CS 61C: Great Ideas in Computer Architecture Lecture 22

CS61C:GreatIdeasinComputerArchitecture

Lecture22:OperatingSystem

BernhardBoser&RandyKatz

http://inst.eecs.berkeley.edu/~cs61c

Memory

CS61Csofar…

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CPU

Caches

MIPSAssembly

CPrograms

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What!kind!of!locality!are!we!taking!advantage!of?!31#

#include <stdlib.h>

int fib(int n) {returnfib(n-1) +fib(n-2);

}

.foolw $t0, 4($r0)addi $t1, $t0, 3beq $t1, $t2, foonop

Project1Project2

Labs

CS61c Lecture22:OperatingSystem

Project3

Sohowisthisanydifferent?

3

Keyboard

Screen

Storage


Memory

AddingI/O

4

CPU

Caches

MIPSAssembly

CPrograms

•  Four##words/block,#cache#size#=#1K#words##!

MulWwordKBlock#DirectKMapped#Cache#

8#Index#

Data#Index# Tag#Valid#

0#

1#

2#

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253#

254#

255#

31#30###.#.#.#################13#12##11####.#.#.####4##3##2##1##0#Byte#

offset#

20#

20#Tag#

Hit# Data#

32#

Block#offset#

What!kind!of!locality!are!we!taking!advantage!of?!31#

#include <stdlib.h>

int fib(int n) {returnfib(n-1) +fib(n-2);

}

.foolw $t0, 4($r0)addi $t1, $t0, 3beq $t1, $t2, foonop

Project1

Project3

I/O(Input/Output)

Screen Keyboard Storage


CPU+$s,Memory

RaspberryPi($40onAmazon)

5

StorageI/O(MicroSDCard)

SerialI/O(USB)

NetworkI/O(Ethernet)ScreenI/O

(HDMI)


It’sarealcomputer!

6CS61c Lecture22:OperatingSystem

Butwait…

7

• That’snotthesame!WhenwerunMARS,itonlyexecutesoneprogramandthenstops.• WhenIswitchonmycomputer,Igetthis:

Yes,butthat’sjust software!TheOperatingSystem(OS)


Well,“justsoftware”

• Thebiggestpieceofsoftwareonyourmachine?• Howmanylinesofcode?Theseareguesstimates:

8

Codebases(inmillionsoflinesofcode).CCBY-NC3.0— DavidMcCandless ©2013http://www.informationisbeautiful.net/visualizations/million-lines-of-code/


OperatingSystem

CS61c Lecture22:OperatingSystem 9

WhatdoestheOSdo?

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• OSisfirstthingthatrunswhencomputerstarts• Findsandcontrolsalldevicesinthemachineinageneralway

− Relyingonhardwarespecific“devicedrivers”

• Startsservices(100+)− Filesystem,− Networkstack(Ethernet,WiFi,Bluetooth,…),− TTY(keyboard),− …

• Loads,runsandmanagesprograms:− Multipleprogramsatthesametime(time-sharing)− Isolateprogramsfromeachother(isolation)− Multiplexresourcesbetweenapplications(e.g.,devices)


Agenda

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• DevicesandI/O• Polling• Interrupts• OSBootSequence• Multiprogramming/time-sharing


Howtointeractwithdevices?

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• AssumeaprogramrunningonaCPU.Howdoesitinteractwiththeoutsideworld?• NeedI/OinterfaceforKeyboards,Network,Mouse,Screen,etc.− Connecttomany typesofdevices− Controlthesedevices,respondtothem,andtransferdata

− Presentthemtouserprogramssotheyareuseful

cmd reg.datareg.

OperatingSystem

Processor Memory

PCIBus

USB


SATA,SAS,…

InstructionSetArchitectureforI/O

• WhatmusttheprocessordoforI/O?− Input:readasequenceofbytes− Output:writeasequenceofbytes

• Interfaceoptionsa) Specialinput/outputinstructions&hardwareb) MemorymappedI/O

§ PortionofaddressspacededicatedtoI/O§ I/Odeviceregistersthere(nomemory)§ Usenormalload/storeinstructions,e.g.lw/sw§ Verycommon,usedbyMIPS


MemoryMappedI/O

• Certainaddressesarenotregularmemory• Instead,theycorrespondtoregistersinI/Odevices

cntrlreg.datareg.

