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名词解释:DMA控制器

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专门用来控制总线进行DMA传送的接口硬件称为DMA控制器。

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AsGilbertWhite,Darwin,andothersobservedlongago,allspeciesappeartohavetheinnatecapacitytoincreasetheirnumbersfromgenerationtogeneration.Thetaskforecologistsistountangletheenvironmentaandbiologicalfactorsthatholdthisintrinsiccapacityforpopulationgrowthincheckoverthelongrun.Thegreatvarietyofdynamicbehaviorsexhibitedbydifferentpopulationmakesthistaskmoredifficult:sompopulationsremainroughlyconstantfromyeartoyear;othersexhibitregularcyclesofabundanceandscarcity;stillothersvarywildly,withoutbreaksandcrashesthatareinsomecasesplainlycorrelatedwiththeweather,andinothercasesnot.Toimposesomeorderonthiskaleidoscopeofpatterns,oneschoolofthoughtproposesdividingpopulationsintotwogroups.Theseecologistspositthattherelativelysteadypopulationshavedensity-dependentgrowthparameters;thatis,ratesofbirth,death,andmigrationwhichdependstronglyonpopulationdensity.Thehighlyvaryingpopulationshavedensity-independentgrowthparameters,withvitalratesbuffetedbyenvironmentalevents;theseratesfluctuateinawaythatiswhollyindependentofpopulationdensity.Thisdichotomyhasitsuses,butitcancauseproblemsiftakentooliterally.Foronething,nopopulationcanbedrivenentirelybydensity-independentfactorsallthetime.Nomatterhowseverelyorunpredictablybirth,death,andmigrationratesmaybefluctuatingaroundtheirlong-termaverages,iftherewerenodensity-dependenteffects,thepopulationwould,inthelongrun,eitherincreaseordecreasewithoutbound(barringamiraclebywhichgainsandlossescanceledexactly)。Putanotherway,itmaybethatonaverage99percentofalldeathsinapopulationarisefromdensity-independentcauses,andonlyonepercentfromfactorsvaryingwithdensity.Thefactorsmakinguptheonepercentmayseemunimportant,andtheircausemaybecorrespondinglyhardtodetermine.Yet,whetherrecognizedornot,theywillusuallydeterminethelong-termaveragepopulationdensity.Inordertounderstandthenatureoftheecologist’sinvestigation,wemaythinkofthedensity-dependenteffectsongrowthparametersasthesignalecologistsaretryingtoisolateandinterpret,onethattendstomakethepopulationincreasefromrelativelylowvaluesordecreasefromrelativelyhighones,whilethedensity-independenteffectsacttoproducenoiseinthepopulationdynamics.Forpopulationsthatremainrelativelyconstant,orthatoscillatearoundrepeatedcycles,thesignalcanbefairlyeasilycharacterizedanditseffectsdescribed,eventhoughthecausativebiologicalmechanismmayremainunknown.Forirregularlyfluctuatingpopulations,wearelikelytohavetoofewobservationstohaveanyhopeofextractingthesignalfromtheoverwhelmingnoise.Butitnowseemsclearthatallpopulationsareregulatedbyamixtureofdensity-dependentanddensity-independenteffectsinvaryingproportions. 1.Theauthorofthetextisprimarilyconcernedwith [A]Discussingtwocategoriesoffactorsthatcontrolpopulationgrowthandassessingtheirrelativeimportance. [B]Describinghowgrowthratesinnaturalpopulationsfluctuateovertimeandexplainingwhythesechangesoccur. [C]Proposingahypothesisconcerningpopulationsizeandsuggestingwaystotestit. [D]Posingafundamentalquestionaboutenvironmentalfactorsinpopulationgrowthandpresentingsomecurrentlyacceptedanswer. 2.Itcanbeinferredfromthetextthattheauthorconsidersthedichotomydiscussedtobe [A]Applicableonlytoerraticallyfluctuatingpopulations. [B]instrumental,butonlyifitslimitationsarerecognized. [C]Dangerouslymisleadinginmostcircumstances. [D]Acompleteandsufficientwaytoaccountforobservedphenomena. 3.tothetext,allofthefollowingbehaviorshavebeenexhibitedbydifferentpopulationsEXCEPT [A]Roughlyconstantpopulationlevelsfromyeartoyear. [B]Regularcyclesofincreasesanddecreasesinnumbers. [C]Erraticincreasesinnumberscorrelatedwiththeweather. [D]Uncheckedincreasesinnumbersovermanygenerations. 4.Thediscussionconcerningpopulationinthethirdparagraphservesprimarilyto [A]Demonstratethedifficultiesecologistsfaceinstudyingdensity-dependentfactorslimitingpopulationgrowth. [B]Advocatemorerigorousstudyofdensity-dependentfactorsinpopulationgrowth. [C]Provethatthedeathratesofanypopulationareneverentirelydensity-independent. [D]underlinetheimportanceofevensmalldensity-dependentfactorsinregulatinglong-termpopulationdensities. 5.Inthetext,theauthordoesallofthefollowingEXCEPT [A]Citetheviewsofotherbiologists. [B]Defineabasicproblemthatthetextaddresses. [C]Presentconceptualcategoriesusedbyotherbiologists. [D]Describetheresultsofaparticularstudy.
