Moving Forth Part 3.docx

Moving Forth Part 3.docx

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MovingForthPart3

MOVINGFORTH

Part3:

DemystifyingDOES>

byBradRodriguez

ThisarticlefirstappearedinTheComputerJournal#62（July/August1993）.

OOPS!

There'sacolossalmistakeinoneofmy6809designdecisionsinthepreviousinstallment.ItbecameevidentwhenIstartedtocodetheForthwordEXECUTE.

EXECUTEcausestheexecutionofasingleForthword,whoseaddressisgivenontheParameterStack.（Tobeprecise:

thecompilationaddress,a.k.a.CodeFieldAddress,isgivenonthestack.）ThiscanbeanykindofForthword:

CODEdefinition,colondefinition,CONSTANT,VARIABLE,ordefinedword.ThisdiffersfromtheusualForthinterpretationprocessinthattheaddressoftheword-to-executeisgivenonthestack,andnottakenfromthe"thread"（aspointedtobyIP）.

Inourdirect-threaded6809codethiscanbeeasilycoded:

EXECUTE:

TFRTOS,WputaddressofwordinW

PULUTOSpopnewTOS

JMP,WjumptoaddressgiveninW

Note:

thisisJMP,WandnotJMP[,W],sincewealreadyhavethecodeaddressoftheword.We'renotfetchingfromthehigh-levelthread.（IfTOSwasn'tinregister,EXECUTEcouldbedonewithsimplyJMP[,PSP++].）NowsupposethatthisEXECUTEdwordisacolondefinition.WwillbepointingtoitsCodeField,whichcontainsJMPENTER.Thisdoesthefollowing（describedinthepreviousarticle）:

JMPENTER

...

ENTER:

PSHSIP

LDX-2,IPre-fetchtheCodeFieldaddress

LEAY3,X

NEXT

Thisisthemistake!

Wearenotexecutingthiswordfromwithinathread,soIPwasnotpointingtoacopyofitsCodeFieldaddress!

（Remember,theaddressoftheword-to-EXECUTEcamefromthestack.）ThisformofENTERwillnotworkwithEXECUTE,becausethereisnowaytofindtheaddressofthewordbeingexecuted!

ThissuggestsanewgeneralruleforDTCForths:

ifNEXTdoesNOTleavetheaddressoftheword-being-executedinaregister,youMUSTuseaCallinthecodefield.

So,the6809ForthisbacktousingaJSRintheCodeField.ButtoavoidthespeedpenaltyforENTER--oneofthemost-usedcodefragmentsinForth--I'llcompletethe"exerciseforthestudent"fromthelastarticle.NotewhathappensifyouswaptheregistersassignedtoRSPandPSP:

withRSP=S,withRSP=U,

andPSP=UandPSP=S

（previous）（new）

JSRENTERJSRENTER

......

ENTER:

PULSWPSHUIPpusholdIPontoRstack

PSHSIPPULSIPpopnewIPfromJSRstack

TFRW,IPNEXT

NEXT

Thenewversionexecutesin31cycles,thesameastheJMPversionIhadwantedtouse.TheimprovementisbecausetheJSRversionofENTERmustusebothForth'sReturnStack,andthe6809subroutine-returnstack（"JSRstack"）.Usingtwodifferentstackpointersmeanswedon'thaveto"swap"thetop-of-stackwithIP,eliminatingtheneedforatemporaryregister.

ThisillustratestheusualdevelopmentprocessforanewForthkernel:

makesomedesigndecisions,writesomesamplecode,discoverabugorabetterwaytodothings,throwoutsomecode,changesomedesigndecisions,rewritesomesamplecode,loopuntilsatisfied.（Thisistheprogrammingequivalentofa"ripup"PCboardautorouter.）

Thisteachesanimportantlesson:

makeEXECUTEoneofyourbenchmarkwords!

OOPS,AGAIN

CareyBloodworthofVanBuren,ARhaspointedoutaminorbutembarassingmistakeinmy6809codeinthepreviousinstallment.Forthe"TOS-in-memory"versionof0=,Ishowedthecodefragment

LDD,PSP

CMPD#0

totestfortop-of-stackequallingzero.Inthiscase,theCMPDinstructioniscompletlysuperfluous,sincetheLDDinstructionwillsettheZeroflagifDiszero!

（TheTOS-in-DversionstillrequirestheCMPDinstruction,butremainsfasterthanTOS-in-memory.）

Now,ontoourmaintopic:

WHAT'SACODEFIELD?

TheDOES>conceptseemstobeoneofthemostmisunderstoodandmystifyingaspectsofForth.YetDOES>isalsooneofForth'smostpowerfulfeatures--inmanyways,itanticipatedobject-orientedprogramming.TheactionandpowerofDOES>hingesuponabrilliantinnovationofForth:

theCodeField.

RecallfromPart1thatthe"body"ofaForthdefinitionconsistsoftwoparts:

theCodeField,andtheParameterField.Youcanthinkofthesetwofieldsinseveralways:

*TheCodeFieldisthe"action"takenbythisForthword,andtheParameterFieldisthedataonwhichitacts.

*TheCodeFieldisasubroutinecall,andtheParameterFieldisparametersthatareincluded"in-line"afterthecall.（Theassemblylanguageprogrammer'sview.）

*TheCodeFieldisthesingle"method"forthis"class"ofwords,andtheParameterFieldcontainsthe"instancevariables"forthisparticularword.（Theobject-orientedprogrammer'sview.）

Commonfeaturesappearinalltheseviews:

*TheCodeFieldroutineisalwayscalledwithatleastoneargument,namely,theaddressoftheParameterFieldfortheForthwordbeingexecuted.TheParameterFieldmaycontainanynumberofparameters.

