students to learning on their own. Both computational power and scientific ad- 9. j. r. Anderson. The ttfMr^q <m&ition groups got the same lectures and read vances in the understanding ofcognition. d Universilv (Mril^ass., the same material. All students complet- m" - We havefocused on the consequencesof ">" ACT* is the most recent"*in a series of theories ed the lessons. However, students work- intelligent tutoring methods for pedago- ff^lSt U^t^eTSr^If ing witn the computer tutor spent 30 gy. However, we should stress that data Thought, a name that has only historical signifi- percent less time doing the problems collected in these pedagogical experi- n. STerown and K. VanLehn. Costive Sci 4 associated with the lemons and scored ments advance the science of human „ "\mQ) - -43 percent better on the final exam than cognition. SiS^^^^ the Students Working On their own. Press. New York. 1982). References and Notes 13. J. R. Anderson and R. Jeffries. Hum. Comput. , in press. i ? c r. wn a fro rid J Grecno, in Research Brief- 14. B. S. Bloom.Educ. Researcher 13. 3 (1984). demy Prt>s Washing- 15> c Boyle and J R Anderson Overalloverall Assessment ofDC ' and7" , ' ncu " mTi'> n ,-., automated instruction of geometry"^"«"proof 2. D Sleeman and J S. Brown. Eds., Intelligent paperpresented at theAmerican Educa- g Sys,ems Academic , „ __ ioo-n < New York, tion Researchiwsi.aii.il rvs>utiauunAssociation meetings, New Or-ur- Researcht> . on ". ... , iS*°?"982) leans. intelligenttutoring is now v . A April 1984. on the threshold of a methodology that ' ' L2^^i£SSS^JKSKS'te will make qualitativechanges abil- M^.r^-S""hlske New York Times < 9 December*' "' gence<i9Bi)i. in our f9'84) ' ity to instruct Students on topics that 5. J Carbonnel] lEEE Trans. Man-Machine W. L SUSdH^, f P^ro"/"fiSlli ' in The many students find difficult. This 1 00 0 b k M Chi results 6. frT^ZTiv (June 1984) * °° - ' from afortuitous convergence R- Anderson « (1984). mfZ?*^^ * * of techno- J- {■ Cognitive Sci. 87 19. Supported by contract NOOOl4-84-K-0064 from 8' S A logical advances in .he availability of k^l^3°&Xi. '" "' ' ically sell for $10,000 to $100,000 and superminicomputers sell for $100,000 to $500,000. 3) A new computer class can be pro- ducedby combining parts in a new way. Multis: A New Class of The supercomputer and various micro- processor-based computers including Multiprocessor Computers workstations and multis emerged in this fashion. In 1964. the supercomputer class was introduced with the CDC-6600, C. Gordon Bell although large computers, including IBM's Stretch, had been built earlier. Seymour Cray's CDC-6600 contained rfx „ . about a half-million denselypacked, Fre- cost and selling pnce, thereby increasing on-cooled transistors connected with g ictecZToevthafrr th£ effectlveness for cu «*«. up For discrete circuits. Since the CDC-6600. «"uSs I ITJ aCCOrdmgly'TT thef : examPle < computers" were es- Cray has designed nearlyall ofthe super- v" tnrt t ?' ,meg ciTTd,^ " tabHshed in 195 With the Univac computers, which have madeuse ofvari- ge U which cost between $300,000 $5 ° and mil-*' ous forms of parallel computation to T>pical> one technology is used until Tni^r"^7 ""T \ hon. The best known family of main- execute a single instruction In Cray's mtr dUCed by IBM m 1%4 *«*■« 'speed is obtained by" o- or anotheftltol "" the"SyStem° 360 Which em ' WaS based Cessine vectors a < over million float- eLTo *"" tech"^V- In the early ing-pomt operationsper second. Cray's manTas' S"JT-^ g? ?" " IBM imrodUCed the 3? SeriesS suPerc"P"ters sell for $4 million to siHconsilicon chip, and microprocessors*" ' chin^ni con- which used integratedcircuits; and° most million V nte" t,y tHe 43XX~3BOX S6rieS' WhiCh In 19?K a Single Chl rocessor the W tS WUh , > P "e^Ta^ S^-l ES r Use f hlgh-Perf°"nance integral- Intel 4004 microprocessor, was intro-- mant «">m ir , an St Sll n Cd drCUitS ° intr dUCed duced and used in a nmge of H ° K° ""*" ' - apph- cZeJ^l Z. 2) ° St d3SS f COmpUt - Cations from calculato Tcost teeT, V"'" n ers WlthV'"'the ' "*' controSL acteris^ d!SSiP same'^Perf° °^nce° as a pre- for microwave ovens. In*1975, Altair packinlden tv rTKT !!7H ' !nH vlous computer can be produced, which introduced the first home- computer, different ways in which technolog.es Seremwa^ the will result in new applications for com- based on Intel's 8080 microprocessor ha t£d na ! PUterS- MinicomPuters a d Phonal Today, microprocessors have" the La" vaTetv of\ZnntmpUter daSSeSr basedJ on comP"ters are the best known" products tures necessary nrice n° for building high-speed fFif of this design path. The first minicom- computers with u/VWhen a new virtual memory that technology becomes puter, the PDP-8, which was the result compare favorably with minicomputers available,there are usually three ways to of second-generation technology, was This high-performance component of lU tW f WhlCh gCnerate introduced in 1965 Djgital Equip- negligible cost has permitted the Ve i ° intro- aSSeS:° nt C By 1972, 91 compa- duction of many new n Th?,ecme hnn v , u classes of comput- ii technology can be used to in- "nies had°?