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ComplexityofI/ODevices
Modernoperatingsystemsareexpectedtosupportawidevarietyofdifferentinputandoutput(I/O)devices.
However,thevarietyofthesedevicesmakestheirconsistent,logical,andefficientsupportdifficult.Inparticular,theI/Otransferratesandsizesareofmost
concernforefficientoperationofanoperatingsystem.
ThetypesofI/Odevicessupportedcanbegroupedroughlyintothreedistinctcategories:
Humaninterfacedevices
communicatewiththeuser,includingkeyboard(10Bps),mouse(50Bps),laserprinter(4MBps),andvideodisplays(150MBps).
Machine-readabledevices
communicatewithinthesinglecomputersystem,orprovidedigitalinputfromexternalsensors,includingdigital-analogueconverters(100KBps),floppy
disks(10KBps),magneto-opticaldisks(1MBps),magnetictape(5MBps),magneticdisk(150MBps),solid-statedisk(600MBps).
Communicationdevices
connectcomputersystemstoeachother,includingmodems(7KBps),standardEthernet(1.25MBps,12.5MBps),ATMnetworks(19.375MBpandvery
fastEthernet(125MBps).
Note,thatthe(approximate)I/OtransferspeedsshownabovearepresentedinmultiplesofBytes-per-second(Bps).
ManyI/Odevices,typicallycommunications/networkingdevices,reporttheirtransferspeedsinbits-per-second(bps).Moreover,communicationspeeds
aregiveninmultiplesof1000bps,not1024bps,sothattransferringonemegabyteofdataat1Mbpsmaytake2.4%longerthanyou'dthink!
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ComplexityofI/ODevices,continued
ThediversityofusestowhichI/Odevicesareplacedmakesitdifficultforanoperatingsystemtomakea(single)uniformandconsistentapproachtotheir
management.
Wecanhighlightmanyofthesedifferences:
Datarate:
ourpreviousexampleshavehighlightedtransferratesspanning7-8ordersofmagnitude.
Application:
theexpecteduseofadeviceaffectsthesoftwarepoliciesandprioritiesemployedbyanoperatingsystem.Forexample,differentoutputdevicesmay
besupportedatdifferentpriorities(particularlyinareal-time,alert-basedsystem)andotherwiseidenticaldiskdrivesmaybemanageddifferentlyifone
istheswappingdevice,andtheother"only"storesusers'files.
Complexityofcontrol:
devicessuchasmiceandkeyboardsrequirelittlecontrol(beingread-onlydevices),whereasbidirectional,mirroreddiskdrivesaremuchmore
complex.
Datatransfermodels:
typicallycharacterstream-based(keyboards),orblock-based(disksandtapes).
Errormanagement:
complexI/Odevicesoftenrecoverfromtheirownerrors,andtheoperatingsystemonlyhearsofcatastrophicfailures.Inaddition,someerrorsmaybe
handledbytheoperatingsystem,whereasothersmust"percolate"totheuser'sapplication.
TheauthorStallingsdescribesI/Omanagementasthe"messiestaspectofoperatingsystemdesign".
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TypesofI/OFunctions
AnoperatingsystemmaybeexpectedtosupportI/Ousingoneofthreemethods.
WhichmethodisemployeddependsonthecomplexityoftheI/Odevice:
Programmed,orpolled,I/O
TheprocessorissuesanI/O-basedinstructiononbehalfofthecurrentlyexecutingprocess.TheprocessloopsincessantlyuntiltheI/Orequestis
satisfied.Ofcourse,inamulti-programmedenvironment,allotherprocessesaredelayed,too.
Thistechniqueisoftentermedbusy-waiting.
Interrupt-drivenI/O
TheprocessoragainissuesanI/O-basedinstructiononbehalfofthecurrentlyexecutingprocess.
Theprocesseithercontinuesexecutionuntil:
itis(asynchronously)informedthattheI/Ohascompleted(termedasynchronousI/O),or
theprocessisblocked(anotherprocessmayexecute)andtheoriginalprocessiseventuallymarkedasReadywhentheoperatingsystem
receivestheappropriate"I/Odone"hardwareinterrupt.
DirectMemoryAccess(DMA)
TheprocessorissuesanI/O-basedinstructiononbehalfofthecurrentlyexecutingprocess,butdirectstherequesttoaDMA(hardware)module.The
DMAmodulemanagesthedatatransferbetweenmainmemoryandtheI/Odevicewithoutprocessorintervention.
Whenthewholedatablockhasbeentransfered,theDMAmoduleinterruptstheprocessor(asdescribedabove).
CITS2002SystemsProgramming,Lecture23,p3,27thOctober2016.
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ResponsibilitiesofanI/OController
Ascomputerhardwareandoperatingsystemshaveevolved,themethodsemployedtomanageI/Ohaveincreasedincomplexity.
Theseevolutionarystepshavebeen:
1. theprocessordirectlycontrolstheI/Odevice.
2. anI/Ocontrollerormoduleisemployed.TheprocessorcommunicateswiththiscontrollerusingprogrammedI/O(nointerrupts),butisunawareofthe
device'sexternalinterface.
3. anI/Ocontrollerisagainused,butefficiencyisincreasedbecausethecontrollerinterruptstheprocessorwhenready.
4. theI/OcontrollercommunicateswiththeDMAmodule,relinquishingtheprocessorofallI/Oresponsibilities.
5. theI/OcontrollersupportsitsownprocessorandI/Oinstructionset.ThemainprocessorinitiatesI/Obyprovidingthecontrollerwiththeaddressofa
sequenceofI/Oinstructionsinmainmemory.WhentheI/Ochannelhasfetchedandexecutedthissequence,itinterruptsthemainprocessor.
