Encyclopedia of Computer Science and Technology

compiler 95as graphics file formats) where there are many alternativesin widespread use, most programs will support multipleformats.PortabilityPortability is the ability to adapt software or hardware toa wide variety of platforms (that is, computer systems oroperating systems). Developers want their products to beportable so they can adapt to an often rapidly changingmarketplace. A typical strategy for portability is to choosea language that is in widespread use and a compiler thatis certified as meeting the ANSI or other standard for thelanguage. The program should be written in such a way thatit makes as few assumptions as possible about hardwaredependentmatters such as how data is stored in memory. Itis also sometimes possible to use standard frameworks thatprovide the same functions in several different operatingsystems such as Windows, Macintosh, and UNIX.However, there is a tradeoff: The more “generic” a programis made in order to be portable, the less optimized itwill be for any given hardware or operating environment.The program will also not be able to take advantage of thespecial features of a given operating system, which may putit at a competitive disadvantage compared to the “nativeversion” of a program. (This is particularly true with Windows,given that operating system’s dominance in personalcomputing.)The Internet has in general been a force for portability.The Java language, in particular, is designed to be platformindependent.A Java program is compiled into an intermediatelanguage called byte code, which is interpreted orcompiled by a “virtual machine” program running on eachplatform. Thus, the same Java program should run in abrowser under Windows, Macintosh, or UNIX (see java).for addition, rather than some binary instruction code). Anassembler is essentially a compiler that needs to make onlyrelatively simple translations, because assembly language isstill at a relatively low level.Moving to higher-level languages with relatively English-likestatements makes programming easier and makesprograms easier to read and maintain. However, the taskof translating high-level statements to machine-level codebecomes a more complex multistep process.The Compilation ProcessCompilers are traditionally thought of as having a “frontend” that analyzes the source code (high-level languagestatements) and a “back end” that generates the appropriatelow-level code. The front end processing begins with lexicalanalysis. The compiler scans the source program looking formatches to valid tokens as defined by the language. A tokenis any word or symbol that has meaning in the language,Further ReadingHakuta, Mitsuari, and Masato Ohminami. “A Study of SoftwarePortability Evaluation.” Journal of Systems and Software 38(August 1997): 145–154.Robinson, John. “Delivering on Standards: Balancing Portabilityand Performance.” Available online. URL: http://ipdps.cc.gatech.edu/1999/papers/it2.pdf. Accessed August 11, 2007.“Software Portability Home Page.” Available online. URL: http://www.cs.wvu.edu/~jdm/research/portability/home.html.Accessed June 11, 2007.compilerA compiler is a program that takes as input a programwritten in a source language and produces as output anequivalent program written in another (target) language.Usually the input program is in a high-level language suchas C++ and the output is in assembly language for the targetmachine (see assembler).Compilers are useful because programming directlyin low-level machine instructions (as had to be done withthe first computers) is tedious and prone to errors. Use ofassembly language helps somewhat by allowing substitutionof symbols (variable names) for memory locations andthe use of mnemonic names for operations (such as “add”A parse tree showing how an assignment statement in Pascal canbe broken down into its component parts. Here ID stands for a variablename, or identifier. An expression can be broken all the waydown to a single number.