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Dirty bit for a page in a page table

  1. helps avoid unnecessary writes on a paging device
  2. helps maintain LRU information
  3. allows only read on a page
  4. None of the above
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Best answer
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57 votes

The dirty bit allows for a performance optimization. A page on disk that is paged in to physical memory, then read from, and subsequently paged out again does not need to be written back to disk, since the page hasn't changed. However, if the page was written to after it's paged in, its dirty bit will be set, indicating that the page must be written back to the backing store answer: (A)

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ans is A...to perform any write operation on page firstly we check the dirty bit status...dirty bit will be set when page will be modified(suppose s=1 set condition) and it will remain s=1(set) until the page modified will write back into the memory..if page was not modified then it will not change the dirty bit value i.e.s=0...now  suppose we perform write operation on page then its dirty bit become s=1..and suppose after some time we were performing write operation on that page then firstly we check the dirty bit value here its value is 1 it means page was modified..so firstly we write that page to memory and then perform any modification by this way we were avoiding unnecessary write operation on paging device
2 votes
2 votes

Suppose if we don't have "Dirty but concept" then whenever page fault occurs, the page has to be written back first in secondary memory in order to assure consistency ( even though the page may or may not be modified ).

Now if we have "Dirty bit concept" then by examining it's status it can be concluded whether the page should be written back in secondary memory or not whenever page fault occurs. In this manner "Dirty bit helps avoid unnecessary writes on paging device"

So (A) is correct 

Answer:

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