CHAPTER 9 Virtual Memory
Practice Exercis
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 verifies
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.
电冰箱原理
9.2 Assume that you have a page-reference string for a process with m
frames (initially all empty). The page-reference string has length p;
n distinct page numbers occur in it. Answer the questions for any
page-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 address and with 256-byte pages. The list of free
page frames is D, E, F (that is, D is at the head of the list, E is cond,
伍氏猪脚
and F is last).
Convert the following virtual address to their equivalent physical
address in hexadecimal. All numbers are given in hexadecimal. (A
色之图吧dash for a page frame indicates that the page is not in memory.)
• 9EF
• 111
2930 Chapter 9 Virtual Memory
• 700
• 0FF初一拜年祝福语
Answer:
• 9E F - 0E F
• 111 - 211
• 700 - D00
• 0F F - EFF
9.4 Consider the following page-replacement algorithms. Rank the algorithms on a five-point scale from “bad〞 to “perfect〞 according to their
page-fault rate. Separate tho algorithms that suffer from Belady’s
anomaly from tho that do not.
a. LRU replacement
b. FIFO replacement
c. Optimal replacement
d. Second-chance replacement
Answer:
Rank Algorithm Suffer from Belady’s anomaly
1 Optimal no
2 LRU no
3 Second-chance yes
4 FIFO yes
9.5 Discuss the hardware support required to support demand paging.
Answer:
For every memory-access operation, the page table needs to be consulted
to check whether the corresponding page is resident or not and whether
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the program has read or write privileges for accessing the page. The
checks have to be performed in hardware. A TLB could rve as a cache
and improve the performance of the lookup operation.
9.6 An operating system supports a paged virtual memory, using a central
processor with a cycle time of 1 microcond. It costs an additional 1
microcond to access a page other than the current one. Pages have 1000
words, and the paging device is a drum that rotates at 3000 revolutions
per minute and transfers 1 million words per cond. The following郭德纲西征梦
重的多音字落实措施statistical measurements were obtained from the system:
• 1 percent of all instructions executed accesd a page other than the
current page.
Of the instructions that accesd another page, 80 percent accesd
a page already in memory.Practice Exercis 31
When a new page was required, the replaced page was modified 50
percent of the time.
Calculate the effective instruction time on this system, assuming that the
system is running one process only and that the processor is idle during
drum transfers.
Answer:
effective access time = 0.99 × (1c + 0.008 × (2 c)
+ 0.002 × (10,000 c + 1,000 c)
+ 0.001 × (10,000 c + 1,000 c)
