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Appendix1:6502InstructionSet

It isconvenientto divide up the 56instructionsinto fourgroups,depending uponhow many of the bits intheirop codes are fixed.

Group1: Five Bits Fixed

Instructionsin thisgrouphave fixed (f) andvariable(v) bits as follows fffvvvff

Thereare twosubgroupstoconsider.

GrouplA:8AddressingModes

Thesemodesare

vvvA10de

000Indexedindirect(seeappendix3)

001

Zeropage

010Immediate(notSTA)

011Absolute

100Indirectindexed

101

Zeropage,indexedX

110Absolute,indexedY

111Absolute,indexedX

Theinstructionsin thisgroupare

ADC,AND,CMP,EaR,LDA,ORA,SBC,STA

225

GroupIB: 5AddressingModes

The modes are

vvvMode

000Immediate(not ASL, LSR,ROL,ROR)

001Zeropage

010Accumulator(not LDX, LDY)

011Absolute

101Zeropage, indexed X (indexed Y in LDX)

111Absolute,indexed X (indexed Y in LDX)

Theinstructionsin this group are

ASL,LDX,LDY, LSR,ROL,

ROR

Group2: Six Bits Fixed

Thereare two subgroups.

Group2A

These have fixed (f) and variable (v) bits as follows fffvvfff

The addressing modes are

vvvMode

00Zeropage

01Absolute

10Zeropage, indexed X (indexed Y in STX)

11Absolute,indexed X (not STX, STY)

Theinstructionsin this group are

DEC,INC, STX, STY

Appendix1:6502InstructionSet

Group2B

Thesehave fixed (f) andvariable(v) bits as follows ffffvvff

Theaddressingmodesare

vvA10de

00Immediate

01

Zeropage

11 l\bsolute

Theinstructionsin thisgroupare

CPX,Cpy

Group3: Seven Bits Fixed

Thereare twosubgroups.

Group3A

This has a fixed (f) andvariable(v) bitpatternof

ffffvfff

Themodesare

vA10de oZeropage 1 l\bsolute

The onlyinstructionin thisgroupis BIT.227

Group3B

This has a fixed (f) andvariable(v) bitpatternof

ffvfffff

Themodesare

vMode ol\bsolute

1Indirect

Theonlyinstructionin thisgroupisJMP.

Group4: All Bits Fixed

Theseare theimpliedandrelativeaddressingmodeinstructions CLV,DEX, DEY,INX,INY,NOP,PHl\,PHP,PLA,PLP,RTI,RTS, modeforJSR.

As anexampleof this,consider

LDA.This is ingrouplA,andthefixed

bits are 101vvv01.Takingeachset ofvaluesfor vvv inturnwearriveat the opcodesAI,A5, A9,AD,B1, B5, B9 andBD. In thedetailedsummarythatfollows, the fixed bytes will be given for eachmnemonic,andtheneachaddressingmodewill haveattachedits own variablebits. Thisinformationis useful if onewishes toconstructan assemblerordisassembler,forexample.

Appendix1:6502InstructionSet

Abbreviationsin TableAI.I

229
t n t v M (M) M 6 M r LOOP N Z C V I D B A X,Y P S PC i t V V

Plus 1 cycle ifpageboundarycrossed

Plus 1 cycle if

branchoccurs;plus 2 cycles ifbranchcrosses into anotherpage

Avariablebit in

theopcode

Anarbitrarymemorylocation(thatis, anaddress)

ThecontentsofM

Thecontentsof bit6ofM

Theone'scomplementof(M)

Asignedbyte(thatis, &00 to &7F is+0to + 127; &80 to &FFis-128to-1)

Anarbitrarylabel(thatis, anaddress)

Thenegativeflag

Thezeroflag

Thecarryflag

Theoverflowflag

Theinterruptdisableflag

Thedecimalmodeflag

Thebreakflag

Theaccumulator

Theindexregisters

Theprocessorstatusregister

Thestackpointer

theinstruction)

Copytomemorylocationorregister

Copytostack(thatis,push)

Transferfromstack(thatis,pull)

OR AND

Exclusive-OR

Signedaddition

(thatis,secondbyteistreatedas asigned byte)

Flagisaffectedbyinstruction

Flagis

notaffectedbyinstruction

230AssemblyLanguageProgrammingfor theBBeMicrocomputer

Table

A.l.lAlphabeticalsummaryofinstructionset

Descriptionof"'Dc.M SymbolicoperationofAoctv{

Addn:,e.,Content.!.ofMAeM)+C _A.

ro\-\,e.

C::CLt't-_,,\eo.....-.C.

H'le..In

o.ncl l"

Flag'affected

5:liIiliJ

DIE]

Fixed bitpattern

OIIV\lVOI

DescriptionofANDM

l-h..\ANO

Op&t-at-ionb,,"en

H,e.c_..b\t"soj

(M)Clnd.. h\t,.I"'"'

OC:C.U!9\U\a

SymbolicoperationofMFlag'affected

5:liIiliJ

Fixedbitpattern

001vvVot

ofMLN\SymbolicoperationofAS.LN\Flags affec.tedFixedbitpattern oooVVV'O

Appendix1:6502 Instruction Set

Bc.e 231

Des'riptionof

eac.c.Loop

IfC-0)bt-o",c.'""0

M,e.i".n.uc.iionlob..lIe.d

LooP

Des'riptionof

&c.$LooP

IfCo:I,b"ot\c.hto

\n..h·uc.no"\o'=»e.lled LooP

Symbolic operation of

&c.c:.t

IfC=C>:

Pc.."Z.....Pc.

IIcI: ho

Symbolic operation of

I-

IfC:.I:

PC. ...'24iDt..t=>c.

Ifc:o nono",

Addressing mode

Addressing modeOpcede

Opcode

Bo ntFlagsaffected tm zc V 22
(ltFlagsaffected zztm

Des,riptionof

BEG»

l.ooP

IfZ.cIIbc-onc.n\"t)

H,..\obc\led

LooP

Symbolic operation of

&E.ca.r

Ifz..i .

..Pc 'Jz.·o no

BE:.ca

Addressing mode

Opcode

FO (ltFlagsaffected 2z t m

SymbolicoperationofBlT""FlagsFixed bit path!m

3=1AND.

it.pe.r.-...u\"" • •tbIifquotesdbs_dbs10.pdfusesText_16

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