0x0

0xFFFFFFFF

0xFFFF0000

address


Processor-I/OSpeedMismatch• 1GHzmicroprocessorI/Othroughput:

− 4Gi-B/s(lw/sw)− TypicalI/Odatarates:

§ 10B/s (keyboard)§ 100Ki-B/s (Bluetooth)§ 60Mi-B/s (USB2)§ 100Mi-B/s (Wifi,dependsonstandard)§ 125Mi-B/s (G-bitEthernet)§ 550Mi-B/s (cuttingedgeSSD)§ 1.25Gi-B/s (USB3.1Gen2)§ 6.4GiB/s (DDR3DRAM)

− Thesearepeakrates– actualthroughputislower• CommonI/Odevicesneitherdelivernoracceptdatamatchingprocessorspeed



NoEndorsement.Justabreakfromthelecture.

Agenda

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ProcessorChecksStatusbeforeActing

• Deviceregistersgenerallyserve2functions:• ControlRegister,saysit’sOKtoread/write(I/Oready)[thinkofaflagmanonaroad]• DataRegister,containsdata

• ProcessorreadsfromControlRegisterinloop−waitingfordevicetosetReady bitinControlreg (0à 1)− Indicates“dataavailable”or“readytoacceptdata”

• Processorthenloadsfrom(input)orwritesto(output)dataregister• I/Odeviceresetscontrolregisterbit(1à 0)

• Procedurecalled“Polling”


• Input:Readfromkeyboardinto$v0lui $t0, 0xffff #ffff0000 (io addr)

Waitloop: lw $t1, 0($t0) #read controlandi $t1,$t1,0x1 #ready bitbeq $t1,$zero, Waitlooplw $v0, 4($t0) #data

• Output:Writetodisplayfrom$a0lui $t0, 0xffff #ffff0000

Waitloop: lw $t1, 8($t0) #write controlandi $t1,$t1,0x1 #ready bitbeq $t1,$zero, Waitloopsw $a0,12($t0) #data

“Ready”bitisfromprocessor’spointofview!

I/OExample(polling)


CostofPolling?

• Assumeforaprocessorwith− 1GHzclockrate− Taking400clockcyclesforapollingoperation

§ Callpollingroutine§ Checkdevice(e.g.keyboardorwifi inputavailable)§ Return

−What’sthepercentageofprocessortimespentpolling?

• Example:−Mouse− Poll30timespersecond

§ Setbyrequirementnottomissanymousemotion(whichwouldleadtochoppymotionofthecursoronthescreen)


PercentProcessorTimetoPollMouse

• MousePolling[instructions/sec]=30[polls/s]*400[instructions/poll]=12K[instructions/s]

• %Processorforpolling:− Totalprocessorthroughput:T=1*109 [instructions/s]− Polling:P=12*103 [instructions/s]− Ratio:P/T=0.0012%à Negligible:

§ Pollingmousehaslittleimpactonprocessorthroughput


ClickerTimeHarddisk:transfersdatain16-Bytechunksandcantransferat16MB/second.Notransfercanbemissed.Whatpercentageofprocessortimeisspentinpolling(assume1GHzclock)?

• A:2%• B:4%• C:20%• D:40%• E:80%


%Processortimeneededpolldisk• FrequencyofPollingDisk

=16[MB/s]/16[B/poll]=1M[polls/s]

• DiskPolling,instructions/sec=1M[polls/s]*400[instructions/poll]=400M[instructions/s]• %Processorforpolling:

400*106 [instructions/s]/1*109 [instructions/s]= 40%à Unacceptable


Whatisthealternativetopolling?

• Pollingwastesprocessorresources• Akintowaitingatthedoorforgueststoshowup

−Whataboutabell?

• Computerlingoforbell:− Interrupt− OccurswhenI/Oisreadyorneedsattention

§ Interruptcurrentprogram§ Transfercontroltospecialcode“interrupthandler”


Agenda

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26

Traps/Interrupts/Exceptions:alteringthenormalflowofcontrol(P&HSection4.9)

Ii-1 HI1

HI2

HIn

Ii

Ii+1

programtraphandler

An external or internal event that needs to be processed - by another program – the OS. The event is often unexpected from original program’s point of view.


