6.42

IDT7005S/L

High-Speed 8K x 8 Dual-Port Static RAM

Military, Industrial and Commercial Temperature Ranges

18

cause either signal (SEM or OE) to go inactive or the output will

never change.

AsequenceWRITE/READmustbeusedbythesemaphoreinorder

to guarantee that no system level contention will occur. A processor

requestsaccesstosharedresourcesbyattemptingtowriteazerointoa

semaphorelocation.Ifthesemaphoreisalreadyinuse,thesemaphore

requestlatchwillcontainazero,yetthesemaphoreflagwillappearasone,

a fact which the processor will verify by the subsequent read (see Truth

TableV).Asanexample,assumeaprocessorwritesazerototheleftport

at a free semaphore location. On a subsequent read, the processor will

verifythatithaswrittensuccessfullytothatlocationandwillassumecontrol

overtheresourceinquestion.Meanwhile,ifaprocessorontherightside

attempts to write a zero to the same semaphore flag it will fail, as will be

verifiedbythefactthataonewillbereadfromthatsemaphoreontheright

side during subsequent read. Had a sequence of READ/WRITE been

usedinstead,systemcontentionproblemscouldhaveoccurredduringthe

gap between the read and write cycles.

Itisimportanttonotethatafailedsemaphorerequestmustbefollowed

by either repeated reads or by writing a one into the same location. The

reasonforthisiseasilyunderstoodbylookingatthesimplelogicdiagram

ofthesemaphoreflaginFigure4.Twosemaphorerequestlatchesfeed

into a semaphore flag. Whichever latch is first to present a zero to the

semaphoreflagwillforceitssideofthesemaphoreflagLOWandtheother

side HIGH. This condition will continue until a one is written to the same

semaphorerequestlatch.Shouldtheotherside’ssemaphorerequestlatch

havebeenwrittentoazerointhemeantime,thesemaphoreflagwillflip

overtotheothersideassoonasaoneiswrittenintothefirstside’srequest

latch.Thesecondside’sflagwillnowstayLOWuntilitssemaphorerequest

latchiswrittentoaone.Fromthisitiseasytounderstandthat,ifasemaphore

is requested and the processor which requested it no longer needs the

resource, the entire system can hang up until a one is written into that

semaphorerequestlatch.

The critical case of semaphore timing is when both sides request a

single token by attempting to write a zero into it at the same time. The

semaphorelogicisspeciallydesignedtoresolvethisproblem.Ifsimulta-

neousrequestsaremade,thelogicguaranteesthatonlyonesidereceives

the token. If one side is earlier than the other in making the request, the

firstsidetomaketherequestwillreceivethetoken.Ifbothrequestsarrive

at the same time, the assignment will be arbitrarily made to one port or

the other.

One caution that should be noted when using semaphores is that

semaphoresalonedonotguaranteethataccesstoaresourceissecure.

Aswithanypowerfulprogrammingtechnique,ifsemaphoresaremisused

or misinterpreted, a software error can easily happen.

Initializationofthesemaphoresisnotautomaticandmustbehandled

viatheinitializationprogramatpower-up.Sinceanysemaphorerequest

flagwhichcontainsazeromustberesettoaone,allsemaphoresonboth

sidesshouldhaveaonewrittenintothematinitializationfrombothsides

to assure that they will be free when needed.

UsingSemaphores—SomeExamples

Perhapsthesimplestapplicationofsemaphoresistheirapplicationas

resourcemarkersfortheIDT7005’sDual-PortRAM.Saythe8Kx8RAM

wastobedividedintotwo4Kx8blockswhichweretobededicatedatany

onetimetoservicingeithertheleftorrightport.Semaphore0couldbeused

toindicatethesidewhichwouldcontrolthelowersectionofmemory,and

configurations.TheIDT7005doesnotuseitssemaphoreflagstocontrol

anyresourcesthroughhardware,thusallowingthesystemdesignertotal

flexibilityinsystemarchitecture.

An advantage of using semaphores rather than the more common

methodsofhardwarearbitrationisthatwaitstatesareneverincurredin

either processor. This can prove to be a major advantage in very

high-speedsystems.

How the Semaphore Flags Work

Thesemaphorelogicisasetofeightlatcheswhichareindependent

oftheDual-PortRAM.Theselatchescanbeusedtopassaflag,ortoken,

fromoneporttotheothertoindicatethatasharedresourceisinuse.The

semaphores provide a hardware assist for a use assignment method

called“TokenPassingAllocation.”Inthismethod,thestateofasemaphore

latchisusedasatokenindicatingthatsharedresourceisinuse.Iftheleft

processorwantstousethisresource,itrequeststhetokenbysettingthe

latch.Thisprocessorthenverifiesitssuccessinsettingthelatchbyreading

it. If it was successful, it proceeds to assume control over the shared

resource.Ifitwasnotsuccessfulinsettingthelatch,itdeterminesthatthe

rightsideprocessorhassetthelatchfirst, hasthetokenandisusingthe

sharedresource.Theleftprocessorcantheneitherrepeatedlyrequest

thatsemaphore’sstatusorremoveitsrequestforthatsemaphoretoperform

anothertaskandoccasionallyattemptagaintogaincontrolofthetokenvia

thesetandtestsequence.Oncetherightsidehasrelinquishedthetoken,

theleftsideshouldsucceedingainingcontrol.

ThesemaphoreflagsareactiveLOW.Atokenisrequestedbywriting

azerointoasemaphorelatchandisreleasedwhenthesamesidewrites

aonetothatlatch.

The eight semaphore flags reside within the IDT7005 in a separate

memoryspacefromtheDual-PortRAM.Thisaddressspaceisaccessed

byplacingaLOWinputontheSEMpin(whichactsasachipselectforthe

semaphore flags) and using the other control pins (Address, OE, and

R/W) as they would be used in accessing a standard static RAM. Each

of the flags has a unique address which can be accessed by either side

the other address pins has any effect.

iswrittenintoanunusedsemaphorelocation,thatflagwillbesettoazero

on that side and a one on the other side (see Truth Table V). That

semaphorecannowonlybemodifiedbythesideshowingthezero.When

aoneiswrittenintothesamelocationfromthesameside,theflagwillbe

settoaoneforbothsides(unlessasemaphorerequestfromtheotherside

ispending)andthencanbewrittentobybothsides. Thefactthattheside

whichisabletowriteazerointoasemaphoresubsequentlylocksoutwrites

fromtheothersideiswhatmakessemaphoreflagsusefulininterprocessor

communications.(Athoroughdiscussionontheuseofthisfeaturefollows

shortly.) A zero written into the same location from the other side will be

storedinthesemaphorerequestlatchforthatsideuntilthesemaphoreis

freed by the first side.

Whenasemaphoreflagisread,itsvalueisspreadintoalldatabitsso

thataflagthatisaonereadsasaoneinalldatabitsandaflagcontaining

azeroreadsasallzeros.Thereadvalueislatchedintooneside’soutput

registerwhenthatside'ssemaphoreselect(SEM)andoutputenable(OE)

signalsgoactive.Thisservestodisallowthesemaphorefromchanging

stateinthemiddleofareadcycleduetoawritecyclefromtheotherside.

Becauseofthislatch,arepeatedreadofasemaphoreinatestloopmust