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Asianeconomieswillcontinuetoexpandrapidlyinthenexttwoyearsonthebackofforeigninvestments,exportsanddomesticdemand,butwillnotbefreeofworries,accordingtoarecentsecuritiescompanyeconomicreport.Massiveinfrastructurespending,thebenefitsofpastinvestmentsandderegulationandgrowingregionaltradewillpushalongregionalgrowth. ThereporttippedChina’sgrossdomesticproducttogrowby9.5percentin1996and10.5percentin1997,andHongKong’stogrowby4.3and5.0percent.Singapore’srealGDPwasforecasttogrowby7.6and7.4percent,Malaysia’sby8.6and8.1percent,Thailand’sby8.8and8.4percent,Indonesia’sby7.3and7.1percentandPhilippines’by5.6and5.8percent. “However,allisnotrosy,”cautionedthereport,whichlistedoverheatingastheregion’sbiggestchallengeintheshorttermandskilled-laborshortagesinthelongrun. Itcitedstrainsfromrapidgrowththathadbeguntostretchcurrentaccountdeficitsandspurinflationinsomeregionaleconomies,whichcouldpreventtheircentralbanksfromloweringinterestrates. “Thisimpliesthateconomicvulnerabilitytounexpectedshockswillremainhigh,”thereportsaid,forecastingafurthertighteningofmonetarypolicyincountriessuchasthePhilippinesandMalaysia,andnosignificanteasinginThailand,IndonesiaandChina. CrosbySecuritiesalsocitedmassivenewinvestmentprogramsgeneratedbydemandsoninfrastructuresuchaspower,telecommunicationsandtransportwhichhadfuelledover-heating. ItsaidAsiawouldalsofaceabiggerchallengefromnewlyderegulatingeconomiesinEasternEuropewhichofferedlowercostsandbetter-educatedworkerstoforeigninvestors. Asia’smainforeigninvestorsshould,however,continuetoexpandtheirpresenceintheregion,thesecuritieshousesaid,notingnewtrendsofintra-Asianinvestment. Singaporeisshiftingitslowervalue-addeddiscdriveandelectronicsindustrytoMalaysia,whichinturnismovingsomeofitsgarment-makingandconsumerelectronicsmanufacturingtoIndonesiaandThailand.Thailand,MalaysiaandSingaporeareincreasinglyinvestinginVietNam,MyanmarandLaos. “Asiawillbenefitfromthistrendinseveralways,”thereportsaid. ()1Asiaeconomywillcontinuetogrowbecauseof_____________. A.foreigninvestmentB.demandsinAsiancountries. C.exportD.alloftheabove ()2Inthenext2years,thehighesteconomicgrowthwillappearin____________. A.SingaporeB.Thailand C.HongKongD.China ()3Yearslater,theprobleminAsianeconomymaybe_________________. A.toolittleinvestmentB.toomuchinvestment C.notenoughqualifiedlaborsD.monetaryinstability ()4Accordingtothisreport,economicchangesinEastEuropeare___________. A.harmfulB.challenging C.helpfulD.useless ()5ThereportisabouttheprospectofAsianeconomy. A.optimisticB.pessimistic C.moreoptimisticthanpessimisticD.morepessimisticthanoptimistic
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1.Thefailureofahigh-profilecholesteroldrughasthrownaspotlightonthecomplicatedmachinerythatregulatescholesterollevels.Butmanyresearchersremainconfidentthatdrugstoboostlevelsof'good'cholesterolarestilloneofthemostpromisingmeanstocombatspirallingheartdisease. 2.DrugcompanyPfizerannouncedon2Decemberthatitwascancellingallclinicaltrialsoftorcetrapib,adrugdesignedtoraiseheart-protectivehigh-densitylipoproteins(HDLs).Inatrialof15000patients,asafetyboardfoundthatmorepeoplediedorsufferedcardiovascularproblemsaftertakingthedrugplusacholesterol-loweringstatinthanthoseinacontrolgroupwhotookthestatinalone. 