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class="vector-toc-list"> <li id="toc-Page_tables" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Page_tables"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1</span> <span>Page tables</span> </div> </a> <ul id="toc-Page_tables-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Paging_supervisor" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Paging_supervisor"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.2</span> <span>Paging supervisor</span> </div> </a> <ul id="toc-Paging_supervisor-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Pinned_pages" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Pinned_pages"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.3</span> <span>Pinned pages</span> </div> </a> <ul id="toc-Pinned_pages-sublist" class="vector-toc-list"> <li id="toc-Virtual-real_operation" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Virtual-real_operation"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.3.1</span> <span>Virtual-real operation</span> </div> </a> <ul id="toc-Virtual-real_operation-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Thrashing" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Thrashing"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.4</span> <span>Thrashing</span> </div> </a> <ul id="toc-Thrashing-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Segmented_virtual_memory" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Segmented_virtual_memory"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Segmented virtual memory</span> </div> </a> <ul id="toc-Segmented_virtual_memory-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Address_space_swapping" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Address_space_swapping"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Address space swapping</span> </div> </a> <ul id="toc-Address_space_swapping-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Notes" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Notes"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>Notes</span> </div> </a> <ul id="toc-Notes-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">9</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Further_reading" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Further_reading"> <div class="vector-toc-text"> <span class="vector-toc-numb">10</span> <span>Further reading</span> </div> </a> <ul id="toc-Further_reading-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">11</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" title="Table of Contents" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Virtual memory</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 43 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-43" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">43 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D8%B0%D8%A7%D9%83%D8%B1%D8%A9_%D8%A7%D9%81%D8%AA%D8%B1%D8%A7%D8%B6%D9%8A%D8%A9" title="ذاكرة افتراضية – Arabic" lang="ar" hreflang="ar" data-title="ذاكرة افتراضية" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-be-x-old mw-list-item"><a href="https://be-tarask.wikipedia.org/wiki/%D0%92%D1%96%D1%80%D1%82%D1%83%D0%B0%D0%BB%D1%8C%D0%BD%D0%B0%D1%8F_%D0%BF%D0%B0%D0%BC%D1%8F%D1%86%D1%8C" title="Віртуальная памяць – Belarusian (Taraškievica orthography)" lang="be-tarask" hreflang="be-tarask" data-title="Віртуальная памяць" data-language-autonym="Беларуская (тарашкевіца)" data-language-local-name="Belarusian (Taraškievica orthography)" class="interlanguage-link-target"><span>Беларуская (тарашкевіца)</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Mem%C3%B2ria_virtual" title="Memòria virtual – Catalan" lang="ca" hreflang="ca" data-title="Memòria virtual" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Virtu%C3%A1ln%C3%AD_pam%C4%9B%C5%A5" title="Virtuální paměť – Czech" lang="cs" hreflang="cs" data-title="Virtuální paměť" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-da mw-list-item"><a href="https://da.wikipedia.org/wiki/Virtuel_hukommelse" title="Virtuel hukommelse – Danish" lang="da" hreflang="da" data-title="Virtuel hukommelse" data-language-autonym="Dansk" data-language-local-name="Danish" class="interlanguage-link-target"><span>Dansk</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Virtuelle_Speicherverwaltung" title="Virtuelle Speicherverwaltung – German" lang="de" hreflang="de" data-title="Virtuelle Speicherverwaltung" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/Virtuaalm%C3%A4lu" title="Virtuaalmälu – Estonian" lang="et" hreflang="et" data-title="Virtuaalmälu" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/%CE%95%CE%B9%CE%BA%CE%BF%CE%BD%CE%B9%CE%BA%CE%AE_%CE%BC%CE%BD%CE%AE%CE%BC%CE%B7" title="Εικονική μνήμη – Greek" lang="el" hreflang="el" data-title="Εικονική μνήμη" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Memoria_virtual" title="Memoria virtual – Spanish" lang="es" hreflang="es" data-title="Memoria virtual" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/Virtuala_memoro" title="Virtuala memoro – Esperanto" lang="eo" hreflang="eo" data-title="Virtuala memoro" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-eu mw-list-item"><a href="https://eu.wikipedia.org/wiki/Alegiazko_memoria" title="Alegiazko memoria – Basque" lang="eu" hreflang="eu" data-title="Alegiazko memoria" data-language-autonym="Euskara" data-language-local-name="Basque" class="interlanguage-link-target"><span>Euskara</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%AD%D8%A7%D9%81%D8%B8%D9%87_%D9%85%D8%AC%D8%A7%D8%B2%DB%8C" title="حافظه مجازی – Persian" lang="fa" hreflang="fa" data-title="حافظه مجازی" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/M%C3%A9moire_virtuelle" title="Mémoire virtuelle – French" lang="fr" hreflang="fr" data-title="Mémoire virtuelle" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-gl mw-list-item"><a href="https://gl.wikipedia.org/wiki/Memoria_virtual" title="Memoria virtual – Galician" lang="gl" hreflang="gl" data-title="Memoria virtual" data-language-autonym="Galego" data-language-local-name="Galician" class="interlanguage-link-target"><span>Galego</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EA%B0%80%EC%83%81_%EB%A9%94%EB%AA%A8%EB%A6%AC" title="가상 메모리 – Korean" lang="ko" hreflang="ko" data-title="가상 메모리" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-hy mw-list-item"><a href="https://hy.wikipedia.org/wiki/%D5%8E%D5%AB%D6%80%D5%BF%D5%B8%D6%82%D5%A1%D5%AC_%D5%B0%D5%AB%D5%B7%D5%B8%D5%B2%D5%B8%D6%82%D5%A9%D5%B5%D5%B8%D6%82%D5%B6" title="Վիրտուալ հիշողություն – Armenian" lang="hy" hreflang="hy" data-title="Վիրտուալ հիշողություն" data-language-autonym="Հայերեն" data-language-local-name="Armenian" class="interlanguage-link-target"><span>Հայերեն</span></a></li><li class="interlanguage-link interwiki-hi mw-list-item"><a href="https://hi.wikipedia.org/wiki/%E0%A4%B5%E0%A4%B0%E0%A5%8D%E0%A4%9A%E0%A5%81%E0%A4%85%E0%A4%B2_%E0%A4%AE%E0%A5%87%E0%A4%AE%E0%A5%8B%E0%A4%B0%E0%A5%80_(%E0%A4%86%E0%A4%AD%E0%A4%BE%E0%A4%B8%E0%A5%80_%E0%A4%B8%E0%A5%8D%E0%A4%AE%E0%A5%83%E0%A4%A4%E0%A4%BF)" title="वर्चुअल मेमोरी (आभासी स्मृति) – Hindi" lang="hi" hreflang="hi" data-title="वर्चुअल मेमोरी (आभासी स्मृति)" data-language-autonym="हिन्दी" data-language-local-name="Hindi" class="interlanguage-link-target"><span>हिन्दी</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Memori_virtual" title="Memori virtual – Indonesian" lang="id" hreflang="id" data-title="Memori virtual" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Memoria_virtuale" title="Memoria virtuale – Italian" lang="it" hreflang="it" data-title="Memoria virtuale" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/%D7%96%D7%99%D7%9B%D7%A8%D7%95%D7%9F_%D7%95%D7%99%D7%A8%D7%98%D7%95%D7%90%D7%9C%D7%99" title="זיכרון וירטואלי – Hebrew" lang="he" hreflang="he" data-title="זיכרון וירטואלי" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-kk mw-list-item"><a href="https://kk.wikipedia.org/wiki/%D0%92%D0%B8%D1%80%D1%82%D1%83%D0%B0%D0%BB%D0%B4%D1%8B%D2%9B_%D1%81%D0%B0%D2%9B%D1%82%D0%B0%D1%83%D1%8B%D1%88_%D2%9B%D2%B1%D1%80%D1%8B%D0%BB%D2%93%D1%8B" title="Виртуалдық сақтауыш құрылғы – Kazakh" lang="kk" hreflang="kk" data-title="Виртуалдық сақтауыш құрылғы" data-language-autonym="Қазақша" data-language-local-name="Kazakh" class="interlanguage-link-target"><span>Қазақша</span></a></li><li class="interlanguage-link interwiki-ky mw-list-item"><a href="https://ky.wikipedia.org/wiki/%D0%92%D0%B8%D1%80%D1%82%D1%83%D0%B0%D0%BB%D0%B4%D1%8B%D0%BA_%D1%8D%D1%81_%D0%B6%D0%B5_%D0%B2%D0%B8%D1%80%D1%82%D1%83%D0%B0%D0%BB%D0%B4%D1%8B%D0%BA_%D1%8D%D1%81%D1%82%D0%B8_%D0%B1%D0%B0%D1%88%D0%BA%D0%B0%D1%80%D1%83%D1%83" title="Виртуалдык эс же виртуалдык эсти башкаруу – Kyrgyz" lang="ky" hreflang="ky" data-title="Виртуалдык эс