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ComputerEnglishChapter2BasicOrganizationofComputersKeypoints:usefultermsandbasicorganizationofcomputersDifficultpoints:describingthebasicorganizationofcomputersRequirements:1.Termsofcomputerhardware2.BasicOrganizationofcomputersandtheirfunctions3.掌握专业词汇的构成规律，特别是常用词缀及复合词的构成NewWords&Expressions:subsystemn.子系统operationn.操作，运算，执行命令(计)microprocessorn.[计]微处理器ystembuses系统总线sequencen.时序,序列fetchvt.取数，取指令decodevt.解码,译解instructionn.指令2.1IntroductionAbbreviations:CPU(CentralProcessingUnit)中央处理器I/O(Input/Output)输入输出(设备)CPUMemorySubsystemI/ODeviceI/ODevice…I/OSubsystemAddressBusDataBusControlBusFig.2-1Genericcomputerorganization2.1IntroductionMostcomputersystems,fromtheembeddedcontrollersfoundinautomobilesandconsumerappliancestopersonalcomputersandmainframes,havethesamebasicorganization.Thisorganizationhasthreemaincomponents:theCPU,thememorysubsystem,andtheI/Osubsystem.ThegenericorganizationofthesecomponentsisshowninFigure2-1.大多数计算机系统，从汽车和日用电器中的嵌入式控制器到个人计算机和大型主机，都具有相同的基本组成。其基本组成包括三个主要部件：CPU、存储器子系统和I/O子系统。这些部件的一般组成如图2-1所示。2.1Introduction2.1IntroductionInthischapter,wefirstdescribethesystembusesusedtoconnectthecomponentsinthecomputersystem.Thenweexaminetheinstructioncycle,thesequenceofoperationsthatoccurswithinthecomputerasitfetches,decodes,andexecutesaninstruction.本章我们首先讲述计算机系统中用来连接计算机各部件的系统总线。然后再来考察指令周期，以及计算机在读取、解码和执行一条指令时所发生的操作顺序。NewWords&Expressions:pinsn.插脚,管脚addressbus地址总线uppermostadj.最高的；adv.在最上controlbus控制总线databus数据总线viaprep.经,通过,经由multibit多位bidirectional双向的unidirectional单向的hierarchyn.层次，层级I/Obus输入输出总线localbusn.局域总线2.2SystemBusesPhysically,abusisasetofwires.Thecomponentsofthecomputerareconnectedtothebuses.Tosendinformationfromonecomponenttoanother,thesourcecomponentoutputsdataontothebus.Thedestinationcomponenttheninputsthisdatafromthebus.Asthecomplexityofacomputersystemincreases,itbecomesmoreefficient(intermsofminimizingconnections)atusingbusesratherthandirectconnectionsbetweeneverypairofdevices.Busesuselessspaceonacircuitboardandrequirelesspowerthanalargenumberofdirectconnections.TheyalsorequirefewerpinsonthechiporchipsthatcomprisetheCPU.从物理上来说，总线就是一组导线。计算机的部件就是连在总线上的。为了将信息从一个部件传到另一个部件，源部件先将数据输出到总线上，然后目标部件再从总线上接受这些数据。随着计算机系统复杂性的不断增长，使用总线比每个设备对之间直接连接要有效得多（就减少连接数量而言）。与大量的直接连接相比，总线使用较少的电路板空间，耗能更少，并且在芯片或组成CPU的芯片组上需要较少的引脚。