CSE 435- Final Exam Prep

Page table w17 p9 consider a paged virtual memory

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Unformatted text preview: d page table. W17 P9 Consider a paged virtual memory system with 32-bit virtual addresses and 1KB pages. Each page table entry requires 32 bits. It is desired to limit the page table size to one page. a. How many levels of page tables are required? b. What is the number of entries of the page table at each level? Hint: One page table size is smaller. c. The smaller page size could be used at the top level or the bottom level of the page table hierarchy. Which strategy consumes the least number of pages? One page consists of 1024 bytes, and each entry in the page table requires 32 bit / 8 bits per byte = 4 bytes. Then a one-page page table can hold 1024/4 = 256 entries. From a 22-bit virtual address, the displacement part takes 10 bits (1KB = 210), and the rest, 22 bits are used to get to the corresponding page table entry. a. The no. levels of page tables required is 22 bits/8 bits (because 256 entries = 28) = 3 levels. b. Level-1 table is 1KB, 256 entries; level-2 table is IKB, 256 entries; and level-3 table is 768 Bytes, 192 entries. c. The smaller page size should be used at the top level of the page table hierarchy to consume the least number of pages. Problem 1 (a) Briefly define alternative fetch ploicies. Demand paging: A page is brought into main memory only when a reference is made to a location on that page. Prepaging: Pages other than the one demanded by a page fault are brought in (b) What is accomplished by page buffering? A replaced page is not lost but rather is assigned to one of two fists, the free page list if the page has not been modified, or the modified page list if it has. Note that the page is not physically moved about in main memory; instead, the entry in the page table for this page is removed and placed in either the free or modified page list. The important aspect of these maneuvers is that the page to be replaced remains in memory. Thus if the process references that page, it is returned to the resident set of that process at little cost. Problem 2 Is it ever possible that FIFO will. outperform LRU? If so, give a scenario. If not, argue why. (Assume that the number of pages allocated is the same). Problem 3 page reference string: Consider the following 12342156212376321236. How many page faults occur for the following page replacement algorithms, in a. static allocation of 4 frames and assuming that the allocation is initially empty. Assume pure demand fetch. Show the execution to identify page faults. LRU (Least recently used): 10 FIFO (First-in-first-out): 14 OPT (Optimal): 8 Clock Problem 1 (a) What is the difference between a resident net and a working set? Resident set: Pages that are in main memory. Working set: Pages that are used by the processes. (b) What is the difference between demand cleaning and precleani ng? Demand cleaning: A page is written out to secondary memory only when it has been selected for replacement. Precleaning: This policy writes modified pages before their page frames are needed so that page...
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This note was uploaded on 09/26/2013 for the course CSE 435 taught by Professor Cheng during the Fall '07 term at Michigan State University.

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