же виртуалдык эсти башкаруу" data-language-autonym="Кыргызча" data-language-local-name="Kyrgyz" class="interlanguage-link-target"><span>Кыргызча</span></a></li><li class="interlanguage-link interwiki-lt mw-list-item"><a href="https://lt.wikipedia.org/wiki/Virtualioji_atmintis" title="Virtualioji atmintis – Lithuanian" lang="lt" hreflang="lt" data-title="Virtualioji atmintis" data-language-autonym="Lietuvių" data-language-local-name="Lithuanian" class="interlanguage-link-target"><span>Lietuvių</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Virtu%C3%A1lis_mem%C3%B3ria" title="Virtuális memória – Hungarian" lang="hu" hreflang="hu" data-title="Virtuális memória" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/Ingatan_maya" title="Ingatan maya – Malay" lang="ms" hreflang="ms" data-title="Ingatan maya" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Virtueel_geheugen" title="Virtueel geheugen – Dutch" lang="nl" hreflang="nl" data-title="Virtueel geheugen" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E4%BB%AE%E6%83%B3%E8%A8%98%E6%86%B6" title="仮想記憶 – Japanese" lang="ja" hreflang="ja" data-title="仮想記憶" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/Virtuell_hukommelse" title="Virtuell hukommelse – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="Virtuell hukommelse" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Pami%C4%99%C4%87_wirtualna" title="Pamięć wirtualna – Polish" lang="pl" hreflang="pl" data-title="Pamięć wirtualna" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Mem%C3%B3ria_virtual" title="Memória virtual – Portuguese" lang="pt" hreflang="pt" data-title="Memória virtual" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%92%D0%B8%D1%80%D1%82%D1%83%D0%B0%D0%BB%D1%8C%D0%BD%D0%B0%D1%8F_%D0%BF%D0%B0%D0%BC%D1%8F%D1%82%D1%8C" title="Виртуальная память – Russian" lang="ru" hreflang="ru" data-title="Виртуальная память" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sq badge-Q70893996 mw-list-item" title=""><a href="https://sq.wikipedia.org/wiki/Virtual_Memory" title="Virtual Memory – Albanian" lang="sq" hreflang="sq" data-title="Virtual Memory" data-language-autonym="Shqip" data-language-local-name="Albanian" class="interlanguage-link-target"><span>Shqip</span></a></li><li class="interlanguage-link interwiki-si mw-list-item"><a href="https://si.wikipedia.org/wiki/%E0%B6%85%E0%B6%AD%E0%B6%AE%E0%B7%8A%E2%80%8D%E0%B6%BA_%E0%B6%B8%E0%B6%AD%E0%B6%9A%E0%B6%BA" title="අතථ්‍ය මතකය – Sinhala" lang="si" hreflang="si" data-title="අතථ්‍ය මතකය" data-language-autonym="සිංහල" data-language-local-name="Sinhala" class="interlanguage-link-target"><span>සිංහල</span></a></li><li class="interlanguage-link interwiki-simple mw-list-item"><a href="https://simple.wikipedia.org/wiki/Virtual_memory" title="Virtual memory – Simple English" lang="en-simple" hreflang="en-simple" data-title="Virtual memory" data-language-autonym="Simple English" data-language-local-name="Simple English" class="interlanguage-link-target"><span>Simple English</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/Virtu%C3%A1lna_pam%C3%A4%C5%A5" title="Virtuálna pamäť – Slovak" lang="sk" hreflang="sk" data-title="Virtuálna pamäť" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-sl mw-list-item"><a href="https://sl.wikipedia.org/wiki/Navidezni_pomnilnik" title="Navidezni pomnilnik – Slovenian" lang="sl" hreflang="sl" data-title="Navidezni pomnilnik" data-language-autonym="Slovenščina" data-language-local-name="Slovenian" class="interlanguage-link-target"><span>Slovenščina</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/%D0%92%D0%B8%D1%80%D1%82%D1%83%D0%B5%D0%BB%D0%BD%D0%B0_%D0%BC%D0%B5%D0%BC%D0%BE%D1%80%D0%B8%D1%98%D0%B0" title="Виртуелна меморија – Serbian" lang="sr" hreflang="sr" data-title="Виртуелна меморија" data-language-autonym="Српски / srpski" 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id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Computer memory management technique</div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">This article is about the computer memory management technique. For the technique of pooling multiple storage devices, see <a href="/wiki/Storage_virtualization" title="Storage virtualization">Storage virtualization</a>. For the TBN game show, see <a href="/wiki/Virtual_Memory_(game_show)" class="mw-redirect" title="Virtual Memory (game show)">Virtual Memory (game show)</a>.</div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Not to be confused with <a href="/wiki/VRAM" class="mw-redirect" title="VRAM">VRAM</a>.</div> <p class="mw-empty-elt"> </p> <figure typeof="mw:File/Thumb"><a href="/wiki/File:Virtual_memory.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6e/Virtual_memory.svg/250px-Virtual_memory.svg.png" decoding="async" width="250" height="395" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6e/Virtual_memory.svg/375px-Virtual_memory.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6e/Virtual_memory.svg/500px-Virtual_memory.svg.png 2x" data-file-width="620" data-file-height="980" /></a><figcaption>Virtual memory combines active <a href="/wiki/Random-access_memory" title="Random-access memory">RAM</a> and inactive memory on <a href="/wiki/Direct-access_storage_device" title="Direct-access storage device">DASD</a><sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>a<span class="cite-bracket">]</span></a></sup> to form a large range of contiguous addresses.</figcaption></figure> <p>In <a href="/wiki/Computing" title="Computing">computing</a>, <b>virtual memory</b>, or <b>virtual storage</b>,<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>b<span class="cite-bracket">]</span></a></sup> is a <a href="/wiki/Memory_management_(operating_systems)" title="Memory management (operating systems)">memory management</a> technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine"<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> which "creates the illusion to users of a very large (main) memory".<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p><p>The computer's <a href="/wiki/Operating_system" title="Operating system">operating system</a>, using a combination of hardware and software, maps <a href="/wiki/Memory_address" title="Memory address">memory addresses</a> used by a program, called <i><a href="/wiki/Virtual_address_space" title="Virtual address space">virtual addresses</a></i>, into <i>physical addresses</i> in <a href="/wiki/Computer_memory" title="Computer memory">computer memory</a>. <a href="/wiki/Main_storage#Primary_storage" class="mw-redirect" title="Main storage">Main storage</a>, as seen by a process or task, appears as a contiguous <a href="/wiki/Address_space" title="Address space">address space</a> or collection of contiguous <a href="/wiki/Memory_segmentation" title="Memory segmentation">segments</a>. The operating system manages <a href="/wiki/Virtual_address_space" title="Virtual address space">virtual address spaces</a> and the assignment of real memory to virtual memory.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> Address translation hardware in the CPU, often referred to as a <a href="/wiki/Memory_management_unit" title="Memory management unit">memory management unit</a> (MMU), automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities, utilizing, e.g., <a href="/wiki/Disk_storage" title="Disk storage">disk storage</a>, to provide a virtual address space that can exceed the capacity of real memory and thus reference more memory than is physically present in the computer. </p><p>The primary benefits of virtual memory include freeing applications from having to manage a shared memory space, ability to share memory used by <a href="/wiki/Library_(computing)" title="Library (computing)">libraries</a> between processes, increased security due to memory isolation, and being able to conceptually use more memory than might be physically available, using the technique of <a href="/wiki/Paging" class="mw-redirect" title="Paging">paging</a> or segmentation. