Consider the page replacement algorithm used by Atlas.In what ways is it different from the clock algorithm discussed in an earlier chapter?
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Answer:The page replacement algorithm used in Atlas is very different from the clock algorithm discussed in earlier chapters.The Atlas system keeps track of whether a page was accessed in each period of 1024instructions for the last 32 periods.Let t1 be the time since the most
recent reference to a page,while t2 is the interval between the last two references of a page.The paging system then discards any page that has t1>t2+1.If it cannot find any such page,it discards the page with the largest t2-t1.This algorithm assumes that programs access memory in
loops and the idea is to retain pages even if it has not been accessed for a long time if there has been a history of accessing the page regularly albeit at long intervals.The clock algorithm,on the other hand,is an approximate version of the least recently used algorithm and therefore discards the least recently used page without taking into account that some of the pages might be infrequently but repeatedly accessed.