《操作系统概念》第六版作业解答3省名师优质课赛课获奖课件市赛课一等奖课件.ppt
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1、Chapter 9n9.3 Describe the following allocation algorithms:na.First fitnb.Best fitnc.Worst fitnFirst fit:搜索可用内存列表,分配第一块足够大nBest fit:搜索整个可用内存块,分配最小足够大nWorst fit:搜索整个可用内存块,分配最大足够大第1页9-cont.n9.5 Given memory partitions of 100K,500K,200K,300K,and 600K(in order),how would each of the First-fit,Best-fit,a
2、nd Worst-fit algorithms place processes of 212K,417K,112K,and 426K(in order)?Which algorithm makes the most efficient use of memory?nFirst fitn212-500(288)n417-600(183)n112-288n426-nonenBest fitn212-300n417-500n112-200 n426-600nWorst fitn212-600(388)n417-500n112-388 n426-none第2页9-cont.n9.10 Consider
3、 a paging system with the page table stored in memory.na.If a memory reference takes 200 nanoseconds,how long does a paged memory reference take?nb.If we add associative registers,and 75 percent of all page-table references are found in the associative registers,what is the effective memory referenc
4、e time?(Assume that finding a page-table entry in the associative registers takes zero time,if the entry is there.)na.400,200(page table)+200(access word)nb.250,75%*200+25%*(200+200)第3页9-cont.n9.16 Consider the following segment table:nWhat are the physical addresses for the following logical addres
5、ses?na.0,430nb.1,10nc.2,500nd.3,400ne.4,112na.430600,219+430=649nb.10100,illegalnd.40096,illegal第4页9-cont.n9.18 In the IBM/370,memory protection is provided through the use of keys.A key is a 4-bit quantity.Each 2K block of memory has a key(the storage key)associated with it.The CPU also has a key(t
6、he protection key)associated with it.A store operation is allowed only if both keys are equal,or if either is zero.Which of the following memory-management schemes could be used successfully with this hardware?na.Bare machinenb.Single-user systemnc.Multiprogramming with a fixed number of processesnd
7、.Multiprogramming with a variable number of processesne.Pagingnf.Segmentationna.Protection not necessary,set system key to 0.nb.Set system key to 0 when in supervisor mode.nc.Region sizes must be fixed in increments of 2k bytes,allocate key with memorynblocks.nd.Same as above.ne.Frame sizes must be
8、in increments of 2k bytes,allocate key with pages.nf.Segment sizes must be in increments of 2k bytes,allocate key with segments.第5页Chapter 10n10.1 Under what circumstances do page faults occur?Describe the actions taken by the operating system when a page fault occurs.nA page fault occurs when an ac
9、cess to a page that has not been brought into main memory takes place.nThe operating system verifies the memory access,naborting the program if it is invalid.nIf it is valid,a free frame is located and I/O is requested to read the needed page into the free frame.Upon completion of I/O,the process ta
10、ble and page table are updated and the instruction is restarted.第6页10-cont.n10.6 Consider the following page-replacement algorithms.Rank these algorithms on a five point scale from“bad”to“perfect”according to their page-fault rate.Separate those algorithms that suffer from Beladys anomaly from those
11、 that do not.na.LRU replacementnb.FIFO replacementnc.Optimal replacementnd.Second-chance replacementnRank Algorithm Suffer from Beladys anomalyn1 Optimal non2 LRU non3 Second-chance yesn4 FIFO yes第7页10-cont.n10.9 Consider a demand-paging system with the following time-measured utilizations:CPU utili
12、zation 20%Paging disk 97.7%Other I/O devices 5%Which(if any)of the following will(probably)improve CPU utilization?Explain your answer.na.Install a faster CPU.nb.Install a bigger paging disk.nc.Increase the degree of multiprogramming.nd.Decrease the degree of multiprogramming.ne.Install more main me
13、mory.nf.Install a faster hard disk or multiple controllers with multiple hard disks.ng.Add prepaging to the page fetch algorithms.nh.Increase the page size.第8页10-cont.n10.10 Consider the two-dimensional array A:int A=new int100100;where A00 is at location 200,in a paged system with pages of size 200
14、.A small process is in page 0(locations 0 to 199)for manipulating the matrix;thus,every instruction fetch will be from page 0.For three page frames,how many page faults are generated by the following array-initialization loops,using LRU replacement,and assuming page frame 1 has the process in it,and
15、 the other two are initially empty:na.for(int j=0;j 100;j+)for(int i=0;i 100;i+)Aij=0;nb.for(int i=0;i 100;i+)for(int j=0;j 100;j+)Aij=0;a.100 x50b.50第9页10-cont.n10.11 Consider the following page reference string:1,2,3,4,2,1,5,6,2,1,2,3,7,6,3,2,1,2,3,6.How many page faults would occur for the follow
16、ing replacement algorithms,assuming one,two,three,four,five,six,or seven frames?Remember all frames are initially empty,so your first unique pages will all cost one fault each.nLRU replacementnFIFO replacementnOptimal replacementnNumber of frames LRU FIFO Optimaln 1 20 20 20n 2 18 18 15n 3 15 16 11n
17、 4 10 14 8n 5 8 10 7n 6 7 10 7n 7 7 7 7第10页Chapter 11n11.3 Why do some systems keep track of the type of a file,while others leave it to the user or simply do not implement multiple file types?Which system is“better?”nSome systems allow different file operations based on the type of the file(for ins
18、tance,an ascii file can be read as a stream while a database file can be read via an index to a block).Other systems leave such interpretation of a files data to the process and provide no help in accessing the data.The method which is“better”depends on the needs of the processes on the system,and t
19、he demands the users place on the operating system.If a system runs mostly database applications,it may be more efficient for the operating system to implement a database-type file and provide operations,rather than making each program implement the same thing(possibly in different ways).For general
20、 purpose systems it may be better to only implement basic file types to keep the operating system size smaller and allow maximum freedom to the processes on the system.第11页11-cont.n11.6 Could you simulate a multilevel directory structure with a single-level directory structure in which arbitrarily l
21、ong names can be used?If your answer is yes,explain how you can do so,and contrast this scheme with the multilevel directory scheme.If your answer is no,explain what prevents your simulations success.How would your answer change if file names were limited to seven characters?nIf arbitrarily long nam
22、es can be used then it is possible to simulate a multilevel directory structure.This can be done,for example,by using the character“.”to indicate the end of a subdirectory.Thus,for example,the name jim.pascal.F1 specifies that F1 is a file in subdirectory pascal which in turn is in the root irectory
23、 jim.If file names were limited to seven characters,then the above scheme could not be utilized and thus,in general,the answer is no.The next best approach in this situation would be to use a specific file as a symbol table(directory)to map arbitrarily long names(such as jim.pascal.F1)into shorter a
24、rbitrary names(such as XX00743),which are then used for actual file access.第12页11-cont.n11.9 Give an example of an application in which data in a file should be accessed in the following order:na.Sequentiallynb.Randomlyna.Print the content of the file.nb.Print the content of record i.This record can
25、 be found using hashing or index techniques.n11.12 Consider a system that supports 5000 users.Suppose that you want to allow 4990 of these users to be able to access one file.na.How would you specify this protection scheme in UNIX?nb.Could you suggest another protection scheme that can be used more
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