微机原理ch03assemblylanguageprogramming

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1、3ASSEMBLYLANGUAGEPROGRAMMING3.1DIRECTIVESANDSAMPLEPROGRAMAgivenassemblylanguageprogramisaseriesofstatements,orlines,whichareeitherassemblylanguageinstructionssuchasMOVorADD,orstatementscalleddirectives.;THEFORMOFANASSEMBLYLANGUAGEPROGRAM;NOTE:USINGSIMPLIFIEDSEGMENTDEFINITION.modelsm

2、all.stack64.datadata1db52hdata2db29hsumdb?.codemainprocfar;thisistheprogramentrypointmovax,@data;loadthedatasegmentmovds,ax;assignvaluetoDSmoval,data1;getthefirstoperandmovbl,data2;getthesecondoperandaddal,bl;addtheoperandsmovsum,al;storetheresultinlocationSUMmovah,4ch;setuptoreturn

3、DOSint21hmainendpp1procfarp1endpp2procfarp2endpendmain;thisistheprogramexitpoint图31SimpleassemblylanguageprogramDirectives(alsocalledpseudo-instructions)givedirectionstotheassemblerabouthowitshouldtranslatetheassemblylanguageinstructionsintomachinecode.Anassemblylanguageinstruction

4、consistsoffourfields:[label:]mnemonic[operands][;comment]3-15Bracketsindicatethatthefieldisoptional.lThelabelfieldallowsprogramtorefertoalineofcodebyname.Thelabelfieldcannotexceed31characters.lLabelsfordirectivesdonotneedendwithacolon.lAlabelmustendwithacolonwhenitreferstoanopcodege

5、neratinginstruction;thecolonindicatestotheassemblerthatthisreferstocodewiththiscodesegment.lTheassemblylanguagemnemonic(instruction)andoperand(s)fieldstogetherperformtherealworkoftheprogramandaccomplishthetasksforwhichtheprogramwaswritten.Inassemblylanguagesuchas:addal,blmovax,6764A

6、DDandMOVarethemnemonicopcodes,and“al,bl”and“ax,6764”aretheoperands.Thesetwofieldscouldcontainassemblerpseudo-instructions,ordirectives.Theyareusedbytheassemblertoorganizetheprogramaswellasotheroutputfiles.Directivesdonotgenerateanymachinecodeandareusedonlybytheassembler,suchasDB,END

7、.lThecommentfieldbeginswitha“;”theassemblerignorescomments3.1.1ModeldefinitionThefirststatementin图31afterthecommentsistheMODELdirective.Thisdirectiveselectsthesizeofthememorymodel.Amongtheoptionsforthememorymodelare:.modelsmall;thisdirectivedefinesthemodeassmallSMALLisoneofthemostw

8、idelyusedmemorymode