Os Assignment 4 Essay

722 WordsJul 24, 20133 Pages
Quiz 4 (Chapter 8) 1 What is the difference between simple paging and virtual memory paging? Simple paging: all the pages of a process must be in main memory for process to run, unless overlays are used. Virtual memory paging: not all pages of a process need be in main memory frames for the process to run; pages may be read in as needed 2 Explain thrashing. Thrashing is a phenomenon in virtual memory schemes in which the processor spends most of its time swapping pieces rather than executing instructions. 3 Why is the principle of locality crucial to the use of virtual memory? Algorithms can be designed to exploit the principle of locality to avoid thrashing. In general, the principle of locality allows the algorithm to predict which resident pages are least likely to be referenced in the near future and are therefore good candidates for being swapped out. 4 What elements are typically found in a page table entry? Briefly define each element. Frame number is the sequential number that identifies a page in main memory; Present bit indicates whether this page is currently in main memory; Modify bit indicates whether this page has been modified since being brought into main memory. 5 What is the purpose of a Translation Lookaside Buffer? The TLB is a cache that contains those page table entries that have been most recently used. Its purpose is to avoid, most of the time, having to go to disk to retrieve a page table entry. 6 Briefly define the alternative page fetch policies. Demand paging - relevant pages are loaded as page

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