最新操作系统第九版部分课后作业习题答案分析解析.pdf

精品文档 CHAPTER 9 Virtual Memory Practice Exercises 9.1 Under what circumstances do page faults occur? Describe the actions taken by the operating system when a page fault occurs. Answer: A page fault occurs when an access to a page that has not been brought into main memory takes place. The operating system veries the memory access, aborting the program if it is invalid. If 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 table and page table are updated and the instruction is restarted. 9.2 Assume that you have a reference string for a process with m frames (initially all empty). The reference string has length p; n distinct page numbers occur in it. Answer these questions for any 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. n b. p 9.3 Consider the page table shown in Figure 9.30 for a system with 12-bit virtual and physical addresses and with 256-byte pages. The list of free 精品文档 精品文档 page 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 physical addresses in hexadecimal. All numbers are given in hexadecimal. (A dash for a page frame indicates that the page is not in memory.) • 9EF • 111 2930 Chapter 9 Virtual Memory • 700 • 0FF Answer: • 9E F - 0E F • 111 - 211 • 700 - D00 • 0F F - EFF 9.4 Consider th following replacement algorithms. Rank thes algorithms on a five-point scale from “bad” to “perfect” according to their fault rate. Separate those algorithms that suffer from Belady’s anomaly from those that do not. a. LRU replacement b. FIFO replacement c. Optimal replacement 精品文档 精品文档 d. Second-chance replacement Answer: Rank Algorithm Suffer from Belady’s anomaly 1 Optimal no