6. theI/Ocontrollersupportsitsownprocessorandlocalmemory.TheI/Ocontrollerisnowdirectedfromthemainprocessorbysimplyprovidinga
descriptionoftheI/Otaskrequired.WhenthisI/Oprocessorhasperformeditstask,itinterruptsthemainprocessor.
Inthisevolutionarysequence,theI/Ocontrollerbecomesincreasingly"intelligent",andthemainprocessorisincreasinglyrelievedofanyI/O
responsibilities,leavingittoperformothercomputations.
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DirectMemoryAccess
TheDirectMemoryAccess(DMA)controllerhastheresponsibilityoftransferringblocksofdatabetweenmainmemoryandanyI/Odevices,without
involvingtheprocessor.Typically,datasizesarelarge-tens,hundredsofkilobytes,ormegabytes.
TheDMAmoduleusesthemainbustoperformthetransfer.Ideallythisusewillbewhentheprocessordoesnotrequirethebus,otherwisetheDMA
modulemustsuspendtheprocessor'sexecution.
Thelattertechniqueisoftentermedcycle-stealingastheDMAmodule'sactionsstealaprocessorcycle.
DMAandInterruptBreakpoints
Ascanbeseen,theprocessordoesnotneedtousethebusallofthetime(andcertainlynotfordatastoredentirelyinitsregisters).Thisprovides
opportunitiesfortheDMAcontrollertotransferdatausingthebuswhenitisotherwiseidle.
TheDMAmoduleisabletosuspendtheprocessorduringitsinstructionfetch-decode-executecycle.DuringtheDMAbreakpoints,theDMAmodule
suspendstheprocessorandtransfersasingleunit(typicallyonlyonebyte)betweenmemoryandanI/Omodule.
NoticethattheDMAmoduledoesnotinterrupttheprocessor,itjustsuspendsit.Thereisnoneedfortheprocessortosaveitsexecutioncontextand
executeanotherroutine,astheDMAmoduledoesnotalterthiscontext.Theprocessorrunsslowerduetoitssuspension,butnotasslowasifitwere
interruptedatthecompletionofeachbyte'stransferorifpollingwerebeingused.
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DirectMemoryAccessOrganisation
Atthehardwarelevel,themainprocessor,DMAmodule,andotherI/Omodulesmaybeconfiguredindifferentways:
InFigure(a),DMAmodulesareseparatedevices,onthesamebusastheprocessorandI/Omodules.
InFigure(b),moresophisticated(faster)deviceshavetheirownDMAcontroller,orasingleDMAcontroller(here,oftentermedanI/Ochannel)may
directlysupportmultipledevices.
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DirectMemoryAccessOrganisation,continued
InFigure(c),theDMAmoduleactsasanI/ObridgebetweenthemainsystembusandanewI/Obus.
ItmaynowbepossibleforasinglesystemtohaveavarietyofdifferentI/Obusstandards,suchas:
IDE(IntegratedDriveElectronics)orSCSI(SmallComputerSystemInterface)fordiskdrives,
aUSBv3.0bus(UniversalSerialBus,600MBps)forexternaldrivesandscanners,and
aFirewirebus(IEEE1394,50MBps)fordigitalmultimedia.
ThisdecreasesthenumberofdirectinterfacesthattheDMAmodulemustsupport,andremoveshalfofthemainbustrafficwhichcanpotentiallyinterfere
withtheprocessor'sexecution.
Moreover,somedevicesmaynowonlybecontactedviatheirDMA(now,bus)controller.
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LogicalOrganisationofI/O
Aswithmostaspectsofoperatingsystemdesign,ahierarchicalstructurecanbeemployedtodecomposeI/Oresponsibilitiesintomanageable
subproblems.
AtthelowestleveloftheI/Ohierarchyaresectionsofcodewhichmustinteractdirectlywithhardware,andcompletetheiractivitiesinafewbillionthsofa
second.
Atthehighestlevel,applicationprogramswishtocommunicatetheirI/Orequirementsatamorelogicallevel,andwishtobeisolatedfromhardware
specifics.Weseethesemore"logical"interfacesrepresentedinaconsistent,smallsetofsystemcalls.
Therearethreedistinctlevelsinthishierarchy:
LogicalI/O:
treatstheI/Odeviceasalogicalresource,whileignoringthecontrolofthedevice.Operatingsysteminterfaces(systemcalls)allowapplication
programstointeractwiththisresponsibilitywithfamiliaroperationssuchasopen(),lseek(),read(),write(),andclose().
DeviceI/O:
therequiredI/Ooperationsandthedata'slocationareconvertedtosequencesofinstructionsandaccesses.
Schedulingandcontrol:
theoperatingsystemschedulestheI/Orequestsinanattempttomaximisetheirthroughput(basedontheI/Odevice'scharacteristics)."Returning"
interruptsarealsohandledatthislevel,withcontrolinformation(status)beingreturnedtotheinvokingprocesses.
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ExamplesofLogicalI/OOrganisation
Howmuchoperatingsystemcodeis"squeezed"aboveDeviceI/Odependsmostlyonhowthedeviceisrepresentedtoanapplication(Stalling'sFigure
11.4).
Forexample,thecommunicationsport'scodemayincludeafullTCP/IPprotocolsuite,requiringtensofthousandsoflinesofcode.
AModelofI/OOrganization
CITS2002SystemsProgramming,Lecture23,p9,27thOctober2016.