charc1willstorethecharacterwhichisreadc2willstoreacopyoftheKSPStatusppointertokeyboardportsp0x62while1c2pifc20x10breakifKSPsayskeyhitbreakc1p2readthecharacterfr ID: 104857
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ofas-ashowsthattheCALLwasato set0x000bofthe.textsegment.Sothelattermusthavestartedat0x804807f-0x000b=0x8048074.TheGDBsessionalsoshowsthatxwasatlocation0x80490a4.Sincexwasatthebeginningofthe.datasegment,thelattermustalsostartat0x80490a4.2.dOurrstquerytoGDBaboutc(ESP)hadbeenjustbeforetheCALL.Atthattime,thetwoargumentstoinit()hadalreadybeenpushedontothestack.Theexe-cutionoftheCALLandthentheREThadnonete ectonESP,sowhenwereachedtheADDwhichimmediatelyfollowedtheCALL,c(ESP)wasstillwhatithadbeenatourlastquery,0xb fcec8.TheADDthenadded8tothatvalue,yielding0xb fced0.ESPneverchangedafterthat.3.ap/x$ebp3.bTheinstructionmovl$12(%ebp),%eaxpointedEAXtothebeginningofargv,i.e.argv[0].Thenextline,addl$4,%eaxmovedEAXtopointtoargv[1].Ifwewantargv[3]instead,thenthenumberaddedshouldbe12insteadof4.4.Ofcourse,justbeforetheCALL,weneedtopushthetwoargumentstoscanf(),whichare$xand$fmt.NotethattherstofthoseshouldNOTbex,incontrasttotheprintf()case;that'sbecauseourcallinthescanf()casewouldbescanf("%",&x)|notetheampersand.Butthere'smore.Wehavetoworryaboutwhatscanf()mightdotoourregistervalues.ItsayslaterinthenotesthattheCcompiler'scallingconventionisasfollows:Themodulebeingcompiled,inthiscasescanf(),mustsavethevaluesoftheESI,EDIandEBXregistersthathadbeenthereatthetimethecallingmodulemakesthecall.Inourcasehere,ourcallingmoduleneednotworrythatitsvalueinEBXmaybelost.Ontheotherhand,ourcallingmoduledoesuseECX,andthecompilerdoesNOTguaranteethatthevalueinthatregisterwillbepreserved;so,wemustsaveitonthestackbeforethecall,andrestoreitafterthecall.WecouldalsosaveandrestoreEDXforthesamereason,incasewelateraddcodewhichusesEDX.Inaddition,incallinganyCfunction,weknowthattheremaybeareturnpassedbackinEAX.ThatisespeciallytruefortheClibraryfunctions,whichgenerallygivearesultasareturnvalue.So,inourcallingmodulewemustprotectEAXtoo,bysavingitbeforethecallandrestoringitafterward.5. charc1,//willstorethecharacterwhichisreadc2,//willstoreacopyoftheKSPStatus*p;//pointertokeyboardports...p=0x62;while(1){c2=*p;if(!(c2&0x10))break;//ifKSPsayskeyhit,break}c1=*(p-2);//readthecharacterfromtheKDPc2|=0x20;*p=c2;//flickACKbitonc2&=0xdf;*p=c2;//restoreACKbitto02