Pre-installed software, but needed to be installed by engineers employed by the OEM. Computer hardware companies not only bundled their software, they also placed demands on the location of the hardware in a refrigerated space called a computer room. Most companies had their software on the books for 0 dollars, unable to claim it as an asset (this is similar to financing of popular music in those days). When Data General introduced the Data General Nova, a company called Digidyne wanted to use its RDOS operating system on its own hardware clone. Data General refused to license their software (which was hard to do, since it was on the books as a free asset), and claimed their "bundling rights". The Supreme Court set a precedent called Digidyne v. Data General in 1985. The Supreme Court let a 9th circuit decision stand, and Data General was eventually forced into licensing the Operating System software because it was ruled that restricting the license to only DG hardware was an illegal tying arrangement.^[5] Soon after, IBM 'published' its DOS source for free,^{[citation needed]} and Microsoft was born. Unable to sustain the loss from lawyer's fees, Data General ended up being taken over by EMC Corporation. The Supreme Court decision made it possible to value software, and also purchase Software patents. The move by IBM was almost a protest at the time. Few in the industry believed that anyone would profit from it other than IBM (through free publicity). Microsoft and Apple were able to thus cash in on 'soft' products. It is hard to imagine today that people once felt that software was worthless without a machine. There are many successful companies today that sell only software products, though there are still many common software licensing problems due to the complexity of designs and poor documentation, leading to patent trolls.

With open software specifications and the possibility of software licensing, new opportunities arose for software tools that then became the de facto standard, such as DOS for operating systems, but also various proprietary word processing and spreadsheet programs. In a similar growth pattern, proprietary development methods became standard Software development methodology.

Types of software[link]

A layer structure showing where the operating system software and application software are situated while running on a typical desktop computer

Software includes all the various forms and roles that digitally stored data may have and play in a computer (or similar system), regardless of whether the data is used as code for a CPU, or other interpreter, or whether it represents other kinds of information. Software thus encompasses a wide array of products that may be developed using different techniques such as ordinary programming languages, scripting languages, microcode, or an FPGA configuration.

The types of software include web pages developed in languages and frameworks like HTML, PHP, Perl, JSP, ASP.NET, XML, and desktop applications like OpenOffice.org, Microsoft Word developed in languages like C, C++, Objective-C, Java, C#, or Smalltalk. Application software usually runs on an underlying software operating systems such as Linux or Microsoft Windows. Software (or firmware) is also used in video games and for the configurable parts of the logic systems of automobiles, televisions, and other consumer electronics.

Practical computer systems divide software systems into three major classes^{[citation needed]}: system software, programming software and application software, although the distinction is arbitrary, and often blurred.

System software[link]

System software is computer software designed to operate the computer hardware to provide basic functionality and to provide a platform for running application software.^[6][7] System software includes device drivers, operating systems, servers, utilities, and window systems.

System software is responsible for managing a variety of independent hardware components, so that they can work together harmoniously. Its purpose is to unburden the application software programmer from the often complex details of the particular computer being used, including such accessories as communications devices, printers, device readers, displays and keyboards, and also to partition the computer's resources such as memory and processor time in a safe and stable manner.

Programming software[link]

Programming software include tools in the form of programs or applications that software developers use to create, debug, maintain, or otherwise support other programs and applications. The term usually refers to relatively simple programs such as compilers, debuggers, interpreters, linkers, and text editors, that can be combined together to accomplish a task, much as one might use multiple hand tools to fix a physical object. Programming tools are intended to assist a programmer in writing computer programs, and they may be combined in an integrated development environment (IDE) to more easily manage all of these functions.

Application software[link]

Application software is developed to perform in any task that benefits from computation. It is a set of programs that allows the computer to perform a specific data processing job for the user. It is a broad category, and encompasses software of many kinds, including the internet browser being used to display this page. This category includes:

• Business software
• Computer-aided design
• Databases
• Decision-making software
• Educational software
• Image editing
• Industrial automation
• Mathematical software
• Medical software
• Molecular modeling software
• Quantum chemistry and solid state physics software
• Simulation software
• Spreadsheets
• Telecommunications (i.e., the Internet and everything that flows on it)
• Video editing software
• Video games
• Word processing

Software topics[link]

Architecture[link]

Users often see things differently than programmers. People who use modern general purpose computers (as opposed to embedded systems, analog computers and supercomputers) usually see three layers of software performing a variety of tasks: platform, application, and user software.