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Properties">Properties</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=1" title="Edit section: Properties"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Virtual memory makes application programming easier by hiding <a href="/wiki/Fragmentation_(computer)" class="mw-redirect" title="Fragmentation (computer)">fragmentation</a> of physical memory; by delegating to the kernel the burden of managing the <a href="/wiki/Computer_data_storage#Hierarchy_of_storage" title="Computer data storage">memory hierarchy</a> (eliminating the need for the program to handle <a href="/wiki/Overlay_(programming)" title="Overlay (programming)">overlays</a> explicitly); and, when each process is run in its own dedicated address space, by obviating the need <a href="/wiki/Relocation_(computer_science)" class="mw-redirect" title="Relocation (computer science)">to relocate</a> program code or to access memory with <a href="/wiki/Addressing_mode#PC-relative" title="Addressing mode">relative addressing</a>. </p><p><a href="/wiki/Memory_virtualization" title="Memory virtualization">Memory virtualization</a> can be considered a generalization of the concept of virtual memory. </p> <div class="mw-heading mw-heading2"><h2 id="Usage">Usage</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=2" title="Edit section: Usage"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Virtual memory is an integral part of a modern <a href="/wiki/Computer_architecture" title="Computer architecture">computer architecture</a>; implementations usually require hardware support, typically in the form of a <a href="/wiki/Memory_management_unit" title="Memory management unit">memory management unit</a> built into the <a href="/wiki/Central_processing_unit" title="Central processing unit">CPU</a>. While not necessary, <a href="/wiki/Emulators" class="mw-redirect" title="Emulators">emulators</a> and <a href="/wiki/Virtual_machine" title="Virtual machine">virtual machines</a> can employ hardware support to increase performance of their virtual memory implementations.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> Older operating systems, such as those for the <a href="/wiki/Mainframe_computer" title="Mainframe computer">mainframes</a> of the 1960s, and those for personal computers of the early to mid-1980s (e.g., <a href="/wiki/DOS" title="DOS">DOS</a>),<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> generally have no virtual memory functionality,<sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Accuracy_dispute#Disputed_statement" title="Wikipedia:Accuracy dispute"><span title="a lot of paging computers were shipped in the 1960 (November 2010)">dubious</span></a> – <a href="/wiki/Talk:Virtual_memory#Prevalence_of_virtual_memory_operating_systems_in_the_1960s_and_early_1970s" title="Talk:Virtual memory">discuss</a></i>]</sup> though notable exceptions for mainframes of the 1960s include: </p> <ul><li>the <a href="/wiki/Atlas_Supervisor" title="Atlas Supervisor">Atlas Supervisor</a> for the <a href="/wiki/Atlas_(computer)" title="Atlas (computer)">Atlas</a></li> <li><a href="/wiki/THE_multiprogramming_system" title="THE multiprogramming system">THE multiprogramming system</a> for the <a href="/wiki/Electrologica_X8" title="Electrologica X8">Electrologica X8</a> (software based virtual memory without hardware support)</li> <li><a href="/wiki/Burroughs_MCP" title="Burroughs MCP">MCP</a> for the <a href="/wiki/Burroughs_Corporation" title="Burroughs Corporation">Burroughs</a> <a href="/wiki/B5000" class="mw-redirect" title="B5000">B5000</a></li> <li><a href="/wiki/Michigan_Terminal_System" title="Michigan Terminal System">MTS</a>, <a href="/wiki/TSS/360" class="mw-redirect" title="TSS/360">TSS/360</a> and <a href="/wiki/CP/CMS" title="CP/CMS">CP/CMS</a> for the <a href="/wiki/IBM_System/360_Model_67" title="IBM System/360 Model 67">IBM System/360 Model 67</a></li> <li><a href="/wiki/Multics" title="Multics">Multics</a> for the <a href="/wiki/GE-600_series" title="GE-600 series">GE 645</a></li> <li>The <a href="/wiki/Time_Sharing_Operating_System" title="Time Sharing Operating System">Time Sharing Operating System</a> for the <a href="/wiki/RCA_Spectra_70" title="RCA Spectra 70">RCA Spectra 70</a>/46</li></ul> <p>During the 1960s and early '70s, computer memory was very expensive. The introduction of virtual memory provided an ability for software systems with large memory demands to run on computers with less real memory. The savings from this provided a strong incentive to switch to virtual memory for all systems. The additional capability of providing virtual address spaces added another level of security and reliability, thus making virtual memory even more attractive to the marketplace. </p><p>Most modern operating systems that support virtual memory also run each <a href="/wiki/Process_(computing)" title="Process (computing)">process</a> in its own dedicated <a href="/wiki/Address_space" title="Address space">address space</a>. Each program thus appears to have sole access to the virtual memory. However, some older operating systems (such as <a href="/wiki/OS/VS1" title="OS/VS1">OS/VS1</a> and <a href="/wiki/OS/VS2_(SVS)" title="OS/VS2 (SVS)">OS/VS2 SVS</a>) and even modern ones (such as <a href="/wiki/IBM_i" title="IBM i">IBM i</a>) are <a href="/wiki/Single_address_space_operating_system" title="Single address space operating system">single address space operating systems</a> that run all processes in a single address space composed of virtualized memory. </p><p><a href="/wiki/Embedded_system" title="Embedded system">Embedded systems</a> and other special-purpose computer systems that require very fast and/or very consistent response times may opt not to use virtual memory due to decreased <a href="/wiki/Deterministic_algorithm" title="Deterministic algorithm">determinism</a>; virtual memory systems trigger unpredictable <a href="/wiki/Trap_(computing)" class="mw-redirect" title="Trap (computing)">traps</a> that may produce unwanted and unpredictable delays in response to input, especially if the trap requires that data be read into main memory from secondary memory. The hardware to translate virtual addresses to physical addresses typically requires a significant chip area to implement, and not all chips used in embedded systems include that hardware, which is another reason some of those systems do not use virtual memory. </p> <div class="mw-heading mw-heading2"><h2 id="History">History</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=3" title="Edit section: History"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In the 1950s, all larger programs had to contain logic for managing primary and secondary storage, such as <a href="/wiki/Overlay_(programming)" title="Overlay (programming)">overlaying</a>. Virtual memory was therefore introduced not only to extend primary memory, but to make such an extension as easy as possible for programmers to use.<sup id="cite_ref-denning_10-0" class="reference"><a href="#cite_note-denning-10"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> To allow for <a href="/wiki/Multiprogramming" class="mw-redirect" title="Multiprogramming">multiprogramming</a> and <a href="/wiki/Computer_multitasking" title="Computer multitasking">multitasking</a>, many early systems divided memory between multiple programs without virtual memory, such as early models of the <a href="/wiki/PDP-10" title="PDP-10">PDP-10</a> via <a href="/wiki/Processor_register" title="Processor register">registers</a>. </p><p>A claim that the concept of virtual memory was first developed by German physicist <a href="/w/index.php?title=Fritz-Rudolf_G%C3%BCntsch&action=edit&redlink=1" class="new" title="Fritz-Rudolf Güntsch (page does not exist)">Fritz-Rudolf Güntsch</a> at the <a href="/wiki/Technische_Universit%C3%A4t_Berlin" title="Technische Universität Berlin">Technische Universität Berlin</a> in 1956 in his doctoral thesis, <i>Logical Design of a Digital Computer with Multiple Asynchronous Rotating Drums and Automatic High Speed Memory Operation</i>,<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-springer_12-0" class="reference"><a href="#cite_note-springer-12"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> does not stand up to careful scrutiny. The computer proposed by Güntsch (but never built) had an address space of 10<sup>5</sup> words which mapped exactly onto the 10<sup>5</sup> words of the drums, i.e. the addresses were real addresses and there was no form of indirect mapping, a key feature of virtual memory. What Güntsch did invent was a form of <a href="/wiki/Cache_memory" class="mw-redirect" title="Cache memory">cache memory</a>, since his high-speed memory was intended to contain a copy of some blocks of code or data taken from the drums. Indeed, he wrote (as quoted in translation<sup id="cite_ref-FOOTNOTEJessen2004_13-0" class="reference"><a href="#cite_note-FOOTNOTEJessen2004-13"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup>): "The programmer need not respect the existence of the primary memory (he need not even know that it exists), for there is only one sort of addresses  [<i><a href="/wiki/Sic" title="Sic">sic</a></i>] by which one can program as if there were only one storage." This is exactly the situation in computers with cache memory, one of the earliest commercial examples of which was the IBM System/360 Model 85.