中科大操作系统原理与实现课件8_memory3

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1、操作系统原理与设计第8章MainMemory3陈香兰中国科学技术大学计算机学院2009年11月17日......提纲Segmentation(分段)BasicMethodHardwareSegmentationwithpagingExample:TheIntelPentium小结和作业......DiscreteMemoryAllocationIpaging（分页）Iinternalfragmentation

2、ion(分段)BasicMethodHardwareSegmentationwithpagingExample:TheIntelPentium小结和作业......SegmentationISegmentation:supportinguserviewofmemoryIAprogramisacollectionofsegments.User’sViewofaProgramAsegmentisalogicalunitsuchas:mainprogram,procedure,function,method,object,localvariables,globalvariables,co

3、mmonblock,stack,symboltable,arrays......LogicaladdressspaceIAcollectionofsegmentsIeachsegmentI2-DaddressspaceIAlogicaladdressconsistsofatwotupleI,orIICompilerautomaticallyconstructssegmentsreflectingtheinputprogram.IPascalcompilerIFORTRANcompilerICc

4、ompiler,suchasgcc,:::......LogicalViewofSegmentation......OutlineSegmentation(分段)BasicMethodHardwareSegmentationwithpagingExample:TheIntelPentium小结和作业......SegmentationArchitectureIISegmenttable–maps2-DLA!1-DPA;eachtableentryhas:Ibase–containsthestartingphysicaladdresswherethesegmentsresideinm

5、emoryIlimit–specifiesthelengthofthesegmentISegment-tablebaseregister(STBR)pointstothesegmenttable’slocationinmemoryISegment-tablelengthregister(STLR)indicatesnumberofsegmentsusedbyaprogram;segmentnumbersislegalifs

6、Witheachentryinsegmenttableassociate:Ivalidationbit=0)illegalsegmentIread/write/executeprivilegesIProtectionbitsassociatedwithsegments;codesharingoccursatsegmentlevelISincesegmentsvaryinlength,memoryallocationisadynamicstorage-allocationproblemIFirst-fit,Best-fit,:::IExternelfragmentation,compac

7、tion,:::......ExampleofSegmentationI!4300+53=4353I!3200+852=4052IWhatabout?......分页和分段的主要区别I角度和目的不同I分页：面向系统，物理上离散，减少外部和内部碎片，提高内存利用率I页是信息的物理单位I分段：面向用户，逻辑上离散，满足用户的需求I段是信息的逻辑单位，意义相对完整I大小不同I分页：大小固定，由