• Platform software: Platform includes the firmware, device drivers, an operating system, and typically a graphical user interface which, in total, allow a user to interact with the computer and its peripherals (associated equipment). Platform software often comes bundled with the computer. On a PC you will usually have the ability to change the platform software.
• Application software: Application software or Applications are what most people think of when they think of software. Typical examples include office suites and video games. Application software is often purchased separately from computer hardware. Sometimes applications are bundled with the computer, but that does not change the fact that they run as independent applications. Applications are usually independent programs from the operating system, though they are often tailored for specific platforms. Most users think of compilers, databases, and other "system software" as applications.
• User-written software: End-user development tailors systems to meet users' specific needs. User software include spreadsheet templates and word processor templates. Even email filters are a kind of user software. Users create this software themselves and often overlook how important it is. Depending on how competently the user-written software has been integrated into default application packages, many users may not be aware of the distinction between the original packages, and what has been added by co-workers.

Documentation[link]

Most software has software documentation so that the end user can understand the program, what it does, and how to use it. Without clear documentation, software can be hard to use—especially if it is very specialized and relatively complex like Photoshop or AutoCAD.

Developer documentation may also exist, either with the code as comments and/or as separate files, detailing how the programs works and can be modified.

Library[link]

An executable is almost always not sufficiently complete for direct execution. Software libraries include collections of functions and functionality that may be embedded in other applications. Operating systems include many standard Software libraries, and applications are often distributed with their own libraries.

Standard[link]

Since software can be designed using many different programming languages and in many different operating systems and operating environments, software standard is needed so that different software can understand and exchange information between each other. For instance, an email sent from a Microsoft Outlook should be readable from Yahoo! Mail and vice versa.

Execution[link]

Computer software has to be "loaded" into the computer's storage (such as the hard drive or memory). Once the software has loaded, the computer is able to execute the software. This involves passing instructions from the application software, through the system software, to the hardware which ultimately receives the instruction as machine code. Each instruction causes the computer to carry out an operation – moving data, carrying out a computation, or altering the control flow of instructions.

Data movement is typically from one place in memory to another. Sometimes it involves moving data between memory and registers which enable high-speed data access in the CPU. Moving data, especially large amounts of it, can be costly. So, this is sometimes avoided by using "pointers" to data instead. Computations include simple operations such as incrementing the value of a variable data element. More complex computations may involve many operations and data elements together.

Quality and reliability[link]

Software quality is very important, especially for commercial and system software like Microsoft Office, Microsoft Windows and Linux. If software is faulty (buggy), it can delete a person's work, crash the computer and do other unexpected things. Faults and errors are called "bugs." Many bugs are discovered and eliminated (debugged) through software testing. However, software testing rarely – if ever – eliminates every bug; some programmers say that "every program has at least one more bug" (Lubarsky's Law). All major software companies, such as Microsoft, Novell and Sun Microsystems, have their own software testing departments with the specific goal of just testing. Software can be tested through unit testing, regression testing and other methods, which are done manually, or most commonly, automatically, since the amount of code to be tested can be quite large. For instance, NASA has extremely rigorous software testing procedures for many operating systems and communication functions. Many NASA based operations interact and identify each other through command programs called software. This enables many people who work at NASA to check and evaluate functional systems overall. Programs containing command software enable hardware engineering and system operations to function much easier together.

License[link]

The software's license gives the user the right to use the software in the licensed environment. Some software comes with the license when purchased off the shelf, or an OEM license when bundled with hardware. Other software comes with a free software license, granting the recipient the rights to modify and redistribute the software. Software can also be in the form of freeware or shareware.

Patents[link]

Software can be patented in some but not all countries; however, software patents can be controversial in the software industry with many people holding different views about it. The controversy over software patents is about specific algorithms or techniques that the software contains, which may not be duplicated by others and considered intellectual property and copyright infringement depending on the severity.

Design and implementation[link]

Design and implementation of software varies depending on the complexity of the software. For instance, design and creation of Microsoft Word software will take much more time than designing and developing Microsoft Notepad because of the difference in functionalities in each one.

