Encyclopedia of Computer Science and Technology

252 interpreterconsidered to be ISPs, the term is generally applied to acommercial, fee-based service.Typically, a user is given an account that is accessed bylogging in through the operating system’s Internet connectionfacility by supplying a user ID and password. Once connected,the user can run Web browsers, e-mail clients, andother programs that are designed to work with an Internetconnection. Most ISPs now charge flat monthly fees rangingfrom $20 or so for dial-up access to around $40–$60for high-speed cable or DSL connections (see broadband).Some services such as America Online and CompuServeinclude ISP service as part of a package that also includessuch features as software libraries, discussion forums, andinstant messaging. Online services tend to be more expensivethan “no frills” ISP services.Most personal ISP accounts include a small allotment ofserver space that users can use to host their personal Webpages. There are generally extra charges for larger allotmentsof space, for sites that generate high traffic, and forcommercial sites. Business-oriented ISPs typically providea more generous starting allotment along with more extensivetechnical support and more reliable and higher-capacityservers that are managed 24 hours a day.The rapid growth in Internet use in the mid-1990sencouraged many would-be entrepreneurs to start ISPs. However,with so many providers entering the field and with theprice for basic Internet connections falling, it soon becameapparent that the survival prospects for “generic” ISPs wouldbe poor. People entering the business today strive to provideadded-value services such as superior Web page hostingfacilities, hosting blogs or wikis, or to focus on specializedservices for particularly industries (such as real estate).Today’s ISPs also face a variety of legal challenges,including customer privacy vs. the war on terrorism (seeprivacy in the digital age), responsibility for copyrightinfringement (see intellectual property and computing),and possible liability for online defamation, harassment,or worse (see cyberstalking and harassment.)Many earlier versions of the BASIC programming languagewere implemented as interpreters. Since an interpreteronly has to hold one program statement at a time inmemory, it could run on early microcomputers that had onlya few tens of thousands of bytes of system memory. However,interpreters run programs considerably more slowlythan a compiled program would run. One reason is that aninterpreter “throws away” each source code statement afterit interprets it. This means that if a statement runs repeatedly(see loop), it must be re-interpreted each time it runs.A compiler, on the other hand, would create only one setof machine code instructions for the loop and then moveon. Also, because a compiler keeps the entire program inmemory, it can analyze the relationship between multiplestatements and recognize ways to rearrange or substitutethem for greater efficiency.Interpretation can also be used to bridge differencesin hardware platforms. For example, in the UCSD Pascalsystem developed in the 1970s, an interpreter first translatesthe Pascal source code into a standardized “P-code”(pseudocode) for a generic processor called a P-machine. ToFurther ReadingBerkowitz, Howard C. Building Service Provider Networks. NewYork: Wiley, 2002.“Everything You Wanted to Know About Internet Service Providers.”Available online. URL: http://www.ispconsumerguide.com/. Accessed February 6, 2008.“ISP Liability.” BitLaw. Available online. URL: http://www.bitlaw.com/internet/isp.html. Accessed August 8, 2007.Nguyen, John V. Designing ISP Architectures. Upper Saddle River,N.J.: Prentice Hall, 2002.interpreterAn interpreter is a program that analyzes (parses) programmingcommands or statements in a high-level language (seeprogramming languages), creates equivalent executableinstructions in machine code (see assembler) and executesthem. An interpreter differs from a compiler in that the latterconverts the entire program to an executable file ratherthan processing and executing it a statement at a time (seecompiler).An interpreter scans a program code or command statement todetermine what each token (word or symbol) represents. Keywordssuch as PRINT are looked up in a dispatch table that containsinstructions for dealing with that function. Variables arelooked up in a symbol table that gives their current value. Valuesand operators make up expressions that are interpreted to yieldtheir final value. In this case the final value of 15 is given as datato the PRINT routine, which is executed to put the number 15 onthe screen.