7024S/L
High-Speed 4K x 16 Dual-Port Static RAM
Military, Industrial and Commercial Temperature Ranges
it. If it was successful, it proceeds to assume control over the shared
Itisimportanttonotethatafailedsemaphorerequestmustbefollowed
resource.Ifitwasnotsuccessfulinsettingthelatch,itdeterminesthatthe byeitherrepeatedreadsorbywritingaoneintothesamelocation. The
rightsideprocessorhassetthelatchfirst, hasthetokenandisusingthe reasonforthisiseasilyunderstoodbylookingatthesimplelogicdiagram
sharedresource.Theleftprocessorcantheneitherrepeatedlyrequest ofthesemaphoreflaginFigure4.Twosemaphorerequestlatchesfeed
thatsemaphore’sstatusorremoveitsrequestforthatsemaphoretoperform into a semaphore flag. Whichever latch is first to present a zero to the
anothertaskandoccasionallyattemptagaintogaincontrolofthetokenvia semaphoreflagwillforceitssideofthesemaphoreflagLOWandtheother
thesetandtestsequence.Oncetherightsidehasrelinquishedthetoken, sideHIGH.Thisconditionwillcontinueuntilaoneiswrittentothesame
theleftsideshouldsucceedingainingcontrol.
semaphorerequestlatch.Shouldtheotherside’ssemaphorerequestlatch
ThesemaphoreflagsareactiveLOW.Atokenisrequestedbywriting havebeenwrittentoazerointhemeantime,thesemaphoreflagwillflip
azerointoasemaphorelatchandisreleasedwhenthesamesidewrites overtotheothersideassoonasaoneiswrittenintothefirstside’srequest
aonetothatlatch.
latch.Thesecondside’sflagwillnowstayLOWuntilitssemaphorerequest
The eight semaphore flags reside within the IDT7024 in a separate latchiswrittentoaone.Fromthisitiseasytounderstandthat,ifasemaphore
memoryspacefromtheDual-PortRAM.This addressspaceisaccessed is requested and the processor which requested it no longer needs the
byplacingaLOWinputontheSEMpin(whichactsasachipselectforthe resource, the entire system can hang up until a one is written into that
semaphore flags) and using the other control pins (Address, OE, and semaphorerequestlatch.
R/W)astheywouldbeusedinaccessingastandardStaticRAM. Each
The critical case of semaphore timing is when both sides request
oftheflagshasauniqueaddresswhichcanbeaccessedbyeitherside a single token by attempting to write a zero into it at the same time.
throughaddresspinsA0–A2.Whenaccessingthesemaphores,noneof The semaphore logic is specially designed to resolve this problem.
theotheraddresspinshasanyeffect.
If simultaneous requests are made, the logic guarantees that only one
Whenwritingtoasemaphore,onlydatapinD0 isused.Ifalowlevel sidereceivesthetoken.Ifonesideisearlierthantheotherinmakingthe
iswrittenintoanunusedsemaphorelocation,thatflagwillbesettoazero request, the first side to make the request will receive the token. If both
on that side and a one on the other side (see Truth Table III). That requestsarriveatthesametime,theassignmentwillbearbitrarilymade
semaphorecannowonlybemodifiedbythesideshowingthezero.When to one port or the other.
aoneiswrittenintothesamelocationfromthesameside,theflagwillbe
One caution that should be noted when using semaphores is that
settoaoneforbothsides(unlessasemaphorerequestfromtheotherside semaphoresalonedonotguaranteethataccesstoaresourceissecure.
ispending)andthencanbewrittentobybothsides.Thefactthattheside Aswithanypowerfulprogrammingtechnique,ifsemaphoresaremisused
whichisabletowriteazerointoasemaphoresubsequentlylocksoutwrites or misinterpreted, a software error can easily happen.
fromtheothersideiswhatmakessemaphoreflagsusefulininterprocessor
Initializationofthesemaphoresisnotautomaticandmustbehandled
communications.(Athoroughdiscussionontheuseofthisfeaturefollows viatheinitializationprogramatpower-up.Sinceanysemaphorerequest
shortly.)Azerowrittenintothesamelocationfromtheothersidewillbe flagwhichcontainsazeromustberesettoaone,allsemaphoresonboth
storedinthesemaphorerequestlatchforthatsideuntilthesemaphoreis sidesshouldhaveaonewrittenintothematinitializationfrombothsides
freedbythefirstside.
to assure that they will be free when needed.
Whenasemaphoreflagisread,itsvalueisspreadintoalldatabitsso
thataflagthatisaonereadsasaoneinalldatabitsandaflagcontaining
azeroreadsasallzeros.Thereadvalueislatchedintooneside’soutput
registerwhenthatside'ssemaphoreselect(SEM)andoutputenable(OE)
signalsgoactive.Thisservestodisallowthesemaphorefromchanging
stateinthemiddleofareadcycleduetoawritecyclefromtheotherside.
Becauseofthislatch,arepeatedreadofasemaphoreinatestloopmust
cause either signal (SEM or OE) to go inactive or the output will never
change.
UsingSemaphores—SomeExamples
Perhapsthesimplestapplicationofsemaphoresistheirapplicationas
resourcemarkersfortheIDT7024’sDual-PortRAM.Saythe4Kx16RAM
wastobedividedintotwo2Kx16blockswhichweretobededicatedat
anyonetimetoservicingeithertheleftorrightport.Semaphore0could
be used to indicate the side which would control the lower section of
memory,andSemaphore1couldbedefinedastheindicatorfortheupper
sectionofmemory.
AsequenceWRITE/READmustbeusedbythesemaphoreinorder
to guarantee that no system level contention will occur. A processor
requestsaccesstosharedresourcesbyattemptingtowriteazerointoa
semaphorelocation.Ifthesemaphoreisalreadyinuse,thesemaphore
requestlatchwillcontainazero,yetthesemaphoreflagwillappearasone,
afactwhichtheprocessorwillverifybythesubsequentread(seeTruth
TableIII).Asanexample,assumeaprocessorwritesazerototheleftport
atafreesemaphorelocation.Onasubsequentread,theprocessorwill
verifythatithaswrittensuccessfullytothatlocationandwillassumecontrol
overtheresourceinquestion.Meanwhile,ifaprocessorontherightside
attemptstowriteazerotothesamesemaphoreflagitwillfail, aswillbe
verifiedbythefactthataonewillbereadfromthatsemaphoreontheright
side during subsequent read. Had a sequence of READ/WRITE been
usedinstead,systemcontentionproblemscouldhaveoccurredduringthe
gap between the read and write cycles.
Totakearesource, inthisexamplethelower2KofDual-PortRAM,
the processor on the left port could write and then read a zero in to
Semaphore0.Ifthistaskweresuccessfullycompleted(azerowasread
back rather than a one), the left processor would assume control of the
lower2K.Meanwhiletherightprocessorwasattemptingtogaincontrolof
theresourceaftertheleftprocessor,itwouldreadbackaoneinresponse
tothezeroithadattemptedtowriteintoSemaphore0. Atthispoint, the
softwarecouldchoosetotryandgaincontrolofthesecond2Ksectionby
writing,thenreadingazerointoSemaphore1.Ifitsucceededingaining
control,itwouldlockouttheleftside.
Once the left side was finished with its task, it would write a one to
Semaphore 0 and may then try to gain access to Semaphore 1. If
Semaphore1wasstilloccupiedbytherightside,theleftsidecouldundo
itssemaphorerequestandperformothertasksuntilitwasabletowrite,then
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