*Therearerelativelyfewdistinctactions,i.e.,relativelyfewdistinctroutinesreferencedbytheCodeField.Eachoftheseroutinesiswidelyshared（exceptforCODEwords,aswewillseelater）.Recall,forexample,theENTERroutinefromPart2:

thiscommonroutineisusedbyallForthcolondefinitions.

*TheinterpretationoftheParameterFieldisimplicitlydeterminedbythecontentsoftheCodeField.I.e.,eachCodeFieldroutineexpectstheParameterFieldtocontainacertainkindofdata.

AtypicalForthkernelwillhaveseveralCodeFieldroutinespredefined.

CodeFieldParameterField

routinecontents

ENTERahigh-level"thread"（seriesofaddresses）

DOCONaconstantvalue

DOVARastoragelocationfordata

DOVOCvocabularyinfo（variesbyimplementation）

WhatmakesthisfeaturepowerfulisthataForthprogramisnotlimitedtothissetofCodeFieldroutines（orwhateversetisprovidedinyourkernel）.TheprogrammercandefinenewCodeFieldroutines,andnewParameterFieldstomatch.Inobject-orientedlingo,new"classes"and"methods"canbecreated（althougheachclasshasonlyonemethod）.And--likeForthwordsthemselves--theCodeFieldactionscanbedefinedineitherassemblylanguageorhigh-levelForth!

TounderstandthemechanismoftheCodeField,andhowparametersarepassed,wewillfirstlookatthecaseofassembly-language（machinecode）actions.We'llstartwithIndirectThreading（ITC）,sinceitistheeasiesttounderstand,andthenseehowthelogicismodifiedinDirect-Threaded（DTC）andSubroutine-Threaded（STC）Forths.Then,we'lllookathowtheCodeFieldactioncanbewritteninhigh-levelForth.

Forthwrightsaresomewhatinconsistentintheirterminology,soI'lldefinemyterms,usingtheITCForthwordillustratedinFigure1.TheHeadercontainsthedictionaryinformation,andisn'tinvolvedintheexecutionoftheForthword.TheBodyisthe"working"partoftheword,andconsistsofthefixed-lengthCodeField,andthevariable-lengthParameterField.Foranygivenword,thelocationsofthesetwofieldsinmemoryaretheCodeFieldAddress（CFA）andtheParameterFieldAddress（PFA）,respectively.TheCodeFieldAddressofawordistheaddressinmemorywhereitsCodeFieldislocated.ThisisnottobeconfusedwiththecontentsoftheCodeField,which,inITCForths,isanotherdifferentaddress.Tobespecific,thecontentsoftheCodeFieldistheaddressofafragmentofmachinecodesomewhereelseinmemory.IwillrefertothisastheCodeAddress.Later,whenindiscussingDTCandSTCForths,Iwillalsorefertothe"CodeFieldcontents,"whichwillincludemorethanjusttheCodeAddress.

MACHINE-CODEACTIONS

ForthCONSTANTsareprobablythesimplestexampleofamachine-codeaction.Let'sconsidersomegoodFrancophoneconstants

1CONSTANTUN

2CONSTANTDEUX

3CONSTANTTROIS

ExecutingthewordUNwillpushthevalue1ontotheForthParameterStack.ExecutingDEUXwillpusha2ontothestack,andsoon.（Don'tconfuseParameterStackwithParameterField;theyareentirelyseparate.）

IntheForthkernelthereisasinglewordcalledCONSTANT.Thisisnotaconstant-typeworditself;itisahigh-levelForthdefinition.CONSTANTisa"definingword":

itcreatesnewwordsintheForthdictionary.Herewecreatethenew"constant-type"wordsUN,DEUX,andTROIS.（Youmaythinkoftheseas"instances"ofthe"class"CONSTANT.）ThesethreewordswillhavetheirCodeFieldspointingtoamachinecodefragmentthatdoestheactionofCONSTANT.

Whatmustthiscodefragmentdo?

Figure2showsthememoryrepresentationofthethreeconstants.Allthreewordspointtoacommonactionroutine.ThedifferenceinthewordsisentirelycontainedintheirParameterFields,which,inthiscase,simplyholdtheconstantvalues（"instancevariables"inobjectlingo）.So,theactionofthesethreewordsshouldbefetchthecontentsoftheParameterField,andpushthisontothestack.Thecodeunderstandsimplicitlythattheparameterfieldcontainsasingle-cellvalue.

Towriteamachine-codefragmenttodothis,weneedtoknowhowtofindtheParameterFieldAddress,aftertheForthinterepreterjumpstothemachinecode.Thatis,howisthePFApassedtothemachine-coderoutine?

This,inturn,dependsonhowtheForthinterpreterNEXThasbeencoded,whichvariesfromimplementationtoimplementation.Towritemachine-codeactions,wemustunderstandNEXT.

TheITCNEXTwasdescribedinpseudo-codeinPart1.Here'soneimplementationforthe6809,usingY=IPandX=W:

NEXT:

LDX,Y++;（IP）->W,andIP+2->IP

JMP[,X];（W）->temp,JMP（temp）

Supposethatwe'reinahigh-levelthread

...SWAPDEUX+...

withtheInterpreterPointer（IP）pointingtotheDEUX"instruction,"whenNEXTisexecuted.（ThiswouldbeattheveryendofSWAP.）Figure3illustrateswhathappens.IP（registerY）