Software is usually designed and created (coded/written/programmed) in integrated development environments (IDE) like Eclipse, Emacs and Microsoft Visual Studio that can simplify the process and compile the program. As noted in different section, software is usually created on top of existing software and the application programming interface (API) that the underlying software provides like GTK+, JavaBeans or Swing. Libraries (APIs) are categorized for different purposes. For instance, JavaBeans library is used for designing enterprise applications, Windows Forms library is used for designing graphical user interface (GUI) applications like Microsoft Word, and Windows Communication Foundation is used for designing web services. Underlying computer programming concepts like quicksort, hashtable, array, and binary tree can be useful to creating software. When a program is designed, it relies on the API. For instance, if a user is designing a Microsoft Windows desktop application, he/she might use the .NET Windows Forms library to design the desktop application and call its APIs like Form1.Close() and Form1.Show()^[8] to close or open the application and write the additional operations him/herself that it need to have. Without these APIs, the programmer needs to write these APIs him/herself. Companies like Sun Microsystems, Novell, and Microsoft provide their own APIs so that many applications are written using their software libraries that usually have numerous APIs in them.

Computer software has special economic characteristics that make its design, creation, and distribution different from most other economic goods.^{[specify][9][10]}

A person who creates software is called a programmer, software engineer, software developer, or code monkey, terms that all have a similar meaning.

Industry and organizations[link]

A great variety of software companies and programmers in the world comprise a software industry. Software can be quite a profitable industry: Bill Gates, the founder of Microsoft was the richest person in the world in 2009 largely by selling the Microsoft Windows and Microsoft Office software products. The same goes for Larry Ellison, largely through his Oracle database software. Through time the software industry has become increasingly specialized.

Non-profit software organizations include the Free Software Foundation, GNU Project and Mozilla Foundation. Software standard organizations like the W3C, IETF develop software standards so that most software can interoperate through standards such as XML, HTML, HTTP or FTP.

Other well-known large software companies include Novell, SAP, Symantec, Adobe Systems, and Corel, while small companies often provide innovation.

See also[link]

	Software portal
	Free software portal

• List of software
• Personal computer hardware

References[link]

External links[link]

Wikimedia Commons has media related to: Software

Look up software in Wiktionary, the free dictionary.

• Software Wikia

Computer

A computer is a general purpose device which can be programmed to carry out a finite set of arithmetic or logical operations. Since a sequence of operations can be readily changed, the computer can solve more than one kind of problem. The essential point of a computer is to implement an idea, the terms of which are satisfied by Alan Turing's Universal Turing machine.

Conventionally, a computer consists of at least one processing element and some form of memory. The processing element carries out arithmetic and logic operations, and a sequencing and control unit that can change the order of operations based on stored information. Peripheral devices allow information to be retrieved from an external source, and the result of operations saved.

A computer's processing unit executes a series of instructions that make it read, manipulate and then store data. Conditional instructions change the sequence of instructions as a function of the current state of the machine or its environment.

In order to interact with such a machine, programmers and engineers developed the concept of a user interface in order to accept input from humans and return results for human consumption.

The first electronic digital computers were developed between 1940 and 1945 in the United Kingdom and United States. Originally, they were the size of a large room, consuming as much power as several hundred modern personal computers (PCs).^[1] In this era mechanical analog computers were used for military applications.

Modern computers based on integrated circuits are millions to billions of times more capable than the early machines, and occupy a fraction of the space.^[2] Simple computers are small enough to fit into mobile devices, and mobile computers can be powered by small batteries. Personal computers in their various forms are icons of the Information Age and are what most people think of as "computers". However, the embedded computers found in many devices from mp3 players to fighter aircraft and from toys to industrial robots are the most numerous.

History of computing[link]

The Jacquard loom, on display at the Museum of Science and Industry in Manchester, England, was one of the first programmable devices.

