计算机组成与结构06-程序的机器级表示：控制.pptx

第3章程序的机器级表示:——控制计算机组成与结构2016年4月主讲教师赵欢******@:ConditioncodesConditionalbranchesLoopsSwitchstatementsHunanUniversityProcessorState(IA32,Partial)InformationaboutcurrentlyexecutingprogramTemporarydata(%eax,…)Locationofruntimestack(%ebp,%esp)Locationofcurrentcodecontrolpoint(%eip,…)Statusofrecenttests(CF,ZF,SF,OF)%eipGeneralpurposeregistersCurrentstacktopCurrentstackframeInstructionpointerCFZFSFOFConditioncodes%eax%ecx%edx%ebx%esi%edi%esp%ebpHunanUniversityConditionCodes(ImplicitSetting)SinglebitregistersCFCarryFlag(forunsigned)SFSignFlag(forsigned)ZFZeroFlag OFOverflowFlag(forsigned)Implicitlyset(thinkofitassideeffect)byarithmeticoperationsExample:addl/addqSrc,Dest↔t=a+bCFsetifcarryoutfrommostsignificantbit(unsignedoverflow)ZFsetift==0SFsetift<0(assigned)OFsetiftwo’plement(signed)overflow(a>0&&b>0&&t<0)||(a<0&&b<0&&t>=0)Notsetbylea/movinstructionHunanUniversityConditionCodes(pare)pareInstructioncmpl/cmpqSrc2,Src1 ;Src1–Src2,影响标志位cmplb,putinga-bwithoutsettingdestinationCFsetifcarryoutfrommostsignificantbit(parisons)ZFsetifa==bSFsetif(a-b)<0(assigned)OFsetiftwo’plement(signed)overflow(a>0&&b<0&&(a-b)<0)||(a<0&&b>0&&(a-b)>0)HunanUniversityConditionCodes(ExplicitSetting:Test)ExplicitSettingbyTestinstructiontestl/testqSrc2,Src1 ;Src1&Src2,影响标志位testlb,putinga&bwithoutsettingdestinationSetsconditioncodesbasedonvalueofSrc1&Src2UsefultohaveoneoftheoperandsbeamaskZFsetwhena&b==0SFsetwhena&b<0OF/binationsofconditioncodesSetXConditionDescriptionseteZFEqual/Zerosetne~ZFNotEqual/NotZerosetsSFNegativesetns~SFNonnegativesetg~(SF^OF)&~ZFGreater(Signed)setge~(SF^OF)GreaterorEqual(Signed)setl(SF^OF)Less(Signed)setle(SF^OF)|ZFLessorEqual(Signed)seta~CF&~ZFAbove(unsigned)setbCFBelow(unsigned)HunanUniversitymovl12(%ebp),%eax #eax=ycmpl%eax,8(%ebp) #Comparex:ysetg%al #al=x>ymovzbl%al,%eax #Zerorestof%eaxReadingConditionCodes(Cont.)SetXInstructions:binationofconditioncodesOneof8addressablebyteregistersDoesnotalterremaining3bytesTypicallyusemovzbltofinish