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 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.
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 these 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 addresses 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 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 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 Belady’s
anomaly from those 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
the program has read or write privileges for accessing the page. These
checks have to be performed in hardware. A TLB could serve 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 microsecond. It costs an additional 1
microsecond 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 second. The following
statistical measurements were obtained from the system:
• 1 percent of all instructions executed accessed a page other than the
current page.
Of the instructions that accessed another page, 80 percent accessed
a page already in memory.Practice Exercises 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 × (1sec + 0.008 × (2 sec)
+ 0.002 × (10,000 sec + 1,000 sec)
+ 0.001 × (10,000 sec + 1,000 sec)
>valid from是什么意思

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