华南理工大学汇编原理课件05-1.ppt

Chapter5,BranchingandLooping分支和循环,Textbook:7ProgramLogicandControl,Outline,Theobjectiveofthischapteristodescribethestructuressuchas:ifthen,ifthenelse(分支结构程序设计)while,until,for(循环结构程序设计),Transfer(控制转移类指令),Instructionsthatcantransfercontroloutsidethenormalsequentialflow.BychangingtheoffsetvalueinIPorchangingthevalueofIPandCS.jmp,jcc,loop,控制转移类指令通过改变IP（或同时修改CS和IP）值，实现程序执行顺序的改变,Short,nearandfaraddress,Theassemblersupportsthreetypesofaddressesthataredistinguishbytheirdistancefromthecurrentlyexecutingaddress.,3typesofaddresses-short,Ashortaddress,limitedtoadistanceof-128(80H)to127(7FH)bytes.Ashortaddressisreachedbya1-byteoffset.,3typesofaddresses-near,Anearaddress,limitedtoadistanceof-32768(8000H)to32767(7FFFH)byteswithinthesamesegment.Anearaddressisreachedbya1-wordor2-wordoffset.,3typesofaddresses-far,Afaraddress,whichmaybewithinthesamesegmentatadistanceover32K,orinanothersegment.Afaraddressisreachedbyasegmentaddressandanoffset.,Transferinstructions(控制转移类指令),Unconditional(无条件转移指令)Jump(跳转)Procedure(过程)Interrupt(中断)Conditional(条件转移指令)Looping(循环指令),UnconditionalJumps8086无条件转移,5.1,JMPInstruction,Unconditional:theoperationtransferscontrolunderallcircumstances.Operation:(IP)EB03,Examples,Theassemblergeneratesonebytefortheoperation(EB)andonebytefortheoperand.,Examples,Jmps-jmp000B-EB03,JMPshort/near/faraddress,UnconditionaltransferinstructionforJMPshort/near/faraddress,JMPInstruction,ForwardjumpBackwardjump,jmpquit;退出程序quit:movax,4c00h;退出并返回dosint21h,Examples,forever:movax,0jmpforever;重复,Examples,TransfertypeofJMP,Directaddressingvs.IndirectaddressingIntra-segmenttransfervs.Inter-segmenttransfer,Intra-segmentvs.Inter-segment,实际编程时，汇编程序会根据目标地址的距离，自动处理成短转移、近转移或远转移程序员可用操作符short、nearptr或farptr强制,Directaddressingvs.Indirectaddressing,Directaddressing(直接寻址方式)转移地址象立即数一样，直接包含在指令的机器代码中，就是直接寻址方式Indirectaddressing(间接寻址方式)转移地址在寄存器或主存单元中，就是通过寄存器或存储器的间接寻址方式,用标号表达,用寄存器或存储器操作数表达,TransfertypeofJMP,4types,Types,Intra-segmentdirectaddressing段内转移直接寻址Intra-segmentindirectaddressing段内转移间接寻址Inter-segmentdirectaddressing段间转移直接寻址Inter-segmentIndirectaddressing段间转移间接寻址,Intra-segmentdirectaddressing,jmpagain;transferagain:deccx;label“again”jmpoutput;transferoutput:movresult,al;label“output”,Examples,实际为相对寻址,JMPlabel;IPIP+shift,(2)Intra-segmentindirectaddressing,jmpax;IPAXjmpwordptr2000h;IP2000h,Examples,JMPr16/m16;IPr16/m16,Inter-segmentdirectaddressing,jmpfarptrotherseg;othersegisinanothersegment,Examples,JMPfarptrlabel