操作系统第九版部分课后作业习题答案分析解析VIP专享

CHAPTER 9 Virtual MemoryPractice Exercises9.1 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 verifiesthe 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 completion of I/O, the process table and page tableare updated and the instruction is restarted.9.2 Assume that you have a page-reference string for a process with mframes (initially all empty). The page-reference string has length p;n distinct page numbers occur in it. Answer these questions for anypage-replacement 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. p9.3 Consider the page table shown in Figure 9.30 for a system with 12-bitvirtual and physical addresses and with 256-byte 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 - EFF9.4 Consider the following page-replacement algorithms. Rank these algorithms on a five-point scale from “bad” to “perfect” according to theirpage-fault rate. Separate those algorithms that suffer from Belady’sanomaly from those that do not.a. LRU...