<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> In the Model 85 all addresses were real addresses referring to the main core store. A semiconductor cache store, invisible to the user, held the contents of parts of the main store in use by the currently executing program. This is exactly analogous to Güntsch's system, designed as a means to improve performance, rather than to solve the problems involved in multi-programming. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:University_of_Manchester_Atlas,_January_1963.JPG" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/d/d9/University_of_Manchester_Atlas%2C_January_1963.JPG/220px-University_of_Manchester_Atlas%2C_January_1963.JPG" decoding="async" width="220" height="146" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/d9/University_of_Manchester_Atlas%2C_January_1963.JPG/330px-University_of_Manchester_Atlas%2C_January_1963.JPG 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/d9/University_of_Manchester_Atlas%2C_January_1963.JPG/440px-University_of_Manchester_Atlas%2C_January_1963.JPG 2x" data-file-width="1788" data-file-height="1188" /></a><figcaption>The University of Manchester <a href="/wiki/Atlas_Computer" class="mw-redirect" title="Atlas Computer">Atlas Computer</a> was the first computer to feature true virtual memory.</figcaption></figure> <p>The first true virtual memory system was that implemented at the <a href="/wiki/University_of_Manchester" title="University of Manchester">University of Manchester</a> to create a one-level storage system<sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> as part of the <a href="/wiki/Atlas_Computer" class="mw-redirect" title="Atlas Computer">Atlas Computer</a>. It used a <a href="/wiki/Paging" class="mw-redirect" title="Paging">paging</a> mechanism to map the virtual addresses available to the programmer onto the real memory that consisted of 16,384 words of primary <a href="/wiki/Magnetic-core_memory" title="Magnetic-core memory">core memory</a> with an additional 98,304 words of secondary <a href="/wiki/Drum_memory" title="Drum memory">drum memory</a>.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> The addition of virtual memory into the Atlas also eliminated a looming programming problem: planning and scheduling data transfers between main and secondary memory and recompiling programs for each change of size of main memory.<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> The first Atlas was commissioned in 1962 but working prototypes of paging had been developed by 1959.<sup id="cite_ref-denning_10-1" class="reference"><a href="#cite_note-denning-10"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page: 2">: 2 </span></sup><sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> </p><p>As early as 1958, <a href="/wiki/Robert_S._Barton" title="Robert S. Barton">Robert S. Barton</a>, working at Shell Research, suggested that main storage should be allocated automatically rather than have the programmer being concerned with overlays from secondary memory, in effect virtual memory.<sup id="cite_ref-Waychoff_1979_20-0" class="reference"><a href="#cite_note-Waychoff_1979-20"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 49">: 49 </span></sup><sup id="cite_ref-IEEE-Computer-Aug-1977_21-0" class="reference"><a href="#cite_note-IEEE-Computer-Aug-1977-21"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> By 1960 Barton was lead architect on the Burroughs B5000 project. From 1959 to 1961, W. R. Lonergan was manager of the Burroughs Product Planning Group which included Barton, <a href="/wiki/Donald_Knuth" title="Donald Knuth">Donald Knuth</a> as consultant, and Paul King. In May 1960, UCLA ran a two-week seminar "Using and Exploiting Giant Computers" to which Paul King and two others were sent. Stan Gill gave a presentation on virtual memory in the Atlas I computer. Paul King took the ideas back to Burroughs and it was determined that virtual memory should be designed into the core of the B5000.<sup id="cite_ref-Waychoff_1979_20-1" class="reference"><a href="#cite_note-Waychoff_1979-20"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page: 3">: 3 </span></sup>. Burroughs Corporation released the B5000 in 1964 as the first commercial computer with virtual memory.<sup id="cite_ref-22" class="reference"><a href="#cite_note-22"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> </p><p>IBM developed<sup id="cite_ref-24" class="reference"><a href="#cite_note-24"><span class="cite-bracket">[</span>c<span class="cite-bracket">]</span></a></sup> the concept of <a href="/wiki/Hypervisor" title="Hypervisor">hypervisors</a> in their <a href="/wiki/IBM_CP-40" title="IBM CP-40">CP-40</a> and <a href="/wiki/CP-67" title="CP-67">CP-67</a>, and in 1972 provided it for the <a href="/wiki/IBM_System/370" title="IBM System/370">S/370</a> as Virtual Machine Facility/370.<sup id="cite_ref-25" class="reference"><a href="#cite_note-25"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> IBM introduced the Start Interpretive Execution (<b>SIE</b>) instruction as part of 370-XA on the 3081, and VM/XA versions of <a href="/wiki/VM_(operating_system)" title="VM (operating system)">VM</a> to exploit it. </p><p>Before virtual memory could be implemented in mainstream operating systems, many problems had to be addressed. Dynamic address translation required expensive and difficult-to-build specialized hardware; initial implementations slowed down access to memory slightly.<sup id="cite_ref-denning_10-2" class="reference"><a href="#cite_note-denning-10"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> There were worries that new system-wide algorithms utilizing secondary storage would be less effective than previously used application-specific algorithms. By 1969, the debate over virtual memory for commercial computers was over;<sup id="cite_ref-denning_10-3" class="reference"><a href="#cite_note-denning-10"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> an <a href="/wiki/IBM" title="IBM">IBM</a> research team led by <a href="/wiki/David_Sayre" title="David Sayre">David Sayre</a> showed that their virtual memory overlay system consistently worked better than the best manually controlled systems.<sup id="cite_ref-26" class="reference"><a href="#cite_note-26"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> Throughout the 1970s, the IBM 370 series running their virtual-storage based operating systems provided a means for business users to migrate multiple older systems into fewer, more powerful, mainframes that had improved price/performance. The first <a href="/wiki/Minicomputer" title="Minicomputer">minicomputer</a> to introduce virtual memory was the Norwegian <a href="/wiki/NORD-1" class="mw-redirect" title="NORD-1">NORD-1</a>; during the 1970s, other minicomputers implemented virtual memory, notably <a href="/wiki/VAX" title="VAX">VAX</a> models running <a href="/wiki/OpenVMS" title="OpenVMS">VMS</a>. </p><p>Virtual memory was introduced to the <a href="/wiki/X86" title="X86">x86</a> architecture with the <a href="/wiki/Protected_mode" title="Protected mode">protected mode</a> of the <a href="/wiki/Intel_80286" title="Intel 80286">Intel 80286</a> processor, but its segment swapping technique scaled poorly to larger segment sizes. The <a href="/wiki/Intel_80386" class="mw-redirect" title="Intel 80386">Intel 80386</a> introduced paging support underneath the existing <a href="/wiki/Segmentation_(memory)" class="mw-redirect" title="Segmentation (memory)">segmentation</a> layer, enabling the page fault exception to chain with other exceptions without <a href="/wiki/Double_fault" title="Double fault">double fault</a>. However, loading segment descriptors was an expensive operation, causing operating system designers to rely strictly on paging rather than a combination of paging and segmentation.<sup id="cite_ref-27" class="reference"><a href="#cite_note-27"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Paged_virtual_memory">Paged virtual memory</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=4" title="Edit section: Paged virtual memory"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">See also: <a href="/wiki/Memory_paging" title="Memory paging">Memory paging</a></div> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-More_citations_needed_section plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This section <b>needs additional citations for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Virtual_memory" title="Special:EditPage/Virtual memory">improve this article</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a> in this section. Unsourced material may be challenged and removed.