The first use of the word "computer" was recorded in 1613, referring to a person who carried out calculations, or computations, and the word continued with the same meaning until the middle of the 20th century. From the end of the 19th century the word began to take on its more familiar meaning, a machine that carries out computations.^[3]

Limited-function early computers[link]

The history of the modern computer begins with two separate technologies, automated calculation and programmability, but no single device can be identified as the earliest computer, partly because of the inconsistent application of that term. A few devices are worth mentioning though, like some mechanical aids to computing, which were very successful and survived for centuries until the advent of the electronic calculator, like the Sumerian abacus, designed around 2500 BC^[4] of which a descendant won a speed competition against a modern desk calculating machine in Japan in 1946,^[5] the slide rules, invented in the 1620s, which were carried on five Apollo space missions, including to the moon^[6] and arguably the astrolabe and the Antikythera mechanism, an ancient astronomical computer built by the Greeks around 80 BC.^[7] The Greek mathematician Hero of Alexandria (c. 10–70 AD) built a mechanical theater which performed a play lasting 10 minutes and was operated by a complex system of ropes and drums that might be considered to be a means of deciding which parts of the mechanism performed which actions and when.^[8] This is the essence of programmability.

Around the end of the 10th century, the French monk Gerbert d'Aurillac brought back from Spain the drawings of a machine invented by the Moors that answered either Yes or No to the questions it was asked.^[9] Again in the 13th century, the monks Albertus Magnus and Roger Bacon built talking androids without any further development (Albertus Magnus complained that he had wasted forty years of his life when Thomas Aquinas, terrified by his machine, destroyed it).^[10]

In 1642, the Renaissance saw the invention of the mechanical calculator,^[11] a device that could perform all four arithmetic operations without relying on human intelligence.^[12] The mechanical calculator was at the root of the development of computers in two separate ways. Initially, it was in trying to develop more powerful and more flexible calculators^[13] that the computer was first theorized by Charles Babbage^[14][15] and then developed.^[16] Secondly, development of a low-cost electronic calculator, successor to the mechanical calculator, resulted in the development by Intel^[17] of the first commercially available microprocessor integrated circuit.

First general-purpose computers[link]

In 1801, Joseph Marie Jacquard made an improvement to the textile loom by introducing a series of punched paper cards as a template which allowed 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.

The Most Famous Image in the Early History of Computing^[18]

This portrait of Jacquard was woven in silk on a Jacquard loom and required 24,000 punched cards to create (1839). It was only produced to order. Charles Babbage owned one of these portraits ; it inspired him in using perforated cards in his analytical engine^[19]

The Zuse Z3, 1941, considered the world's first working programmable, fully automatic computing machine.

It was the fusion of automatic calculation with programmability that produced the first recognizable computers. In 1837, Charles Babbage was the first to conceptualize and design a fully programmable mechanical computer, his analytical engine.^[20] Limited finances and Babbage's inability to resist tinkering with the design meant that the device was never completed—nevertheless his son, Henry Babbage, completed a simplified version of the analytical engine's computing unit (the mill) in 1888. He gave a successful demonstration of its use in computing tables in 1906. This machine was given to the Science museum in South Kensington in 1910.

In the late 1880s, Herman Hollerith invented the recording of data on a machine-readable medium. Earlier uses of machine-readable media had been for control, not data. "After some initial trials with paper tape, he settled on punched cards ..."^[21] To process these punched cards he invented the tabulator, and the keypunch machines. These three inventions were the foundation of the modern information processing industry. Large-scale automated data processing of punched cards was performed for the 1890 United States Census by Hollerith's company, which later became the core of IBM. By the end of the 19th century a number of ideas and technologies, that would later prove useful in the realization of practical computers, had begun to appear: Boolean algebra, the vacuum tube (thermionic valve), punched cards and tape, 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.

Alan Turing is widely regarded as the father of modern computer science. In 1936 Turing provided an influential formalisation of the concept of the algorithm and computation with the Turing machine, providing a blueprint for the electronic digital computer.^[22] Of his role in the creation of the modern computer, Time magazine in naming Turing one of the 100 most influential people of the 20th century, states: "The fact remains that everyone who taps at a keyboard, opening a spreadsheet or a word-processing program, is working on an incarnation of a Turing machine".^[22]

The ENIAC, which became operational in 1946, is considered to be the first general-purpose electronic computer.