°T*tion.formed to build minicomputers __ crease the performance of an existing with the third-generation technology,in- _c - Gordon Bell is vice Chairman ofTechnology ofcomputers ration class while maintainingthe tegratedcircuits (2). Minicomputers typ- Ma"sachu^ensPSiCorP° - We,les,ey Hills 462 SCIENCE. VOL. 228 3* j Interact., ■ «, skills", Press,■— »-v.^ ium, nuii r>ew ,T ," ' a/., 8, t S C!I ' ,atCSt trans,Stor S $20 reC rS W3S C reSU ■ ' -ft ers (2), including lap and home comput- asymmetric processing is that all the permits a trade-off between expensive ers ($2OO to personal computers operating-system functions are per- high-speed memory, and large, cheap to multiple-user formed by a single processor, which can low-speed memory. Since the content of (shared) microcomputers ($6OOO to create a bottleneck in that processor, each cache is associated with a proces- workstations to IBM developed both symmetric and sor, extra logic must be added to the and microprocessor-based asymmetric dual processors, including system to delete "cached" copies of multiprocessors, or multis to the System 370 and System 308Xseries data or instructions that have been modi- It is with this newest class of computers (4). The IBM Attached Pro- fied by anotherprocessor. If a processor computers, the multis, that we will be cessor however, is asymmetric uses stale data, incorrect calculations i concerned. and, addition, requires f in the master can occur. i processor to perform the input-output function. Multiprocessors, Multicomputers, In the past, uniprocessors had a cost The Structure of a Multi and Multis advantageover multiprocessorsbecause the cost of switching and cabling is pro- Multis are based on advances in mi- Multiprocessors are computers that portional to the number of processors croprocessor technology and the cache two or moreprocessors capable the (5). memory 2). ! contain and memories in system As a (Fig. They use a single set of of independently executing instructions result ofprogram sharing, a multiproces- wires called a bus for all communication and gainingaccess to programs and data held in a common memory. A processor is the part of the computer that carries Summary. Multis are a new class of computers based on multiplemicroprocessors. out computational work by retrieving The small size, low cost, and highperformance of microprocessors allowthe design instructions from memory and perform- and construction of computer structures that offer significant advantages in manufac- ing operations on data that were also ture, price-performance ratio, and reliability over traditional computer families. Cur- retrieved from memory. Thus informa- rently, commercialmultis consist of 4 to 28 modules, which include microprocessors, tion in a multiprocessor can be freely common memories, and input-output devices, all of which communicatethrough a used by and exchanged among all pro- single set of wires called a bus. Adding microprocessors together increases the cessors. performance of multis in direct proportion to their price and allows multis to offer a In contrast, multicomputers consist of performancerange that spans that of smallminicomputersto mainframe computers. ! interconnected, independentcomputers, Multis are commercially available for applications ranging from real-time industrial each ofwhich has a processor, thatcom- control to transaction processing. Traditional batch, time-sharing, and transaction j municate by passing messages through systems process a number of independent jobs that can be distributedamong the fixed links or a switch such as a local- microprocessors of a multi with a resulting increased throughput (number of jobs area network. Intel recently introduced a completedper unit of time). Many scientific applications(such as the solvingof partial series of multicomputers,called the Intel differentialequations) and engineeringapplications(such as thechecking of integrat- iPSC Family of Concurrent Supercom- ed circuit designs) are speededup by this parallelcomputation; thus, multis produce puters, which have 32, 64, or 128 com- results at supercomputer speedbut at a fraction of the cost. Multis are likelyto be the puters connected in a hypercube config- basis for the next, the generationof computers a generationbased on parallel uration. Within a model, each computer processing. — is connected to five, six, or seven other computers through message-passing links that can transfer dataat arate of 10 sor with N processors requires less between processors, memories, and in- megabits per second. memory than N independentcomputers, put-output devices. This bus and corn- Since a multiprocessor can be parti- but the multiprocessor's memory must puter structure was pioneered in Digital tioned into independent computers and be faster and larger than a uniproces- Equipment Corporation's single-proces- use common memoryfor intercommuni- sor's.