Dirty bit

<h2 id="page-replacement">Page replacement</h2>
<p>When speaking about page replacement, each <a href="/facts/Page_(computer_memory)/CNGruG1H">page</a> may have a modify bit associated with it in the hardware.  The dirty bit for a page is set by the hardware whenever any word or <a href="/facts/Byte/BNHUp9Qq">byte</a> in the page is written into, indicating that the page has been modified.  When a page is selected for replacement, the modify bit is examined.  If the bit is set, the page has been modified since it was read in from the <a href="/facts/Hard_disk_drive/kekLSGSE">disk</a>.  In this case, <i>the page must be written to the disk</i>.  If the dirty bit is not set, however, the page has not been modified since it was read into <a href="/facts/Random-access_memory/DuD4bxh6">memory</a>.  Therefore, if the copy of the page on the disk has not been overwritten (by some other page, for example), then there is no need to write the memory page to the disk: it is already there.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
</p>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Laplante, Philip A. (2001). Dictionary of Computer Science, Engineering, and Technology. CRC Press. p. 138. ISBN 0-8493-2691-5. <a href="0-8493-2691-5" target="_blank">0-8493-2691-5</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Silberschatz, Abraham; Galvin, Peter Baer; Gagne, Greg (2002). Operating System Concepts: Sixth Edition. p. 333. ISBN 0-471-41743-2. <a href="0-471-41743-2" target="_blank">0-471-41743-2</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
</ol>

Dirty bit open-in-new

Dirty bit