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

The Atanasoff–Berry Computer (ABC) was the world's first electronic digital computer, albeit not programmable.^[23] Atanasoff is considered to be one of the fathers of the computer.^[24] Conceived in 1937 by Iowa State College physics professor John Atanasoff, and built with the assistance of graduate student Clifford Berry,^[25] the machine was not programmable, being designed only to solve systems of linear equations. The computer did employ parallel computation. A 1973 court ruling in a patent dispute found that the patent for the 1946 ENIAC computer derived from the Atanasoff–Berry Computer.

The first program-controlled computer was invented by Konrad Zuse, who built the Z3, an electromechanical computing machine, in 1941.^[26] The first programmable electronic computer was the Colossus, built in 1943 by Tommy Flowers.

George Stibitz is internationally recognized as a father of the modern digital computer. While working at Bell Labs in November 1937, Stibitz invented and built a relay-based calculator he dubbed the "Model K" (for "kitchen table", on which he had assembled it), which was the first to use binary circuits to perform an arithmetic operation. Later models added greater sophistication including complex arithmetic and programmability.^[27]

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:

• Konrad Zuse's electromechanical "Z machines". The Z3 (1941) was the first working machine featuring binary arithmetic, including floating point arithmetic and a measure of programmability. In 1998 the Z3 was proved to be Turing complete, therefore being the world's first operational computer.^[28]
• The non-programmable Atanasoff–Berry Computer (commenced in 1937, completed in 1941) which used vacuum tube based computation, binary numbers, and regenerative capacitor memory. The use of regenerative memory allowed it to be much more compact than its peers (being approximately the size of a large desk or workbench), since intermediate results could be stored and then fed back into the same set of computation elements.
• The secret British Colossus computers (1943),^[29] which had limited programmability but demonstrated that a device using thousands of tubes could be reasonably reliable and electronically reprogrammable. It was used for breaking German wartime codes.
• The Harvard Mark I (1944), a large-scale electromechanical computer with limited programmability.^[30]
• The U.S. Army's Ballistic Research Laboratory ENIAC (1946), which used decimal arithmetic and is sometimes called the first general purpose electronic computer (since Konrad Zuse's Z3 of 1941 used electromagnets instead of electronics). Initially, however, ENIAC had an inflexible architecture which essentially required rewiring to change its programming.

Stored-program architecture[link]

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, distributed in 1945. A number of projects to develop computers based on the stored-program architecture commenced around this time, the first of which was completed in 1948 at the University of Manchester in England, the Manchester Small-Scale Experimental Machine (SSEM or "Baby"). The Electronic Delay Storage Automatic Calculator (EDSAC), completed a year after the SSEM at Cambridge University, was the first practical, non-experimental implementation of the stored-program design and was put to use immediately for research work at the university. 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. 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.

Die of an Intel 80486DX2 microprocessor (actual size: 12×6.75 mm) in its packaging.

Beginning in the 1950s, Soviet scientists Sergei Sobolev and Nikolay Brusentsov conducted research on ternary computers, devices that operated on a base three numbering system of −1, 0, and 1 rather than the conventional binary numbering system upon which most computers are based. They designed the Setun, a functional ternary computer, at Moscow State University. The device was put into limited production in the Soviet Union, but supplanted by the more common binary architecture.

Semiconductors and microprocessors[link]

Computers using vacuum tubes as their electronic elements were in use throughout the 1950s, but by the 1960s had been largely replaced by semiconductor transistor-based machines, which were smaller, faster, cheaper to produce, required less power, and were more reliable. The first transistorised computer was demonstrated at the University of Manchester in 1953.^[31] In the 1970s, integrated circuit technology and the subsequent creation of microprocessors, such as the Intel 4004, further decreased size and cost and further increased speed and reliability of computers. By the late 1970s, many products such as video recorders contained dedicated computers called microcontrollers, and they started to appear as a replacement to mechanical controls in domestic appliances such as washing machines. The 1980s witnessed home computers and the now ubiquitous personal computer. With the evolution of the Internet, personal computers are becoming as common as the television and the telephone in the household^{[citation needed]}.

Modern smartphones are fully programmable computers in their own right, and as of 2009 may well be the most common form of such computers in existence^{[citation needed]}.

