Microprocessor and InterfacesMicroprocessorandInterfaces

InternalRegisters

Internal Registers of 8086InternalRegistersof8086

The8086hasfourgroupsoftheuseraccessibleg pinternalregisters InstructionpointerData registers (4) Dataregisters(4)

Pointerregister(4) Index registerg Segmentregisters(4)

The 8086has atotal offourteen 16bitregistersi l di 16 bit i t ll d th t tincluding a16bitregistercalledthe statusregister,with9ofbitsimplementedforstatusandcontrolflagsg

Segment registersSegmentregisters

Mostofthe registers containdata/instructionost o t e eg ste s co ta data/ st uct ooffsetswithin64KBmemorysegment

Therearefourdifferent64 KBsegmentsforginstructions(code),stack,dataandextradata

Tospecifywherein1MBof processormemorythese4segmentsarelocatedtheprocessorusesfoursegmentregistersIt i ibl t h d f lt t d b Itispossibletochangedefaultsegmentsusedbygeneralandindexregistersbyprefixinginstructions with a CS SS DS or ES prefixinstructionswithaCS,SS,DSorESprefix

Code segment (CS)Codesegment(CS)

16bit register containing address of 64 KB16 bit registercontainingaddressof64KBsegmentwithprocessorinstructions

The processor uses CS segment for allTheprocessorusesCSsegmentforallaccessestoinstructionsreferencedbyinstructionpointer(IP)register

CSregistercannotbechangeddirectly The CS register is automatically updatedTheCSregisterisautomaticallyupdatedduringfarjump,farcall andfarreturninstructions

Stack segment (SS)Stacksegment(SS)

16bit register containing address of 64KB16 bitregistercontainingaddressof64KBsegmentwithprogramstack

By default the processor assumes that all data Bydefault,theprocessorassumesthatalldatareferencedbythestack pointer(SP)andbasepointer (BP) registers is located in the stackpointer(BP)registersislocatedinthestacksegmentSS i b h d di l i POP SSregister canbechangeddirectlyusingPOPinstruction

Data segment (DS)Datasegment(DS)

16bit register containing address of 64KB16 bitregister containingaddressof64KBsegmentwithprogramdata

By default the processor assumes that all Bydefault,theprocessorassumesthatalldata referencedbygeneralregisters(AX,BX,CX DX) and index register (SI DI) is located inCX,DX)andindexregister(SI,DI)islocatedinthedatasegmentDS i b h d di l i POP DSregistercanbechangeddirectlyusingPOPandLDSinstructions

Extra segment (ES)Extrasegment(ES)

16bit register containing address of 64KB16 bitregistercontainingaddressof64KBsegment,usuallywithprogramdata

By default the processor assumes that the DI Bydefault,theprocessorassumesthattheDIregisterreferencestheESsegmentinstringmanipulation instructionsmanipulationinstructions

ESregistercanbechangeddirectlyusingPOPd LES i iandLESinstructions

General RegistersGeneralRegisters

All general registers of the 8086Allgeneralregistersofthe8086microprocessorcanbeusedforarithmeticandlogic operationslogic operations

Accumulator registerAccumulatorregister

Accumulator register consists of two 8bitAccumulatorregisterconsistsoftwo8 bitregistersALandAH,whichcanbecombinedtogether and used as a 16bit register AXtogetherandusedasa16 bitregisterAX

AL inthiscasecontainstheloworderbyteofthe word and AH contains the high order bytetheword,andAH containsthehighorderbyte

AccumulatorcanbeusedforI/Ooperationsd i i l iandstringmanipulation

Base registerBaseregister

Base register consists of two 8bit registers BLBaseregisterconsistsoftwo8 bitregistersBLandBH,whichcanbecombinedtogetherandusedas a16bitregisterBX

BLin thiscasecontainstheloworderbyteof theword,andBHcontainsthe highorderbyte

BXregisterusuallycontainsadatapointerusedforbased,basedindexedorregisterindirectaddressing

Count registerCountregister

Count register consists of two 8bit registersCountregisterconsistsoftwo8 bitregistersCL andCH,whichcanbecombinedtogetherandusedas a16bitregisterCX

Whencombined,CLregistercontainstheloworderbyteoftheword,andCH containsthehighorderbyte

CountregistercanbeusedinLoop,shift/rotateinstructionsandasacounterinstringmanipulation

Data registerDataregister

Dataregisterconsistsoftwo8bitregistersDLg gandDH,whichcanbecombinedtogetherandusedas a16bitregisterDX

When combined DL register contains the low Whencombined,DLregistercontainstheloworderbyteoftheword,andDH containsthehighorderbyte

Dataregistercanbeusedasa portnumberinI/OoperationsI i 32 bi l i l d di id i i Ininteger32bitmultiplyanddivideinstructiontheDXregistercontainshighorderwordoftheinitialorresultingnumberg

General and Index registersGeneraland Indexregisters

StackPointer(SP)isa16bitregisterpointingStac o te (S ) s a 6 b t eg ste po t gto programstack

BasePointer(BP)isa16bitregisterpointingg p gto datainstacksegment.BPregisterisusuallyusedforbased,basedindexedorregisterindirectdd iaddressing

SourceIndex(SI)isa16bitregister.SIisusedforindexed based indexed and register indirectindexed,basedindexedandregisterindirectaddressing,aswellasasourcedataaddressin stringmanipulationinstructionsg p

General and Index registersGeneraland Indexregisters

Destination Index (DI) is a 16bit register DI isDestinationIndex(DI)isa16 bitregister.DIisusedforindexed,basedindexedandregisterindirect addressing as well as a destinationindirectaddressing,aswellasadestinationdataaddressinstringmanipulationinstructionsinstructions

InstructionPointer(IP)isa16bitregister

FlagsFlags

OverflowFlag(OF):Setiftheresultistoolargeg ( ) gpositivenumber,oristoosmallnegativenumbertofit intodestinationoperand

Direction Flag (DF) if set then string DirectionFlag(DF): ifset thenstringmanipulationinstructionswillautodecrementindexregisters.Ifclearedthentheindexregisterswill beautoincremented

Interruptenable Flag(IF) setting this bit enablesmaskable interruptsmaskable interrupts

SinglestepFlag(TF)ifsetthensinglestepinterruptwilloccurafterthenext instructionp

FlagsFlags

SignFlag(SF)setifthemostsignificantbitoftheg g ( ) gresultisset

ZeroFlag(ZF)setifthe resultiszero( ) AuxiliarycarryFlag(AF) setif there was

a carry fromorborrowtobits03intheALregisterregister

ParityFlag(PF)setifparity(thenumberof"1"bits)intheloworderbyteoftheresultis even

CarryFlag(CF)setiftherewasacarryfromorborrowtothemostsignificantbitduringlastresult calculationresultcalculation