;IPlabel的偏移地址;CSlabel的段地址,Inter-segmentdirectaddressing,jmpfarptrs,Examples,Inter-segmentIndirectaddressing,movwordptrbx,0movwordptrbx+2,1500hJMPfarptrbx;转移到1500h:0,Examples,JMPfarptrmem;IPmem，CSmem+2,JMP指令：段内相对转移,JMP指令：段内寄存器间接转移,JMP指令：段间直接转移,JMP指令：段间间接转移,ConditionalJumps,CompareInstructions,andifStructure条件转移、比较指令和if结构,5.2,Conditionaljumpinstruction,Theprocessorsupportsavarietyofconditionaljumpinstructionsthattransfercontroldependingonsettingsintheflagregister.,DECCXJNZA20;testthesettingofZF,Examples,label:Jnnnshort-address;条件满足，发生转移：IPIP8位位移量;条件不满足，顺序执行,Conditionaljumpinstruction:CMP,cmpal,100jbbelow;ifal100,jumptobelowsubal,100;else(al100),alal-100below:,执行比较指令之后，可以根据标志判断两个数是否相等、大小关系等,Examples,Conditionaljumpinstruction,Forthe8086286,thedistanceforaconditionaljumpmustbeashort,within128to+127byte.Iftheoperationexceedsthelimit,theassemblerissuesamessage“relativejumpoutofrange”.The80386andlaterprocessorsallowreachinganyaddresswithin32K(=215).,label:Jnnnshort-address,JnnnInstructions,JnnninstructionstransfercontroldependingonsettingsintheFlagsregister.(Table)notaffectflags1.JumpsbasedonUnsigned(logical)data2.Jumpbasedonsigned(Arithmetic)data3.Specialarithmetictests1比较无符号数高低2比较有符号数大小3判断单个标志位状态及其他,JnnnInstructions,实际虽然指令只有16条，但却有30个助记符采用多个助记符，只是为了方便记忆和使用,JumpsbasedonUnsigned(logical)data(无符号数高低),Above-Equal-Below,JumpsbasedonUnsigned(logical)data,cmpax,bxjnbnext;jumpifaxbxxchgax,bxnext:.,Examples,JumpsbasedonSigned(Arithmetic)data(有符号数大小),Greater-Equal-Less,JumpsbasedonSigneddata,cmpax,bxjnlnext;jumptonextifaxbxxchgax,bxnext:.,Examples,Specialarithmetictests,JZ/JNZ,testal,80h;测试最高位jznext0;D70（ZF1），转移movah,0ffh;D71，顺序执行jmpdone;无条件转向next0:movah,0done:.testal,80h;测试最高位jnznext1;D71（ZF0），转移movah,0;D70，顺序执行jmpdone;无条件转向next1:movah,0ffhdone:.,Examples,JS/JNS,;计算|XY|（绝对值）;X和Y为存放16位操作数的变量;结果存入变量resultmovax,Xsubax,Yjnsnonnegnegax;neg是求补指令nonneg:movresult,ax,Examples,JO/JNO,;计算XY;X和Y为存放16位操作数的变量;若溢出，则转移到overflow处理movax,Xsubax,Yjooverflow.;无溢出，结果正确overflow:.;有溢出处理,Examples,JP/JNP,;设字符的ASCII码在AL寄存器中;将字符加上奇校验位;在字符ASCII码中为“1”的个数已为奇数时;则令其最高位为“0”;否则令最高位为“1”andal,7fh;最高位置0，同时判断1的个数jnpnext;个数已为奇数，则转向nextoral,80h;否则，最高位置“1”next:.,Examples,JC/JNC,;记录BX中1的个数xoral,al;AL0，CF0again:testbx,0ffffh;等价于cmpbx,0jenextshlbx,1jncagainincaljmpagainnext:.;AL保存1的个数,Examples,记录BX中“1”的个数,xoral,al;AL0，CF0again:cmpbx,0jznextshlbx,1;也可使用shrbx,1adcal,0jmpagainnext:.;AL保存1的个数,Examples,法2,JCXZ,JCXZjustteststhecontentsoftheCXregisterforzero,notaffecta