2.2SystemBusesThesystemshowninFigure2-1hasthreebuses.Theuppermostbusinthisfigureistheaddressbus.WhentheCPUreadsdataorinstructionsfromorwritesdatatomemory,itmustspecifytheaddressofthememorylocationitwishestoaccess.Itoutputsthisaddresstotheaddressbus;memoryinputsthisaddressfromtheaddressbusanduseittoaccessthepropermemorylocation.EachI/Odevices,suchasakeyboard,monitor,ordiskdrive,hasauniqueaddressaswell.WhenaccessinganI/Odevice,theCPUplacestheaddressofthedeviceontheaddressbus.EachdevicecanreadtheaddressoffofthebusanddeterminewhetheritisthedevicebeingaccessedbytheCPU.Unliketheotherbuses,theaddressbusalwaysreceivesdatafromtheCPU;theCPUneverreadstheaddressbus.图2-1所示的系统包括三组总线。最上面的是地址总线。当CPU从存储器读取数据或指令，或写数据到存储器时，它必须指明将要访问的存储器单元地址。CPU将地址输出到地址总线上，而存储器从地址总线上读取地址，并且用它来访问正确的存储单元。每个I/O设备，比如键盘、显示器或者磁盘，同样都有一个唯一的地址。当访问某个I/O设备时，CPU将此设备的地址放到地址总线上。每一个设备均从总线上读取地址并且判断自己是否就是CPU正要访问的设备。与其他总线不同，地址总线总是从CPU上接收信息，而CPU从不读取地址总线。2.2SystemBusesDataistransferredviathedatabus.WhentheCPUfetchesdatafrommemory,itfirstoutputsthememoryaddressonitsaddressbus.Thenmemoryoutputsthedataontothedatabus;theCPUcanthenreadthedatafromthedatabus.Whenwritingdatatomemory,theCPUfirstoutputstheaddressontotheaddressbus,thenoutputsthedataontothedatabus.Memorythenreadsandstoresthedataattheproperlocation.TheprocessesforreadingdatafromandwritingdatatotheI/Odevicesaresimilar.数据是通过数据总线传送的。当CPU从存储器中取数据时，它首先把存储器地址输出到地址总线上，然后存储器将数据输出到数据总线上，这样CPU就可以从数据总线上读取数据了。当CPU向存储器中写数据时，它首先将地址输出到地址总线上，然后把数据输出到数据总线上，这样存储器就可以从数据总线上读取数据并将它存储到正确的单元中。对I/O设备读写数据的过程与此类似。2.2SystemBusesThecontrolbusisdifferentfromtheothertwobuses.Theaddressbusconsistsofnlines,whichcombinetotransmitonen-bitaddressvalue.Similarly,thelinesofthedatabusworktogethertotransmitasinglemultibitvalue.Incontrast,thecontrolbusisacollectionofindividualcontrolsignals.ThesesignalsindicatewhetherdataistobereadintoorwrittenoutoftheCPU,whethertheCPUisaccessingmemoryoranI/Odevice,andwhethertheI/Odeviceormemoryisreadytotransferdata.AlthoughthisbusisshownasbidirectionalinFigure2-1,itisreallyacollectionof(mostly)unidirectionalsignals.MostofthesesignalsareoutputfromtheCPUtothememoryandI/Osubsystems,althoughafewareoutputbythesesubsystemstotheCPU.Weexaminethesesignalsinmoredetailwhenwelookattheinstructioncycleandthesubsysteminterface.控制总线与以上两种总线都不相同。地址总线由n根线构成，n根线联合传送一个n位的地址值。类似地，数据总线的各条线合起来传输一个单独的多位值。相反，控制总线是单根控制信号的集合。这些信号用来指示数据是要读入CPU还是要从CPU写出，CPU是要访问存储器还是要访问I/O设备，是I/O设备还是存储器已就绪要传送数据等等。虽然图2-1所示的控制总线看起来是双向的，但它实际上（主要）是单向（大多数都是）信号的集合。大多数信号是从CPU输出到存储器与I/O子系统的，只有少数是从这些子系统输出到CPU的。在介绍指令周期和子系统接口时，我们将详细地讨论这些信号。2.2SystemBusesAsystemmayhaveahierarchyofbuses.Forexample,itmayuseitsaddress,data,andcontrolbusestoaccessmemory,andanI/Ocontroller.TheI/Ocontroller,inturn,mayaccessallI/Odevicesusingasecondbus,oftencalledanI/Obusoralocalbus.一个系统可能