Implementing Subprograms.ppt

ImplementingSubprogramsInText:essingnonlocals(staticscoping)essingnonlocalswithdynamicscopingChapter9:ImplementingSubprogramsImplementingSubprogramsThesubprogramcallandreturnoperationsofalanguagearetogethercalleditssubprogramlinkageFirst,let’slookatimplementingFORTRAN77subprogramsChapter9:ImplementingSubprogramsImplementingFORTRAN77SubprogsCallSemantics:SavetheexecutionstatusofthecallerCarryouttheparameter-passingprocessPassthereturnaddressTransfercontroltothecalleeReturnSemantics:Ifpass-by-value-resultisused,movecurrentvaluesofparameterstotheircorrespondingactualsIfitisafunction,movereturnvaluetoaplacethecallercangetitRestoretheexecutionstatusofthecallerTransfercontrolbacktothecallerRequiredStorage:Statusinformationofthecaller,parameters,returnaddress,andfunctionalvalue(ifitisafunction)Chapter9:ImplementingSubprogramsFORTRANActivationInfocanallocateallmemorystaticallymain... f(...)...procf(...)... g(...)...procg()... g(...)--!!!monMainfg}}Chapter9:ImplementingSubprogramsActivationRecordsTheformat,orlayout,ofthenoncodepartofanexecutingsubprogramiscalledanactivationrecord(AR)Anactivationrecordinstance(ARI)isaconcreteexampleofanactivationrecord(thecollectionofdataforaparticularsubprogramactivation)FORTRAN77subprogramscanhavenomorethanoneactivationrecordinstanceatanygiventimeThecodeofalloftheprogramunitsofaFORTRAN77programmayresidetogetherinmemory,withthedataforallunitsstoredtogetherelsewhereThealternativeistostorealllocalsubprogramdatawiththesubprogramcodeChapter9:ImplementingSubprogramsImplementingSubprogramsinALGOL-plicatedthanFORTRAN77:ParametersareoftenpassedbytwomethodsLocalvariablesareoftendynamicallyallocatedRecursionmustbesupportedStaticscopingmustbesupportedChapter9:ImplementingSubprogramsActivationRecordStructureAtypicalactivationrecordforanALGOL-likelanguage:LocalvariablesParametersDynamiclinkStaticlinkstacktopReturnaddressChapter9:ImplementingSubprogramsActivationRecordDetailsTheactivationrecordformatisstatic,butitssizemaybedynamicThestaticlinkpoint