3.Thenewscameasakickintheteethtomanycardiologistsbecauseearliertestsinanimalsandpeoplesuggesteditwouldlowerratesofcardiovasculardisease."Therehavebeennoredflagstomyknowledge,"saysJohnChapman,aspecialistinlipoproteinsandatherosclerosisattheNationalInstituteforHealthandMedicalResearch(INSERM)inPariswhohasalsostudiedtorcetrapib."Thiscancellationcameasacompleteshock." 4.TorcetrapibisoneofthemostadvancedofanewbreedofdrugsdesignedtoraiselevelsofHDLs,whichferrycholesteroloutofartery-cloggingplaquestotheliverforremovalfromthebody.Specifically,torcetrapibblocksaproteincalledcholesterolestertransferprotein(CETP),whichnormallytransfersthecholesterolfromhigh-densitylipoproteinstolowdensity,plaque-promotingones.Statins,incontrast,mainlyworkbyloweringthe'bad'low-densitylipoproteins. Underpressure 5.Researchersarenowtryingtoworkoutwhyandhowthedrugbackfired,somethingthatwillnotbecomeclearuntiltheclinicaldetailsarereleasedbyPfizer.Onehintliesinevidencefromearliertrialsthatitslightlyraisesbloodpressureinsomepatients.Itwasthoughtthatthismildproblemwouldbeoffsetbytheheartbenefitsofthedrug.Butitispossiblethatitactuallyprovedfatalinsomepatientswhoalreadysufferedhighbloodpressure.Ifbloodpressureistheexplanation,itwouldactuallybegoodnewsfordrugdevelopersbecauseitsuggeststhattheproblemsarespecifictothiscompound.OtherprototypedrugsthatarebeingdevelopedtoblockCETPworkinaslightlydifferentwayandmightnotsufferthesamedownfall. 6.ButitisalsopossiblethatthewholeideaofblockingCETPisflawed,saysMotiKashyap,whodirectsatherosclerosisresearchattheVAMedicalCenterinLongBeach,California.WhenHDLsexcretecholesterolintheliver,theyactuallyrelyonLDLsforpartofthisprocess.SoinhibitingCETP,whichpreventsthetransferofcholesterolfromHDLtoLDL,mightactuallycauseanabnormalandirreversibleaccumulationofcholesterolinthebody."You'reblockingaphysiologicmechanismtoeliminatecholesterolandeffectivelyconstipatingthepathway,"saysKashyap.Goingup 7.Mostresearchersremainconfidentthatelevatinghighdensitylipoproteinslevelsbyonemeansoranotherisoneofthebestroutesforhelpingheartdiseasepatients.ButHDLsarecomplexandnotentirelyunderstood.Oneapproveddrug,calledniacin,isknowntobothraiseHDLandreducecardiovascularriskbutalsocausesanunpleasantsensationofheatandtingling.Researchersareexploringwhethertheycanbypassthissideeffectandwhetherniacincanlowerdiseaseriskmorethanstatinsalone.Scientistsarealsoworkingonseveralothermeanstobumpuphigh-densitylipoproteinsby,forexample,introducingsyntheticHDLs."Theonlythingweknowisdeadinthewateristorcetrapib,notthewholeideaofraisingHDL,"saysMichaelMiller,directorofpreventivecardiologyattheUniversityofMarylandMedicalCenter,Baltimore. Questions7-13 Matchtorcetrapib,HDLs,statinandCETPwiththeirfunctions(Questions8-13)..WritethecorrectletterA,B,CorDinboxes8-13onyouranswersheet.NBYoumayuseanylettermorethanonce. 7.Ithasbeenadministeredtoover10,000subjectsinaclinicaltrial. 8.Itcouldhelpridhumanbodyofcholesterol. 9.Researchersareyettofindmoreaboutit. 10.Itwasusedtoreducethelevelofcholesterol. 11.AccordingtoKashyap,itmightleadtounwantedresultifit'sblocked. 12.Itproducedcontradictoryresultsindifferenttrials. 13.ItcouldinhibitLDLs.Listofchoices A.TorcetrapicB.HDLSC.StatinD.CETP
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