Interrupt-drivenI/O

Label: sll $t1,$s3,2addu $t1,$t1,$s5lw $t1,0($t1) or $s1,$s1,$t1addu $s3,$s3,$s4bne $s3,$s2,Label

StackFrame

StackFrame

StackFrame

handler: lui $t0, 0xfffflw $t1, 0($t0)andi $t1,$t1,0x1lw $v0, 4($t0)sw $t1, 8($t0)ret

Interrupt(SPI0)

CPUVector InterruptTable

SPI0 handler

… …

HandlerExecution1. Incominginterruptsuspendsinstructionstream2. Looksupthevector(functionaddress)ofahandlerin

aninterruptvectortablestoredwithintheCPU3. Performajal tothehandler(savePCinEPC*register)4. Handlerrunoncurrentstackandreturnsonfinish

(threaddoesn’tnoticethatahandlerwasrun)


Codeforsaving/restoring$t0,$t1,$v0notshown

*EPC:Exceptionprogramcounter

TerminologyInCS61C(otherdefinitionsinuseelsewhere):• Interrupt – causedbyaneventexternal tocurrentrunningprogram− E.g.keypress,disk− Asynchronoustocurrentprogram

§ Canhandleinterruptonanyconvenientinstruction§ “Wheneverit’sconvenient,justdon’twaittoolong”

• Exception – causedbysomeeventduring executionofoneinstructionofcurrentrunningprogram− E.g.,overflow,buserror,illegalinstruction− Synchronous

§ Musthandleexceptionprecisely oninstructionthatcausesexception§ “Dropwhateveryouaredoingandactnow”

• Trap – actionofservicinginterruptorexceptionbyhardwarejumpto“interruptortraphandler”code

PreciseTraps

• Traphandler’sviewofmachinestateisthateveryinstructionpriortothetrappedone(e.g.overflow)hascompleted,andnoinstructionafterthetraphasexecuted.• Impliesthathandlercanreturnfromaninterruptbyrestoringuserregistersandjumpingbacktointerruptedinstruction− Interrupthandlersoftwaredoesn’tneedtounderstandthepipelineofthemachine,orwhatprogramwasdoing!

− Morecomplextohandletrapcausedbyanexceptionthaninterrupt

• Providingprecisetrapsistrickyinapipelinedsuperscalarout-of-orderprocessor!− Butarequiremente.g.for

§ Virtualmemorytofunctionproperly(seenextlecture)


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TrapHandlingin5-StagePipeline

Exceptionsarehandledlikepipelinehazards1) Completeexecutionofinstructionsbeforeexceptionoccurred2) Flushinstructionscurrentlyinpipeline

(converttonops or“bubbles”)3) Optionallystoreexceptioncauseinstatusregister(e.g.MIPS)

− Indicatetypeofexception− Note:severalexceptionscanoccurinasingleclockcycle!

4) Transferexecutiontotraphandler

PCInst. Mem D Decode E M

Data Mem W+

Illegal Opcode Overflow Data address

ExceptionsPC address Exception

Asynchronous Interrupts


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TrapPipelineDiagramtimet0 t1 t2 t3 t4 t5 t6 t7 . . . .

(I1) 096: ADD IF1 ID1 EX1 MA1 - overflow!(I2) 100: XOR IF2 ID2 EX2 - -(I3) 104: SUB IF3 ID3 - - -(I4) 108: ADD IF4 - - - -(I5) Trap Handler code* IF5 ID5 EX5 MA5 WB5

*EPC=100(instructionfollowingoffendingADD)


Agenda

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Whathappensatboot?

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• Whenthecomputerswitcheson,itdoesthesameasMARS:theCPUexecutesinstructionsfromsomestartaddress(storedinFlashROM)

CPU

PC=0x2000(somedefaultvalue) AddressSpace

0x2000:addi $t0, $zero, 0x1000lw $t0, 4($r0)…

(Code to copy firmware into regular memory and jump into it)

Memorymapped


Whathappensatboot?

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1.BIOS*:Findastoragedeviceandloadfirstsector(blockofdata)

2.Bootloader (storedon,e.g.,disk):LoadtheOSkernel fromdiskintoalocationinmemoryandjumpintoit.

3.OSBoot:Initializeservices,drivers,etc.

