计算机组成与结构：第二部分-程序和数据的机器级表达 03-信息存储

第2章信息的表示与处理

——信息存储

计算机组成与结构

2016年4月主讲教师Today:Information

StorageRepresentationsinmemory,pointers,stringsByte-OrientedMemoryOrganizationProgramsrefertodatabyaddressConceptually,envisionitasaverylargearrayofbytesInreality,it’snot,butcanthinkofitthatwayAnaddressislikeanindexintothatarrayand,apointervariablestoresanaddressNote:systemprovidesprivateaddressspacestoeach“process”ThinkofaprocessasaprogrambeingexecutedSo,aprogramcanclobberitsowndata,butnotthatofothers•••00•••0FF•••FMachineWordsAnygivencomputerhasa“WordSize”Nominalsizeofinteger-valueddataandofaddressesMostcurrentmachinesuse32bits(4bytes)aswordsizeLimitsaddressesto4GB(232bytes)Becomingtoosmallformemory-intensiveapplicationsleadingtoemergenceofcomputerswith64-bitwordsizeMachinesstillsupportmultipledataformatsFractionsormultiplesofwordsizeAlwaysintegralnumberofbytesWord-OrientedMemoryOrganizationAddressesSpecifyByteLocationsAddressoffirstbyteinwordAddressesofsuccessivewordsdifferby4(32-bit)or8(64-bit)00000001000200030004000500060007000800090010001132-bitWordsBytesAddr.001200130014001564-bitWordsAddr=??Addr=??Addr=??Addr=??Addr=??Addr=??000000040008001200000008Forotherdatarepresentationstoo…CDataTypeTypical32-bitIntelIA32x86-64char111short222int444long448longlong888float444double888longdouble810/1210/16pointer448ByteOrderingSo,howarethebyteswithinamulti-bytewordorderedinmemory?ConventionsBigEndian:Sun,PPCMac,InternetLeastsignificantbytehashighestaddressLittleEndian:x86LeastsignificantbytehaslowestaddressByteOrderingExampleExampleVariablexhas4-bytevalueof0x01234567Addressgivenby&xis0x1000x1000x1010x1020x103012345670x1000x1010x1020x10367452301BigEndianLittleEndian0123456767452301RepresentingIntegersDecimal: 15213Binary:

0011101101101101Hex:

3B6D6D3B0000IA32,x86-643B6D0000SunintA=15213;93C4FFFFIA32,x86-64C493FFFFSunTwo’scomplementrepresentationintB=-15213;longintC=15213;000000006D3B0000x86-643B6D0000Sun6D3B0000IA32ExaminingDataRepresentationsCodetoPrintByteRepresentationofDataCastingpointertounsignedchar*allowstreatmentasabytearrayPrintfdirectives:%p: Printpointer%x: PrintHexadecimaltypedefunsignedchar*pointer;voidshow_bytes(pointerstart,intlen){inti;for(i=0;i<len;i++)printf(”%p\t0x%.2x\n",start+i,start[i]);printf("\n");}show_bytesExecutionExampleinta=15213;printf("inta=15213;\n");show_bytes((pointer)&a,sizeof(int));Result(Linux):inta=15213;0x11ffffcb8 0x6d0x11ffffcb9 0x3b0x11ffffcba 0x000x11ffffcbb 0x00RepresentingPointersDifferentcompilers&machinesassigndifferentlocationstoobjectsintB=-15213;int*P=&B;x86-64SunIA32EFFFFB2CD4F8FFBF0C89ECFFFF7F0000charS[6]="18243";RepresentingStringsStringsinCRepresentedbyarrayofcharactersEachcharacterencodedinASCIIformatStandard7-bitencodingofcharactersetCharacter“0”hascode0x30