Programs[link]

Replica of the Small-Scale Experimental Machine (SSEM), the world's first stored-program computer, at the Museum of Science and Industry in Manchester, England

The defining feature of modern computers which distinguishes them from all other machines is that they can be programmed. That is to say that some type of instructions (the program) can be given to the computer, and it will process them. While some computers may have strange concepts "instructions" and "output" (see quantum computing), modern computers based on the von Neumann architecture often have machine code in the form of an imperative programming language.

In practical terms, a computer program may be just a few instructions or extend to many millions of instructions, as do the programs for word processors and web browsers for example. A typical modern computer can execute billions of instructions per second (gigaflops) and rarely makes a mistake over many years of operation. Large computer programs consisting of several million instructions may take teams of programmers years to write, and due to the complexity of the task almost certainly contain errors.

Stored program architecture[link]

This section applies to most common RAM machine-based computers.

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 No. 0, sum     ; set sum to 0
      mov No. 1, num     ; set num to 1
loop: add num, sum    ; add num to sum
      add No. 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.^[32]

Bugs[link]

The actual first computer bug, a moth found trapped on a relay of the Harvard Mark II computer

Errors in computer programs are called "bugs". They 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 or the entire system to "hang" – become unresponsive to input such as mouse clicks or keystrokes – to completely fail, or to crash. Otherwise benign bugs may sometimes be harnessed for malicious intent by an unscrupulous user writing an exploit, code designed to take advantage of a bug and disrupt a computer'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.^[33]

Rear Admiral Grace Hopper is credited for having first used the term "bugs" in computing after a dead moth was found shorting a relay in the Harvard Mark II computer in September 1947.^[34]

Machine code[link]

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 these instructions) can be represented as lists of numbers and can themselves be manipulated inside the computer in the same way as 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 while this technique was used with many early computers,^[35] it is extremely tedious and potentially error-prone 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.

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.

Programming language[link]

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 code 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.

Low-level languages[link]

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.^[36]

Higher-level languages[link]

Though considerably easier than in machine language, writing long programs in assembly language is often difficult and is also error prone. Therefore, most practical programs are written in more abstract high-level programming languages that are able to express the needs of the 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.^[37] High level languages are less related to the workings of the target computer than assembly language, and more related to the language and structure of the problem(s) to be solved by the final program. It is therefore often 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.

Program design[link]

Program design of small programs is relatively simple and involves the analysis of the problem, collection of inputs, using the programming constructs within languages, devising or using established procedures and algorithms, providing data for output devices and solutions to the problem as applicable. As problems become larger and more complex, features such as subprograms, modules, formal documentation, and new paradigms such as object-oriented programming are encountered. Large programs involving thousands of line of code and more require formal software methodologies. The task of developing large software systems presents a significant intellectual challenge. Producing software with an acceptably high reliability within a predictable schedule and budget has historically been difficult; the academic and professional discipline of software engineering concentrates specifically on this challenge.

Components[link]

A general purpose computer has four main components: the arithmetic 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.

Inside each of these parts are thousands to trillions of small electrical circuits which can be turned off or on by means of an electronic switch. Each circuit represents a bit (binary digit) of information so that when the circuit is on it represents a "1", and when off it represents a "0" (in positive logic representation). The circuits are arranged in logic gates so that one or more of the circuits may control the state of one or more of the other circuits.

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[link]

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

The control unit (often called a control system or central controller) manages the computer's various components; it reads and interprets (decodes) the program instructions, transforming them into a series of control signals which activate other parts of the computer.^[38] 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.^[39]

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).

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, which runs a microcode program that causes all of these events to happen.

Arithmetic logic unit (ALU)[link]

The ALU is capable of performing two classes of operations: arithmetic and logic.^[40]

The set of arithmetic operations that a particular ALU supports may be limited to addition and subtraction, or might include multiplication, division, trigonometry functions such as 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 for creating complicated conditional statements and processing boolean logic.

Superscalar computers may contain multiple ALUs, allowing them to process several instructions simultaneously.^[41] Graphics processors and computers with SIMD and MIMD features often contain ALUs that can perform arithmetic on vectors and matrices.

