操作系统第九版部分课后作业习题答案解析.docx
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操作系统第九版部分课后作业习题答案解析.docx
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操作系统第九版部分课后作业习题答案解析
CHAPTER9VirtualMemory
PracticeExercises
9.1Underwhatcircumstancesdopagefaultsoccur?
Describetheactions
takenbytheoperatingsystemwhenapagefaultoccurs.
Answer:
Apagefaultoccurswhenanaccesstoapagethathasnotbeen
broughtintomainmemorytakesplace.Theoperatingsystemverifies
thememoryaccess,abortingtheprogramifitisinvalid.Ifitisvalid,a
freeframeislocatedandI/Oisrequestedtoreadtheneededpageinto
thefreeframe.UponcompletionofI/O,theprocesstableandpagetable
areupdatedandtheinstructionisrestarted.
9.2Assumethatyouhaveapage-referencestringforaprocesswithm
frames(initiallyallempty).Thepage-referencestringhaslengthp;
ndistinctpagenumbersoccurinit.Answerthesequestionsforany
page-replacementalgorithms:
a.Whatisalowerboundonthenumberofpagefaults?
b.Whatisanupperboundonthenumberofpagefaults?
Answer:
a.n
b.p
9.3ConsiderthepagetableshowninFigure9.30forasystemwith12-bit
virtualandphysicaladdressesandwith256-bytepages.Thelistoffree
pageframesisD,E,F(thatis,Disattheheadofthelist,Eissecond,
andFislast).
Convertthefollowingvirtualaddressestotheirequivalentphysical
addressesinhexadecimal.Allnumbersaregiveninhexadecimal.(A
dashforapageframeindicatesthatthepageisnotinmemory.)
•9EF
•111
2930Chapter9VirtualMemory
•700
•0FF
Answer:
•9EF-0EF
•111-211
•700-D00
•0FF-EFF
9.4Considerthefollowingpage-replacementalgorithms.Rankthesealgorithmsonafive-pointscalefrom“bad”to“perfect”accordingtotheir
page-faultrate.SeparatethosealgorithmsthatsufferfromBelady’s
anomalyfromthosethatdonot.
a.LRUreplacement
b.FIFOreplacement
c.Optimalreplacement
d.Second-chancereplacement
Answer:
RankAlgorithmSufferfromBelady’sanomaly
1Optimalno
2LRUno
3Second-chanceyes
4FIFOyes
9.5Discussthehardwaresupportrequiredtosupportdemandpaging.
Answer:
Foreverymemory-accessoperation,thepagetableneedstobeconsulted
tocheckwhetherthecorrespondingpageisresidentornotandwhether
theprogramhasreadorwriteprivilegesforaccessingthepage.These
checkshavetobeperformedinhardware.ATLBcouldserveasacache
andimprovetheperformanceofthelookupoperation.
9.6Anoperatingsystemsupportsapagedvirtualmemory,usingacentral
processorwithacycletimeof1microsecond.Itcostsanadditional1
microsecondtoaccessapageotherthanthecurrentone.Pageshave1000
words,andthepagingdeviceisadrumthatrotatesat3000revolutions
perminuteandtransfers1millionwordspersecond.Thefollowing
statisticalmeasurementswereobtainedfromthesystem:
•1percentofallinstructionsexecutedaccessedapageotherthanthe
currentpage.
•
Oftheinstructionsthataccessedanotherpage,80percentaccessed
apagealreadyinmemory.PracticeExercises31
•
Whenanewpagewasrequired,thereplacedpagewasmodified50
percentofthetime.
Calculatetheeffectiveinstructiontimeonthissystem,assumingthatthe
systemisrunningoneprocessonlyandthattheprocessorisidleduring
drumtransfers.
Answer:
effectiveaccesstime=0.99×(1sec+0.008×(2sec)
+0.002×(10,000sec+1,000sec)
+0.001×(10,000sec+1,000sec)
=(0.99+0.016+22.0+11.0)sec
=34.0sec
9.7Considerthetwo-dimensionalarrayA:
intA[][]=newint[100][100];
whereA[0][0]isatlocation200inapagedmemorysystemwithpages
ofsize200.Asmallprocessthatmanipulatesthematrixresidesinpage
0(locations0to199).Thus,everyinstructionfetchwillbefrompage0.
Forthreepageframes,howmanypagefaultsaregeneratedby
thefollowingarray-initializationloops,usingLRUreplacementand
assumingthatpageframe1containstheprocessandtheothertwo
areinitiallyempty?
a.for(intj=0;j<100;j++)
for(inti=0;i<100;i++)
A[i][j]=0;
b.for(inti=0;i<100;i++)
for(intj=0;j<100;j++)
A[i][j]=0;
Answer:
a.5,000
b.50
9.8Considerthefollowingpagereferencestring:
1,2,3,4,2,1,5,6,2,1,2,3,7,6,3,2,1,2,3,6.
Howmanypagefaultswouldoccurforthefollowingreplacement
algorithms,assumingone,two,three,four,five,six,orsevenframes?
Rememberallframesareinitiallyempty,soyourfirstuniquepageswill
allcostonefaulteach.
•
LRUreplacement
•FIFOreplacement
•
Optimalreplacement32Chapter9VirtualMemory
Answer:
NumberofframesLRUFIFOOptimal
1202020
2181815
3151611
410148
58107
67107
7777
9.9Supposethatyouwanttouseapagingalgorithmthatrequiresareference
bit(suchassecond-chancereplacementorworking-setmodel),but
thehardwaredoesnotprovideone.Sketchhowyoucouldsimulatea
referencebitevenifonewerenotprov
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