4.Init:Launchanapplicationthatwaitsforinputinloop(e.g.,Terminal/Desktop/...


*BIOS:BasicInputOutputSystem

LaunchingApplications

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• Applicationsarecalled“processes”inmostOSs.− Thread:sharedmemory− Process:separatememory− Boththreadsandprocessesrun(pseudo)simultaneously

• Appsarestartedbyanotherprocess(e.g.shell)callinganOSroutine(usinga“syscall”)− DependsonOS,butLinuxusesfork tocreateanewprocess,andexecve toloadapplication.

• Loadsexecutablefilefromdisk(usingthefilesystemservice)andputsinstructions&dataintomemory(.text,.datasections),preparesstackandheap.• Setargc andargv,jumptostartofmain.• Shellwaitsformaintoreturn(join)


SupervisorMode

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• Ifsomethinggoeswronginanapplication,itcouldcrashtheentiremachine.Andwhataboutmalware,etc.?• TheOSenforcesresourceconstraintstoapplications(e.g.,accesstomemory,devices).• TohelpprotecttheOSfromtheapplication,CPUshaveasupervisormode(sete.g.byastatusbit).− Aprocesscanonlyaccessasubsetofinstructionsand(physical)memorywhennotinsupervisormode(usermode).

− Processcanchangeoutofsupervisormodeusingaspecialinstruction,butnotintoitdirectly– onlyusinganinterrupt.

− Supervisorymodeisabitlike“superuser”§ Butusedmuchmoresparingly(mostofOScodedoesnot runinsupervisorymode)

§ Errorsinsupervisorymodeoftencatastrophic(blue“screenofdeath”,or“Ijustcorruptedyourdisk”)


Syscalls

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• WhatifwewanttocallanOSroutine?E.g.,− toreadafile,− launchanewprocess,− askformorememory(malloc),− senddata,etc.

• Needtoperformasyscall:− setupfunctionargumentsinregisters,− raisesoftwareinterrupt(withspecialassemblyinstruction)

• OSwillperformtheoperationandreturntousermode• Thisway,theOScanmediateaccesstoallresources,anddevices.


Agenda

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Multiprogramming

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• TheOSrunsmultipleapplicationsatthesametime.• Butnotreally(unlessyouhaveacoreperprocess)• Switchesbetweenprocessesveryquickly(onhumantimescale).Thisiscalleda“contextswitch”.• Whenjumpingintoprocess,settimerinterrupt.

−Whenitexpires,storePC,registers,etc.(processstate).− Pickadifferentprocesstorunandloaditsstate.− Settimer,changetousermode,jumptothenewPC.

• Decidingwhatprocesstoruniscalledscheduling.


Protection,Translation,Paging

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• SupervisormodealoneisnotsufficienttofullyisolateapplicationsfromeachotherorfromtheOS.− Applicationcouldoverwriteanotherapplication’smemory.− Typicallyprogramsstartatsomefixedaddress,e.g.0x1000

§ Howcan100’sofprogramssharememoryatlocation0x1000?− Also,maywanttoaddressmorememorythanweactuallyhave(e.g.,forsparsedatastructures).

• Solution:VirtualMemory.− Giveseachprocesstheillusion ofafullmemoryaddressspacethatithascompletelyforitself.


And,inConclusion,…

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• Basicmachine(datapath,memory,IOdevices)areapplicationagnostic• Sameconcepts/processorarchitectureapplytolargevarietyofapplications.E.g.− OSwithcommandlineandgraphicalinterface(Linux,…)− Embeddedprocessorinnetworkswitch,carenginecontrol,…

• Input/output(I/O)− Memorymapped:appearslike“specialkindofmemory”− Accesswithusualload/storeinstructions(e.g.lw, sw)

• Exceptions− Notifyprocessorofspecialevents,e.g.overflow,pagefault(nextlecture)− “precise”handling:immediatelyatoffendinginstruction

• Interrupts− Notificationofexternalevents,e.g.keyboardinputorEthernettraffic

• Multiprogrammingandsupervisorymode− Enablesandisolatesmultipleprograms

• TakeCS162tolearnmoreaboutoperatingsystemsCS61c Lecture22:OperatingSystem

CS 61C: Great Ideas in Computer Architecture Lecture 22

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