Not Recently Used (NRU) page replacement algorithm

It is a page replacement algorithm. This algorithm removes a page at random from the lowest numbered non-empty class. Implicit in this algorithm is that it is better to remove a modified page that has not been referenced in atleast one clock tick than a clean page that is in heavy use.

It is easy to understand, moderately efficient to implement and gives a performance that while certainly not optimal, may be adequate. When page is modified, a modified bit is set. When a page needs to be replaced, the Operating System divides pages into 4 classes.

• 0:- Not Referenced, Not Modified
• 1:- Not Referenced, Modified
• 2:- Referenced, Not Modified
• 3:- Referenced, Modified

Out of above 4 categories, NRU will replace a Not Referenced, Not Modified page, if such page exists. Note that this algorithm implies that a Modified but Not Referenced is less important than a Not Modified and Referenced.

Example –

Page	Referenced	Modified
0	1	0
1	0	1
2	0	0
3	1	1

• Case-1 implies to Not Referenced and Modified.
• Case-2 implies to Not Referenced and Not Modified.
• Case-3 implies to Referenced and Modified.
• Case-0 implies to Referenced and Not Modified.

Algorithm :
From the given reference string NRU will remove a page at random from the lowest numbered nonempty class. Implicit in this algorithm is that it is better to remove a modified page that has not been referenced in at least one clock tick (typically 20 msec) than a clean page that is in heavy use.

Example –

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