</span> <span class="date-container"><i>(<span class="date">December 2010</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>Nearly all current implementations of virtual memory divide a <a href="/wiki/Virtual_address_space" title="Virtual address space">virtual address space</a> into <a href="/wiki/Page_(computer_memory)" title="Page (computer memory)">pages</a>, blocks of contiguous virtual memory addresses. Pages on contemporary<sup id="cite_ref-28" class="reference"><a href="#cite_note-28"><span class="cite-bracket">[</span>d<span class="cite-bracket">]</span></a></sup> systems are usually at least 4 <a href="/wiki/Kilobyte" title="Kilobyte">kilobytes</a> in size; systems with large virtual address ranges or amounts of real memory generally use larger page sizes.<sup id="cite_ref-29" class="reference"><a href="#cite_note-29"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Page_tables">Page tables</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=5" title="Edit section: Page tables"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Page_table" title="Page table">Page tables</a> are used to translate the virtual addresses seen by the application into <a href="/wiki/Physical_address" title="Physical address">physical addresses</a> used by the <a href="/wiki/Computer_hardware" title="Computer hardware">hardware</a> to process instructions;<sup id="cite_ref-30" class="reference"><a href="#cite_note-30"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> such hardware that handles this specific translation is often known as the <a href="/wiki/Memory_management_unit" title="Memory management unit">memory management unit</a>. Each entry in the page table holds a flag indicating whether the corresponding page is in real memory or not. If it is in real memory, the page table entry will contain the real memory address at which the page is stored. When a reference is made to a page by the hardware, if the page table entry for the page indicates that it is not currently in real memory, the hardware raises a <a href="/wiki/Page_fault" title="Page fault">page fault</a> <a href="/wiki/Trap_(computing)" class="mw-redirect" title="Trap (computing)">exception</a>, invoking the paging supervisor component of the <a href="/wiki/Operating_system" title="Operating system">operating system</a>. </p><p>Systems can have, e.g., one page table for the whole system, separate page tables for each address space or process, separate page tables for each segment; similarly, systems can have, e.g., no segment table, one segment table for the whole system, separate segment tables for each address space or process, separate segment tables for each <i>region</i> in a tree<sup id="cite_ref-32" class="reference"><a href="#cite_note-32"><span class="cite-bracket">[</span>e<span class="cite-bracket">]</span></a></sup> of region tables for each address space or process. If there is only one page table, different applications <a href="/wiki/Multiprogramming" class="mw-redirect" title="Multiprogramming">running at the same time</a> use different parts of a single range of virtual addresses. If there are multiple page or segment tables, there are multiple virtual address spaces and concurrent applications with separate page tables redirect to different real addresses. </p><p>Some earlier systems with smaller real memory sizes, such as the <a href="/wiki/SDS_940" title="SDS 940">SDS 940</a>, used <i><a href="/wiki/Page_address_register" title="Page address register">page registers</a></i> instead of page tables in memory for address translation. </p> <div class="mw-heading mw-heading3"><h3 id="Paging_supervisor">Paging supervisor</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=6" title="Edit section: Paging supervisor"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This part of the operating system creates and manages page tables and lists of free page frames. In order to ensure that there will be enough free page frames to quickly resolve page faults, the system may periodically steal allocated page frames, using a <a href="/wiki/Page_replacement_algorithm" title="Page replacement algorithm">page replacement algorithm</a>, e.g., a <a href="/wiki/Least_recently_used" class="mw-redirect" title="Least recently used">least recently used</a> (LRU) algorithm. Stolen page frames that have been modified are written back to auxiliary storage before they are added to the free queue. On some systems the paging supervisor is also responsible for managing translation registers that are not automatically loaded from page tables. </p><p>Typically, a page fault that cannot be resolved results in an abnormal termination of the application. However, some systems allow the application to have exception handlers for such errors. The paging supervisor may handle a page fault exception in several different ways, depending on the details: </p> <ul><li>If the virtual address is invalid, the paging supervisor treats it as an error.</li> <li>If the page is valid and the page information is not loaded into the MMU, the page information will be stored into one of the page registers.</li> <li>If the page is uninitialized, a new page frame may be assigned and cleared.</li> <li>If there is a stolen page frame containing the desired page, that page frame will be reused.</li> <li>For a fault due to a write attempt into a read-protected page, if it is a copy-on-write page then a free page frame will be assigned and the contents of the old page copied; otherwise it is treated as an error.</li> <li>If the virtual address is a valid page in a memory-mapped file or a paging file, a free page frame will be assigned and the page read in.</li></ul> <p>In most cases, there will be an update to the page table, possibly followed by purging the Translation Lookaside Buffer (TLB), and the system restarts the instruction that causes the exception. </p><p>If the free page frame queue is empty then the paging supervisor must free a page frame using the same <a href="/wiki/Page_replacement_algorithm" title="Page replacement algorithm">page replacement algorithm</a> for page stealing. </p> <div class="mw-heading mw-heading3"><h3 id="Pinned_pages">Pinned pages</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=7" title="Edit section: Pinned pages"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Operating systems have memory areas that are <i>pinned</i> (never swapped to secondary storage). Other terms used are <i>locked</i>, <i>fixed</i>, or <i>wired</i> pages. For example, <a href="/wiki/Interrupt" title="Interrupt">interrupt</a> mechanisms rely on an array of pointers to their handlers, such as <a href="/wiki/I/O" class="mw-redirect" title="I/O">I/O</a> completion and <a href="/wiki/Page_fault" title="Page fault">page fault</a>. If the pages containing these pointers or the code that they invoke were pageable, interrupt-handling would become far more complex and time-consuming, particularly in the case of page fault interruptions. Hence, some part of the page table structures is not pageable. </p><p>Some pages may be pinned for short periods of time, others may be pinned for long periods of time, and still others may need to be permanently pinned. For example: </p> <ul><li>The paging supervisor code and drivers for secondary storage devices on which pages reside must be permanently pinned, as otherwise paging would not even work because the necessary code would not be available.</li> <li>Timing-dependent components may be pinned to avoid variable paging delays.</li> <li><a href="/wiki/Data_buffer" title="Data buffer">Data buffers</a> that are accessed directly by peripheral devices that use <a href="/wiki/Direct_memory_access" title="Direct memory access">direct memory access</a> or <a href="/wiki/I/O_channel" class="mw-redirect" title="I/O channel">I/O channels</a> must reside in pinned pages while the I/O operation is in progress because such devices and the <a href="/wiki/Bus_(computing)" title="Bus (computing)">buses</a> to which they are attached expect to find data buffers located at physical memory addresses; regardless of whether the bus has a <a href="/wiki/IOMMU" class="mw-redirect" title="IOMMU">memory management unit for I/O</a>, transfers cannot be stopped if a page fault occurs and then restarted when the page fault has been processed. For example, the data could come from a measurement sensor unit and lost real time data that got lost because of a page fault can not be recovered.