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dirty bit

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Dirty bit in page table is used for

1.implementing NRU page replacement

2.implementing optimal page replacement

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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 ...
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