vshingala.github.io/NRU.html at master · VShingala/vshingala.github.io · GitHub

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<title>Page Replacement Algorithms</title>
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	  <a href="pagereplacementpage.html">Optimal Page Repalcement</a>
	  <a href="NRU.html">NRU Page Replacement</a>
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	  <a href="SC.html">Second Chance Page Replacement</a>
	  <a href="LRU.html">LRU Page Replacement</a>

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		<br>
		<h4>Not Recently Used Page Replacement</h4>

			The NRU (Not Recently Used) algorithm removes a page at random from the
			lowest-numbered nonempty class. Implicit in this algorithm is the idea that it is
			better to remove a modified page that has not been referenced in at least one clock
			tick (typically about 20 msec) than a clean page that is in heavy use. The main
			attraction of NRU is that it is easy to understand, moderately efficient to implement,
			and gives a performance that, while certainly not optimal, may be adequate.<br><br>

			When a page fault occurs, the operating system inspects all the pages and
			divides them into four categories based on the current values of their R and M bits:<br>

			<ul>
				<li>Class 0: not referenced, not modified.</li>
				<li>Class 1: not referenced, modified.</li>
				<li>Class 2: referenced, not modified.</li>
				<li>Class 3: referenced, modified.</li>
			</ul>
		<br>

			In order to allow the operating system to collect useful page usage statistics,
			most computers with virtual memory have two status bits, R and M, associated
			with each page. R is set whenever the page is referenced (read or written). M is
			set when the page is written to (i.e., modified).<br>
			If the hardware does not have these bits, they can be simulated using the operating
			system’s page fault and clock interrupt mechanisms<br>

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