</li></ul> <p>In IBM's operating systems for <a href="/wiki/System/370" class="mw-redirect" title="System/370">System/370</a> and successor systems, the term is "fixed", and such pages may be long-term fixed, or may be short-term fixed, or may be unfixed (i.e., pageable). System control structures are often long-term fixed (measured in wall-clock time, i.e., time measured in seconds, rather than time measured in fractions of one second) whereas I/O buffers are usually short-term fixed (usually measured in significantly less than wall-clock time, possibly for tens of milliseconds). Indeed, the OS has a special facility for "fast fixing" these short-term fixed data buffers (fixing which is performed without resorting to a time-consuming <a href="/wiki/Supervisor_Call_instruction" title="Supervisor Call instruction">Supervisor Call instruction</a>). </p><p><a href="/wiki/Multics" title="Multics">Multics</a> used the term "wired". <a href="/wiki/OpenVMS" title="OpenVMS">OpenVMS</a> and <a href="/wiki/Microsoft_Windows" title="Microsoft Windows">Windows</a> refer to pages temporarily made nonpageable (as for I/O buffers) as "locked", and simply "nonpageable" for those that are never pageable. The <a href="/wiki/Single_UNIX_Specification" title="Single UNIX Specification">Single UNIX Specification</a> also uses the term "locked" in the specification for <code class="mw-highlight mw-highlight-lang-c mw-content-ltr" style="" dir="ltr"><span class="n">mlock</span><span class="p">()</span></code>, as do the <code class="mw-highlight mw-highlight-lang-c mw-content-ltr" style="" dir="ltr"><span class="n">mlock</span><span class="p">()</span></code> <a href="/wiki/Man_pages" class="mw-redirect" title="Man pages">man pages</a> on many <a href="/wiki/Unix-like" title="Unix-like">Unix-like</a> systems. </p> <div class="mw-heading mw-heading4"><h4 id="Virtual-real_operation">Virtual-real operation</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=8" title="Edit section: Virtual-real operation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In <a href="/wiki/OS/VS1" title="OS/VS1">OS/VS1</a> and similar OSes, some parts of systems memory are managed in "virtual-real" mode, called "V=R". In this mode every virtual address corresponds to the same real address. This mode is used for <a href="/wiki/Interrupt" title="Interrupt">interrupt</a> mechanisms, for the paging supervisor and page tables in older systems, and for application programs using non-standard I/O management. For example, IBM's z/OS has 3 modes (virtual-virtual, virtual-real and virtual-fixed).<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2022)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading3"><h3 id="Thrashing">Thrashing</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=9" title="Edit section: Thrashing"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>When <a href="/wiki/Paging" class="mw-redirect" title="Paging">paging</a> and <a href="/wiki/Paging#Page_stealing" class="mw-redirect" title="Paging">page stealing</a> are used, a problem called "<a href="/wiki/Thrashing_(computer_science)" title="Thrashing (computer science)">thrashing</a>"<sup id="cite_ref-Thrashing_33-0" class="reference"><a href="#cite_note-Thrashing-33"><span class="cite-bracket">[</span>28<span class="cite-bracket">]</span></a></sup> can occur, in which the computer spends an unsuitably large amount of time transferring pages to and from a backing store, hence slowing down useful work. A task's <a href="/wiki/Working_set" title="Working set">working set</a> is the minimum set of pages that should be in memory in order for it to make useful progress. Thrashing occurs when there is insufficient memory available to store the working sets of all active programs. Adding real memory is the simplest response, but improving application design, scheduling, and memory usage can help. Another solution is to reduce the number of active tasks on the system. This reduces demand on real memory by swapping out the entire working set of one or more processes. </p><p>A system thrashing is often a result of a sudden spike in page demand from a small number of running programs. Swap-token<sup id="cite_ref-34" class="reference"><a href="#cite_note-34"><span class="cite-bracket">[</span>29<span class="cite-bracket">]</span></a></sup> is a lightweight and dynamic thrashing protection mechanism. The basic idea is to set a token in the system, which is randomly given to a process that has page faults when thrashing happens. The process that has the token is given a privilege to allocate more physical memory pages to build its working set, which is expected to quickly finish its execution and to release the memory pages to other processes. A time stamp is used to handover the token one by one. The first version of swap-token was implemented in Linux 2.6.<sup id="cite_ref-swap-token-page_35-0" class="reference"><a href="#cite_note-swap-token-page-35"><span class="cite-bracket">[</span>30<span class="cite-bracket">]</span></a></sup> The second version is called preempt swap-token and is also in Linux 2.6.<sup id="cite_ref-swap-token-page_35-1" class="reference"><a href="#cite_note-swap-token-page-35"><span class="cite-bracket">[</span>30<span class="cite-bracket">]</span></a></sup> In this updated swap-token implementation, a priority counter is set for each process to track the number of swap-out pages. The token is always given to the process with a high priority, which has a high number of swap-out pages. The length of the time stamp is not a constant but is determined by the priority: the higher the number of swap-out pages of a process, the longer the time stamp for it will be. </p> <div class="mw-heading mw-heading2"><h2 id="Segmented_virtual_memory">Segmented virtual memory</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=10" title="Edit section: Segmented virtual memory"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Some systems, such as the <a href="/wiki/Burroughs_Corporation" title="Burroughs Corporation">Burroughs</a> B5500,<sup id="cite_ref-36" class="reference"><a href="#cite_note-36"><span class="cite-bracket">[</span>31<span class="cite-bracket">]</span></a></sup> and the current Unisys MCP systems<sup id="cite_ref-“MCP-VM_37-0" class="reference"><a href="#cite_note-“MCP-VM-37"><span class="cite-bracket">[</span>32<span class="cite-bracket">]</span></a></sup> use segmentation instead of paging, dividing virtual address spaces into variable-length segments. Using segmentation matches the allocated memory blocks to the logical needs and requests of the programs, rather than the physical view of a computer, although pages themselves are an artificial division in memory. The designers of the B5000 would have found the artificial size of pages to be <a href="/wiki/Procrustes" title="Procrustes">Procrustean</a> in nature, a story they would later use for the exact data sizes in the <a href="/wiki/Burroughs_B1700" title="Burroughs B1700">B1000</a>.<sup id="cite_ref-“B1000-Procrustes_38-0" class="reference"><a href="#cite_note-“B1000-Procrustes-38"><span class="cite-bracket">[</span>33<span class="cite-bracket">]</span></a></sup> </p><p>In the Burroughs and Unisys systems, each memory segment is described by a master <a href="/wiki/Data_descriptor" title="Data descriptor">descriptor</a> which is a single absolute descriptor which may be referenced by other relative (copy) descriptors, effecting sharing either within a process or between processes. Descriptors are central to the working of virtual memory in MCP systems. Descriptors contain not only the address of a segment, but the segment length and status in virtual memory indicated by the 'p-bit' or 'presence bit' which indicates if the address is to a segment in main memory or to a secondary-storage block. When a non-resident segment (p-bit is off) is accessed, an interrupt occurs to load the segment from secondary storage at the given address, or if the address itself is 0 then allocate a new block. In the latter case, the length field in the descriptor is used to allocate a segment of that length. </p><p>A further problem to thrashing in using a segmented scheme is checkerboarding,<sup id="cite_ref-Checkerboarding_39-0" class="reference"><a href="#cite_note-Checkerboarding-39"><span class="cite-bracket">[</span>34<span class="cite-bracket">]</span></a></sup> where all free segments become too small to satisfy requests for new segments. The solution is to perform memory compaction to pack all used segments together and create a large free block from which further segments may be allocated. Since there is a single master descriptor for each segment the new block address only needs to be updated in a single descriptor, since all copies refer to the master descriptor. </p><p>Paging is not free from fragmentation – the fragmentation is internal to pages (<a href="/wiki/Internal_fragmentation" class="mw-redirect" title="Internal fragmentation">internal fragmentation</a>). If a requested block is smaller than a page, then some space in the page will be wasted. If a block requires larger than a page, a small area in another page is required resulting in large wasted space. The fragmentation thus becomes a problem passed to programmers who may well distort their program to match certain page sizes. With segmentation, the fragmentation is external to segments (<a href="/wiki/External_fragmentation" class="mw-redirect" title="External fragmentation">external fragmentation</a>) and thus a system problem, which was the aim of virtual memory in the first place, to relieve programmers of such memory considerations. In multi-processing systems, optimal operation of the system depends on the mix of independent processes at any time. Hybrid schemes of segmentation and paging may be used. </p><p>The <a href="/wiki/Intel_80286" title="Intel 80286">Intel 80286</a> supports a similar segmentation scheme as an option, but it is rarely used. </p><p>Segmentation and paging can be used together by dividing each segment into pages; systems with this memory structure, such as <a href="/wiki/Multics" title="Multics">Multics</a> and <a href="/wiki/IBM_System/38" title="IBM System/38">IBM System/38</a>, are usually paging-predominant, segmentation providing memory protection.