Operating System Long Questions and AnswersHere in this section of Operating System Long Questions and Answers,We have listed out some of the important Long Questions with Answers on NRU (Not Recently Used) Page Replacement Algorithm which will help students to answer it correctly in their University Written Exam.


Lists of Long Descriptive type Questions that may be asked in Written Exams.

  • (1) Explain NRU (Not Recently Used) Page Replacement Algorithm


Question-1 Explain NRU (Not Recently Used) Page Replacement Algorithm

NRU (Not Recently Used)

  • NRU makes approximation to replace the page based on R and M bits.
  • When a process is started up, both page bits for all pages are set to 0 by operating system.
  • Periodically, the R bit is cleared, to distinguish pages that have not been referenced recently from those that have been.
  • When page fault occurs, the operating system inspects all the pages and divide them into 4 categories based on current values of their R and M bits
    • Case 0 : not referenced, not modified
    • Case 1 : not referenced, modified
    • Case 2 : referenced, not modified
    • Case 3 : referenced, modified
  • The NRU (Not Recently Used) algorithm removes a page at random from the lowest numbered nonempty class.
  • In given example,
    • Page-0 is of class-2 (referenced, not modified)
    • Page-1 is of class-1 (not referenced, modified)
    • Page-2 is of class-0 ( not referenced, not modified)
    • Page-3 is of class-3 (referenced, modified)

So lowest class page-2 needs to be replaced by NRU.

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