Encyclopedia of Computer Science and Technology

402 regular expressioncompetitive and other pressures led their designers to startadding complexity.One way processor designers coped with the demandfor more complicated instructions was to give the main processora microprocessor with its own set of simple instructions.When the main processor received one of the complexinstructions, it would be executed by being broken downinto simpler instructions or “microcode” to be executed bythe sub-processor.This approach gave processor designers greater flexibility.It also made things easier for compiler designers,because the compiler could translate higher-level languagestatements into fewer, more complex instructions, leavingit to the hardware with its micro engine to break themdown into the ultimate machine operations. However, italso meant that the processor had to decode and executemore instructions in every processor cycle, making it lessefficient and slower and losing some of the benefits of thefaster processors that were becoming available.In 1975, John Cocke and his colleagues at IBM decidedto build a new minicomputer architecture from the groundup. Instead of using complex instructions and decodingthem with a micro engine, they would use only simpleinstructions that could be executed one per cycle. The clock(and thus the cycle time) would be much faster than forexisting machines, and the processor would use pipeliningso it could decode the next instruction while still executingthe previous one. Similarly, in many cases the next itemof data needed could be fetched at the same time the datafrom the previous step was being written (stored). Thisapproach became known as reduced instruction set computing(RISC), because the number of instructions had beenreduced compared to exiting systems, which then becameknown as complex instruction set computing (CISC).Since the RISC system had only simple instructions,compilers could no longer use many complicated but handyinstructions. The compiler would have to take over the jobof the micro engine and break all statements down intothe basic instructions. It became important that the compilerbe able to generate the optimal set of instructions byanalyzing how data would have to be moved around inthe machine’s registers and memory. In other words, RISChardware gained higher performance through simplificationat the hardware level but at the cost of making compilersmore complicated. Fortunately, both hardware and softwaredesigners were able to meet the challenge and in the processlearn how to get the most out of new technology.RISC would also play a part in the design of the microprocessorsthat began to power personal computers. Forexample, the DEC Alpha, a “pure” RISC chip introducedin 1992, provided a level of power that made it suitable forhigh-performance workstations. Another successful RISCbaseddevelopment has been the SPARC (Scalable ProcessorARChitecture) developed by Sun Microsystems for servers,computer clusters, and workstations.Perhaps the most interesting development, however, hasbeen the gradual application of RISC principles to mainstreamprocessors such as the Intel 80×86 series used inmost personal computers today. Increasingly, the recentPentium series chips, while supporting their legacy of CISCinstructions, are processing them using an inner architecturethat uses RISC principles and takes advantage of pipelining,as well as using more registers and a larger datacache. However, the sheer increase in clock cycle speed andperformance in the newer chips has made the old tradeoffbetween complicated and simple instructions less relevant.Further ReadingDandamudi, Sivarama P. Guide to RISC Processors for Programmersand Engineers: Introduction to Assembly Language Programmingfor Pentium and RISC Processors. New York: Springer, 2005.Knuth, Donald E. MMIX—A RISC Computer for the New Millennium.Vol. 1, fascicle 1 of Art of Computer Programming. UpperSaddle River, N.J.: Addison-Wesley Professional, 2005.regular expressionMany users of UNIX and the old MS-DOS are familiar withthe ability to use “wildcards” to find filenames that matchspecified patterns. For example, suppose a user wants to listall of the TIF graphics files in a particular directory. Sincethese files have the extension .tif, a UNIX ls command or aDOS dir command, when given the pattern *.tif, will matchand list all the TIF files. (One does have to be aware ofwhether the operating system in question is case-sensitive.UNIX is, while MS-DOS is not.)The specification *.tif tells the command “match all fileswhose names consist of one or more characters and that endwith a period followed by the letters tif.” It is one of manypossible regular expressions. (See the accompanying tablefor more examples.) The asterisk here is a “metacharacter.”This means that it is not treated as a literal character, but asa pattern that will be matched in a specified way.Most operating systems that have command processors(see shell) allow for some form of regular expressions, butdon’t necessarily implement all of the metacharacters. UNIXprovides the most extensive use for regular expressions (seeUNIX). UNIX has an operating system facility called globthat expands regular expressions (that is, substitutes forthem whatever matches) and passes them on to the manyUNIX tools or utilities designed to work with regular expressions.These tools include editors such as ex and vi, thecharacter translation utility (tr), the “stream editor” (sed),and the string-searching tool grep. For example, sed can beused to remove all blank lines from a file by specifyingsed ‘s/^$/d’ list.txtThis command finds all lines with no characters (^$) inthe file list.txt and deletes them from the output. Even moreextensive use of pattern-matching with regular expressionsis found in many scripting languages (see scripting languages,awk, and Perl).It is true that most of today’s computer users don’t enteroperating system commands in text form but instead usemenus and manipulate icons (see user interface andMicrosoft Windows). If such a user wants to change oneword to another throughout a word processing document,he or she is likely to open the Edit menu, select Find,