<sup id="cite_ref-40" class="reference"><a href="#cite_note-40"><span class="cite-bracket">[</span>35<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-41" class="reference"><a href="#cite_note-41"><span class="cite-bracket">[</span>36<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-42" class="reference"><a href="#cite_note-42"><span class="cite-bracket">[</span>37<span class="cite-bracket">]</span></a></sup> </p><p>In the <a href="/wiki/Intel_80386" class="mw-redirect" title="Intel 80386">Intel 80386</a> and later <a href="/wiki/IA-32" title="IA-32">IA-32</a> processors, the segments reside in a <a href="/wiki/32-bit" class="mw-redirect" title="32-bit">32-bit</a> linear, paged address space. Segments can be moved in and out of that space; pages there can "page" in and out of main memory, providing two levels of virtual memory; few if any operating systems do so, instead using only paging. Early non-hardware-assisted <a href="/wiki/X86_virtualization" title="X86 virtualization">x86 virtualization</a> solutions combined paging and segmentation because x86 paging offers only two protection domains whereas a VMM, guest OS or guest application stack needs three.<sup id="cite_ref-43" class="reference"><a href="#cite_note-43"><span class="cite-bracket">[</span>38<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 22">: 22 </span></sup> The difference between paging and segmentation systems is not only about memory division; segmentation is visible to user processes, as part of memory model semantics. Hence, instead of memory that looks like a single large space, it is structured into multiple spaces. </p><p>This difference has important consequences; a segment is not a page with variable length or a simple way to lengthen the address space. Segmentation that can provide a single-level memory model in which there is no differentiation between process memory and file system consists of only a list of segments (files) mapped into the process's potential address space.<sup id="cite_ref-44" class="reference"><a href="#cite_note-44"><span class="cite-bracket">[</span>39<span class="cite-bracket">]</span></a></sup> </p><p>This is not the same as the mechanisms provided by calls such as <a href="/wiki/Mmap" title="Mmap">mmap</a> and <a href="/wiki/Win32" class="mw-redirect" title="Win32">Win32</a>'s MapViewOfFile, because inter-file pointers do not work when mapping files into semi-arbitrary places. In Multics, a file (or a segment from a multi-segment file) is mapped into a segment in the address space, so files are always mapped at a segment boundary. A file's linkage section can contain pointers for which an attempt to load the pointer into a register or make an indirect reference through it causes a trap. The unresolved pointer contains an indication of the name of the segment to which the pointer refers and an offset within the segment; the handler for the trap maps the segment into the address space, puts the segment number into the pointer, changes the tag field in the pointer so that it no longer causes a trap, and returns to the code where the trap occurred, re-executing the instruction that caused the trap.<sup id="cite_ref-45" class="reference"><a href="#cite_note-45"><span class="cite-bracket">[</span>40<span class="cite-bracket">]</span></a></sup> This eliminates the need for a <a href="/wiki/Linker_(computing)" title="Linker (computing)">linker</a> completely<sup id="cite_ref-denning_10-4" class="reference"><a href="#cite_note-denning-10"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> and works when different processes map the same file into different places in their private address spaces.<sup id="cite_ref-46" class="reference"><a href="#cite_note-46"><span class="cite-bracket">[</span>41<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Address_space_swapping">Address space swapping</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=11" title="Edit section: Address space swapping"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Some operating systems provide for swapping entire <a href="/wiki/Address_space" title="Address space">address spaces</a>, in addition to whatever facilities they have for paging and segmentation. When this occurs, the OS writes those pages and segments currently in real memory to swap files. In a swap-in, the OS reads back the data from the swap files but does not automatically read back pages that had been paged out at the time of the swap out operation. </p><p>IBM's <a href="/wiki/MVS" title="MVS">MVS</a>, from <a href="/wiki/OS/360_and_successors#OS/VS2_SVS_and_MVS" title="OS/360 and successors">OS/VS2 Release 2</a> through <a href="/wiki/Z/OS" title="Z/OS">z/OS</a>, provides for marking an address space as unswappable; doing so does not pin any pages in the address space. This can be done for the duration of a job by entering the name of an eligible<sup id="cite_ref-47" class="reference"><a href="#cite_note-47"><span class="cite-bracket">[</span>42<span class="cite-bracket">]</span></a></sup> main program in the Program Properties Table with an unswappable flag. In addition, privileged code can temporarily make an address space unswappable using a SYSEVENT <a href="/wiki/Supervisor_Call_instruction" title="Supervisor Call instruction">Supervisor Call instruction</a> (SVC); certain changes<sup id="cite_ref-48" class="reference"><a href="#cite_note-48"><span class="cite-bracket">[</span>43<span class="cite-bracket">]</span></a></sup> in the address space properties require that the OS swap it out and then swap it back in, using SYSEVENT TRANSWAP.<sup id="cite_ref-49" class="reference"><a href="#cite_note-49"><span class="cite-bracket">[</span>44<span class="cite-bracket">]</span></a></sup> </p><p>Swapping does not necessarily require memory management hardware, if, for example, multiple jobs are swapped in and out of the same area of storage. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=12" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1184024115">.mw-parser-output .div-col{margin-top:0.3em;column-width:30em}.mw-parser-output .div-col-small{font-size:90%}.mw-parser-output .div-col-rules{column-rule:1px solid #aaa}.mw-parser-output .div-col dl,.mw-parser-output .div-col ol,.mw-parser-output .div-col ul{margin-top:0}.mw-parser-output .div-col li,.mw-parser-output .div-col dd{page-break-inside:avoid;break-inside:avoid-column}</style><div class="div-col" style="column-width: 30em;"> <ul><li><a href="/wiki/Processor_design" title="Processor design">Processor design</a></li> <li><a href="/wiki/Page_(computer_memory)" title="Page (computer memory)">Page (computer memory)</a></li> <li><a href="/wiki/Cache_replacement_policies" title="Cache replacement policies">Cache replacement policies</a></li> <li><a href="/wiki/Memory_management" title="Memory management">Memory management</a></li> <li><a href="/wiki/Memory_management_(operating_systems)" title="Memory management (operating systems)">Memory management (operating systems)</a></li> <li><a href="/wiki/Protected_mode" title="Protected mode">Protected mode</a>, an <a href="/wiki/X86" title="X86">x86</a> mode that allows for virtual memory.</li> <li><a href="/wiki/CUDA" title="CUDA">CUDA Pinned memory</a></li> <li><a href="/wiki/Heterogeneous_System_Architecture" title="Heterogeneous System Architecture">Heterogeneous System Architecture</a>, a series of specifications intended to unify CPU and GPU memory</li></ul></div> <div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=13" title="Edit section: Notes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist reflist-lower-alpha"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">Early systems used <a href="/wiki/Drum_memory" title="Drum memory">drums</a>; contemporary systems use <a href="/wiki/Disk_storage" title="Disk storage">disks</a> or <a href="/wiki/Solid-state_drive" title="Solid-state drive">solid state memory</a></span> </li> <li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text">IBM uses the term <b>virtual storage</b> on mainframe operating systems. This usage runs from <a href="/wiki/TSS_(operating_system)" title="TSS (operating system)">TSS</a><sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> on the <a href="/wiki/IBM_System/360_Model_67" title="IBM System/360 Model 67">360/67</a> through <a href="/wiki/Z/OS" title="Z/OS">z/OS</a><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> on <a href="/wiki/Z/Architecture" title="Z/Architecture">z/Architecture</a>.