Memory[link]

Magnetic core memory was the computer memory of choice throughout the 1960s, until it was 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 the software's responsibility 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 (2^8 = 256); 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 if it can be represented numerically. 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. As 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, therefore 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 are erased when the power to the computer is turned off, but 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 required software may be stored in ROM. Software 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, as it retains its data when turned off but is also rewritable. It is typically much slower than conventional ROM and RAM however, so its use is restricted to applications where high speed is unnecessary.^[42]

In more sophisticated computers there may be one or more RAM cache memories, which are slower than registers but faster than main memory. Generally computers with this sort of cache are designed to move frequently needed data into the cache automatically, often without the need for any intervention on the programmer's part.

Input/output (I/O)[link]

Hard disk drives are common storage devices used with computers.

I/O is the means by which a computer exchanges information with the outside world.^[43] Devices that provide input or output to the computer are called peripherals.^[44] 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.

I/O devices are often 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.

Multitasking[link]

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 multitasking i.e. having the computer switch rapidly between running each program in turn.^[45]

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.^[46]

Before the era of cheap computers, the principal 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, but 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 simultaneously without unacceptable speed loss.

Multiprocessing[link]

Cray designed many supercomputers that used multiprocessing heavily.

Some computers are designed to distribute their work across several CPUs in a multiprocessing configuration, a technique once employed only in large and powerful machines such as supercomputers, mainframe computers and servers. Multiprocessor and multi-core (multiple CPUs on a single integrated circuit) personal and laptop computers are now widely available, and are being increasingly used 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.^[47] 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.

Networking and the Internet[link]

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 such as Sabre.^[48]

In the 1970s, computer engineers at research institutions throughout the United States began to link their computers together using telecommunications technology. The effort was funded by ARPA (now DARPA), and the computer network that resulted was called the ARPANET.^[49] 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.

Computer architecture paradigms[link]

There are many types of computer architectures:

• Quantum computer vs Chemical computer
• Scalar processor vs Vector processor
• Non-Uniform Memory Access (NUMA) computers
• Register machine vs Stack machine
• Harvard architecture vs von Neumann architecture
• Cellular architecture

The quantum computer architecture holds the most promise to revolutionize computing.^[50]

Logic gates are a common abstraction which can apply to most of the above digital or analog paradigms.

The ability to store and execute lists of instructions called programs makes computers extremely versatile, distinguishing them from calculators. The Church–Turing thesis is a mathematical statement of this versatility: any computer with a minimum capability (being Turing-complete) is, in principle, capable of performing the same tasks that any other computer can perform. Therefore any type of computer (netbook, supercomputer, cellular automaton, etc.) is able to perform the same computational tasks, given enough time and storage capacity.

Misconceptions[link]

A computer does not need to be electronic, nor even have a processor, nor RAM, nor even a hard disk. While popular usage of the word "computer" is synonymous with a personal computer, the definition of a computer is literally "A device that computes, especially a programmable [usually] electronic machine that performs high-speed mathematical or logical operations or that assembles, stores, correlates, or otherwise processes information."^[51] Any device which processes information qualifies as a computer, especially if the processing is purposeful.

Required technology[link]

Historically, computers evolved from mechanical computers and eventually from vacuum tubes to transistors. However, conceptually computational systems as flexible as a personal computer can be built out of almost anything. For example, a computer can be made out of billiard balls (billiard ball computer); an oft-quoted example. More realistically, modern computers are made out of transistors made of photolithographed semiconductors.

There is active research to make computers out of many promising new types of technology, such as optical computers, DNA computers, neural computers, and quantum computers. Most computers are universal, and are able to calculate any computable function, and are limited only by their memory capacity and operating speed. However different designs of computers can give very different performance for particular problems, for example quantum computers can potentially break some modern encryption algorithms by quantum factoring) very quickly.

Further topics[link]

• Glossary of computers

Artificial intelligence[link]

A computer will solve problems in exactly the way it is programmed to, without regard to efficiency, alternative solutions, possible shortcuts, or possible errors in the code. Computer programs that learn and adapt are part of the emerging field of artificial intelligence and machine learning.

Hardware[link]

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[link]

Other hardware topics[link]

Software[link]

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.

Languages[link]

There are thousands of different programming languages—some intended to be general purpose, others useful only for highly specialized applications.

Professions and organizations[link]

As the use of computers has spread throughout society, there are an increasing number of careers involving computers.

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.

See also[link]

Information technology portal

Notes[link]