操作系統(tǒng)第九版部分課后作業(yè)習(xí)題答案(已修改)
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【正文】 CHAPTER 9 Virtual MemoryPractice Exercises Under what circumstances do page faults occur? Describe the actionstaken by the operating system when a page fault occurs.Answer:A page fault occurs when an access to a page that has not beenbrought into main memory takes place. The operating system veri?esthe memory access, aborting the program if it is invalid. If it is valid, afree frame is located and I/O is requested to read the needed page intothe free frame. Upon pletion of I/O, the process table and page tableare updated and the instruction is restarted. Assume that you have a pagereference string for a process with mframes (initially all empty). The pagereference string has length p。n distinct page numbers occur in it. Answer these questions for anypagereplacement algorithms:a. What is a lower bound on the number of page faults?b. What is an upper bound on the number of page faults?Answer:a. nb. p Consider the page table shown in Figure for a system with 12bitvirtual and physical addresses and with 256byte pages. The list of freepage frames is D, E, F (that is, D is at the head of the list, E is second,and F is last).Convert the following virtual addresses to their equivalent physicaladdresses in hexadecimal. All numbers are given in hexadecimal. (Adash for a page frame indicates that the page is not in memory.)? 9EF? 1112930 Chapter 9 Virtual Memory? 700? 0FFAnswer:? 9E F 0E F? 111 211? 700 D00? 0F F EFF Consider the following pagereplacement algorithms. Rank these algorithms on a ?vepoint scale from “bad” to “perfect” according to theirpagefault rate. Separate those algorithms that suffer from Belady’sanomaly from those that do not.a. LRU replacementb. FIFO replacementc. Optimal replacementd. Secondchance replacementAnswer:Rank Algorithm Suffer from Belady’s anomaly1 Optimal no2 LRU no3 Secondchance yes4 FIFO yes Discuss the hardware support required to support demand paging.Answer:For every memoryaccess operation, the page table needs to be consultedto check whether the corresponding page is resident or not and whetherthe program has read or write privileges for accessing the page. Thesechecks have to be performed in hardware. A TLB could serve as a cacheand improve the performance of the lookup operation. An operating system supports a paged virtual memory, using a centralprocessor with a cycle time of 1 microsecond. It costs an additional 1microsecond to access a page other than the current one. Pages have 1000words, and the paging device is a drum that rotates at 3000 revolutionsper minute and transfers 1 million words per second. The followingstatistical measurements were obtained from the system:? 1 percent of all instructions executed accessed a page other than thecurrent page.?Of the instructions that accessed another page, 80 percent accesseda page already in Exercises 31?When a new page was required, the replaced page was modi?ed 50percent of the time.Calculate the effective instruction time on this system, assuming that thesystem is running one process only and that the processor is idle duringdrum transfers.Answer:effective access time = (1sec + (2 sec)+ (10,000 sec + 1,000 sec)+ (10,000 sec + 1,000 sec)= ( + + + )sec= sec Consider the twodimensional array A:int A[][] = new int[100][100]。where A[0][0] is at location 200 in a paged memor
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