Encyclopedia of Computer Science and Technology

Forth 201dows and Macintosh usually come with an assortment offonts, and applications can register additional fonts to makethem available to the system.Fonts are often presented as a “family” that includesthe same type design with different attributes such as boldfaceand italic. The spacing of letters could be uniform(monospace) as in the Courier font often used for printingcomputer program code or proportional (as with most textfonts). For proportional fonts the design can include kerning,or the precise fitting together of adjacent letters for amore attractive appearance. Fonts are also described as serifif they have small crossbars on the ends of letters such as atthe end of the crossbar on a T in the Times Roman font.Other fonts such as Arial lack the tiny bars and are calledsans serif (without serif).There are two basic ways to store font data in thecomputer system. Bitmapped fonts store the actual patternof tiny dots that make up the letters in the font. Thishas the advantage of allowing each letter in each pointsize to be precisely designed. The primary disadvantageis the amount of memory and system resources requiredto store a font in many point sizes. In practice, this considerationresults in only a relatively few fonts and sizesbeing available.The alternative, an outline or vector font uses a “pagedescription language” such as Adobe PostScript or TrueTypeto provide graphics commands that specify the drawingof each letter in a font. When the user specifies a font,the text is rendered by processing the graphics commandsin an interpreter. Since the actual bitmap doesn’t need to bestored and all point sizes of a font can be generated fromone description, outline fonts save memory and disk space(although they require additional processor resources forrendering). While sophisticated scaling techniques are usedto maintain a pleasing appearance as the font size changes,outline fonts will not look as polished as bitmapped fontsthat are hand-designed at each point size. (For use of fontssee typography, computerized.)Strictly speaking, a particular type design is called a typeface, anda font is a rendering of a typeface with specified characteristicssuch as height in points and possibly width or pitch in charactersper inch. Thus there are usually many fonts for each typeface.Further ReadingAaron, B. TrueType Display Fonts. San Francisco: Sybex, 1993.Adobe Systems. Adobe Type Library Reference Book. 2nd ed. MountainView, Calif.: Adobe Press, 2003.Headley, Gwyn. The Encyclopedia of Fonts. London: Cassell Illustrated,2005.Lupton, Ellen. Thinking with Type: A Critical Guide for Designers,Writers, Editors & Students. New York: Princeton ArchitecturalPress, 2007.Microsoft Typography. Available online. URL: http://www.microsoft.com/typography/default.mspx. Accessed August 4, 2007.TrueType Typography. Available online. URL: http://www.truetype-typography.com/. Accessed August 4, 2007.ForthThe unusual Forth programming language was designedby Charles H. Moore in 1970. An astronomer, Moore wasinterested in developing a compact language for controllingmotors to drive radio telescopes and other equipment.Language StructureForth has a very simple structure. The Forth system consistsof a collection of words. Each word is a sequence ofoperations (which can include other existing words). Forexample, the DUP word makes a copy of a data value. Datais held by a stack. For example, the arithmetic expressionwritten as 2 + 3 in most languages would be written inForth as + 2 3. When the + operator (which in Forth is apre-defined word) executes, it adds the next two numbersit encounters (2 and 3) together, and puts the sum on thestack (where in turn it might be fetched for further processingby the next word in the program (see stack). This representationis also called postfix notation and is familiar tomany users of scientific calculators.The words in the dictionary are “threaded” or linked sothat each word contains the starting address of the next one.The Forth interpreter runs a simple loop where it fetchesthe next token (one or more characters delimited by spaces)and scans the dictionary to see if it matches a defined word(including variables). If a word is found, the code in theword is executed. If no word is found, the interpreter interpretsthe token as a numeric constant, loads it on the stack,and proceeds to the next word.A key feature of Forth is its extensibility. Once you havedefined a word, the new word can be used in exactly the sameway as the predefined words. The various forms of definingwords allow for great control over what happens when a newword is created and when the word is later executed. (Inmany ways Forth anticipated the principles of object-orientedprogramming, with words as objects with implicit constructorsand methods. A well-organized Forth program builds upfrom “primitive” operations to the higher-level words, withthe program itself being the highest-level word.)Forth has always attracted an enthusiastic following ofprogrammers who appreciate a close communion with theflow of data in the machine and the ability to preciselytailor programs. The language is completely interactive,since any word can be typed at the keyboard to executeit and display the results. Forth was also attractive in the