One small modification to the clock algorithm (also originally suggested by Corbato“A Paging Experiment with the Multics System” by F.J. Corbato. Included in a Festschrift published in honor of Prof. P.M. Morse. MIT Press, Cambridge, MA, 1969. The original (and hard to find!) reference to the clock algorithm, though not the first usage of a use bit. Thanks to H. Balakrishnan of MIT for digging up this paper for us.) that is commonly made is the additional consideration of whether a page has been modified or not while in memory. The reason for this: if a page has been modified and is thus dirty, it must be written back to disk to evict it, which is expensive. If it has not been modified (and is thus clean), the eviction is free. The physical frame can simply be reused for other purposes without additional I/O. Thus, some VM systems prefer to evict clean pages over dirty pages.
Modified/Dirty bit
To support this behavior, the hardware should include a modified bit (a.k.a. dirty bit). This bit is set any time a page is written and thus can be incorporated into the page-replacement algorithm. The clock algorithm, for example, could be changed to scan for pages that are both unused and clean to evict first; failing to find those, then for unused pages that are dirty, and so forth.