Question

Dirty bit in page table is used for

1.implementing NRU page replacement

2.implementing optimal page replacement

Comments

NRU???

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I think it will be MRU

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Both.

Because whenever the page fault occurs, then we have to check the status ( modified or not) of the page selected to be replaced, thus every time dirty bit status should be checked before replacement of that particular page.

ref :- https://en.wikipedia.org/wiki/Dirty_bit

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no matter which replacement algorithm is used, dirty bit is always checked, if unset new page directly overwrite older one, if set old page is first copied to MM and then it is replaced with new page

Answer 1

any page replacement algorithm is there ....it must see the dirty bit .....actually pages in CAHCHE or in main memory are copies of actual pages...now say during working....if CPU makes any changes...then such a changes are needed to be reflected in actual copy...so a page may be modified or may not....

HENCE BUT BEFORE SWAP OUT ...ALL ALGOS WILL LOOK OUT FOR DIRTY BIT...so that if dirty bit=1....write the changes in actual copy ...if dirty bit=0...no need of making any change in actual copy ...