</span> </li> <li id="cite_note-24"><span class="mw-cite-backlink"><b><a href="#cite_ref-24">^</a></b></span> <span class="reference-text">IBM had previously used the term 'hypervisor' for the <a href="/wiki/IBM_System/360_Model_65" title="IBM System/360 Model 65">360/65</a>,<sup id="cite_ref-23" class="reference"><a href="#cite_note-23"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> but that did not involve virtual memory.</span> </li> <li id="cite_note-28"><span class="mw-cite-backlink"><b><a href="#cite_ref-28">^</a></b></span> <span class="reference-text">IBM <a href="/wiki/DOS/360_and_successors#DOS/VS" title="DOS/360 and successors">DOS/VS</a>, <a href="/wiki/OS/VS1" title="OS/VS1">OS/VS1</a> and <a href="/wiki/DOS/VS" class="mw-redirect" title="DOS/VS">DOS/VS</a> only supported 2 KB pages.</span> </li> <li id="cite_note-32"><span class="mw-cite-backlink"><b><a href="#cite_ref-32">^</a></b></span> <span class="reference-text">On <a href="/wiki/IBM_Z" title="IBM Z">IBM Z</a><sup id="cite_ref-31" class="reference"><a href="#cite_note-31"><span class="cite-bracket">[</span>27<span class="cite-bracket">]</span></a></sup> there is a 3-level tree of regions for each address space.</span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=14" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239543626"><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation book cs1"><a rel="nofollow" class="external text" href="http://bitsavers.org/pdf/ibm/360/tss/C20-1647-0_360-67_TSS_Tech.pdf#page=21">"SYSTEM COMPONENTS: Dynamic Relocation"</a> <span class="cs1-format">(PDF)</span>. <a rel="nofollow" class="external text" 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Archived from <a rel="nofollow" class="external text" href="https://web.cse.ohio-state.edu/~zhang.574/swaptoken-PE-05.html">the original</a> on 7 December 2023.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Swap+Token+effectively+minimizes+system+thrasing+effects+and+is+adopted+in+OS+kernels&rft.pub=Ohio+State+University&rft.aulast=Zhang&rft.aufirst=Xiaodong&rft_id=https%3A%2F%2Fweb.cse.ohio-state.edu%2F~zhang.574%2Fswaptoken-PE-05.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-36"><span class="mw-cite-backlink"><b><a href="#cite_ref-36">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBurroughs1964" class="citation book cs1">Burroughs (1964). <a rel="nofollow" class="external text" href="http://bitsavers.org/pdf/burroughs/B5000_5500_5700/1021326_B5500_RefMan_May67.pdf"><i>Burroughs B5500 Information Processing System Reference Manual</i></a> <span class="cs1-format">(PDF)</span>. <a href="/wiki/Burroughs_Corporation" title="Burroughs Corporation">Burroughs Corporation</a>. 1021326<span class="reference-accessdate">. Retrieved <span class="nowrap">28 November</span> 2013</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Burroughs+B5500+Information+Processing+System+Reference+Manual&rft.pub=Burroughs+Corporation&rft.date=1964&rft.au=Burroughs&rft_id=http%3A%2F%2Fbitsavers.org%2Fpdf%2Fburroughs%2FB5000_5500_5700%2F1021326_B5500_RefMan_May67.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-“MCP-VM-37"><span class="mw-cite-backlink"><b><a href="#cite_ref-“MCP-VM_37-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://public.support.unisys.com/aseries/docs/ClearPath-MCP-20.0/86000387-514/section-000023206.html">"Unisys MCP Virtual Memory"</a>. <i>Unisys</i>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Unisys&rft.atitle=Unisys+MCP+Virtual+Memory&rft_id=https%3A%2F%2Fpublic.support.unisys.com%2Faseries%2Fdocs%2FClearPath-MCP-20.0%2F86000387-514%2Fsection-000023206.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-“B1000-Procrustes-38"><span class="mw-cite-backlink"><b><a href="#cite_ref-“B1000-Procrustes_38-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFWilner1972" class="citation book cs1">Wilner, W. 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(1972). <a rel="nofollow" class="external text" href="https://dl.acm.org/doi/10.1145/1479992.1480060">"Design of the Burroughs B1700"</a>. <i>Proceedings of the December 5–7, 1972, fall joint computer conference, Part I on – AFIPS '72 (Fall, part I)</i>. pp. <span class="nowrap">489–</span>497. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1145%2F1479992.1480060">10.1145/1479992.1480060</a>. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-1-4503-7912-0" title="Special:BookSources/978-1-4503-7912-0"><bdi>978-1-4503-7912-0</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=Design+of+the+Burroughs+B1700&rft.btitle=Proceedings+of+the+December+5%E2%80%937%2C+1972%2C+fall+joint+computer+conference%2C+Part+I+on+%E2%80%93+AFIPS+%2772+%28Fall%2C+part+I%29&rft.pages=%3Cspan+class%3D%22nowrap%22%3E489-%3C%2Fspan%3E497&rft.date=1972&rft_id=info%3Adoi%2F10.1145%2F1479992.1480060&rft.isbn=978-1-4503-7912-0&rft.aulast=Wilner&rft.aufirst=W.+T.&rft_id=https%3A%2F%2Fdl.acm.org%2Fdoi%2F10.1145%2F1479992.1480060&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span> <span class="cs1-visible-error citation-comment"><code class="cs1-code">{{<a href="/wiki/Template:Cite_book" title="Template:Cite book">cite book</a>}}</code>: </span><span class="cs1-visible-error citation-comment"><code class="cs1-code">|website=</code> ignored (<a href="/wiki/Help:CS1_errors#periodical_ignored" title="Help:CS1 errors">help</a>)</span></span> </li> <li id="cite_note-Checkerboarding-39"><span class="mw-cite-backlink"><b><a href="#cite_ref-Checkerboarding_39-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://public.support.unisys.com/aseries/docs/ClearPath-MCP-20.0/86000387-514/section-000023184.html">"Checkerboarding"</a>. <i>Unisys</i>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Unisys&rft.atitle=Checkerboarding&rft_id=https%3A%2F%2Fpublic.support.unisys.com%2Faseries%2Fdocs%2FClearPath-MCP-20.0%2F86000387-514%2Fsection-000023184.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-40"><span class="mw-cite-backlink"><b><a href="#cite_ref-40">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation book cs1"><a rel="nofollow" class="external text" href="http://www.bitsavers.org/pdf/ge/GE-645/LSB0468_GE-645_System_Manual_Jan1968.pdf"><i>GE-645 System Manual</i></a> <span class="cs1-format">(PDF)</span>. January 1968. pp. <span class="nowrap">21–</span>30<span class="reference-accessdate">. Retrieved <span class="nowrap">25 February</span> 2022</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=GE-645+System+Manual&rft.pages=%3Cspan+class%3D%22nowrap%22%3E21-%3C%2Fspan%3E30&rft.date=1968-01&rft_id=http%3A%2F%2Fwww.bitsavers.org%2Fpdf%2Fge%2FGE-645%2FLSB0468_GE-645_System_Manual_Jan1968.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-41"><span class="mw-cite-backlink"><b><a href="#cite_ref-41">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFCorbatóVyssotsky" class="citation web cs1"><a href="/wiki/Fernando_J._Corbat%C3%B3" title="Fernando J. Corbató">Corbató, F. J.</a>; <a href="/wiki/Victor_A._Vyssotsky" title="Victor A. Vyssotsky">Vyssotsky, V. A.</a> <a rel="nofollow" class="external text" href="http://www.multicians.org/fjcc1.html">"Introduction and Overview of the Multics System"</a><span class="reference-accessdate">. Retrieved <span class="nowrap">13 November</span> 2007</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Introduction+and+Overview+of+the+Multics+System&rft.aulast=Corbat%C3%B3&rft.aufirst=F.+J.&rft.au=Vyssotsky%2C+V.+A.&rft_id=http%3A%2F%2Fwww.multicians.org%2Ffjcc1.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-42"><span class="mw-cite-backlink"><b><a href="#cite_ref-42">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFGlaserCouleurOliver" class="citation web cs1">Glaser, Edward L.; Couleur, John F. & Oliver, G. 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(14 August 2005). <a rel="nofollow" class="external text" href="https://old.hotchips.org/wp-content/uploads/hc_archives/hc17/1_Sun/HC17.T1P2.pdf">"<i>Virtual Machines: Architectures, Implementations and Applications</i>, HOTCHIPS 17, Tutorial 1, part 2"</a> <span class="cs1-format">(PDF)</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Virtual+Machines%3A+Architectures%2C+Implementations+and+Applications%2C+HOTCHIPS+17%2C+Tutorial+1%2C+part+2&rft.date=2005-08-14&rft.aulast=Smith&rft.aufirst=J.+E.&rft.au=Uhlig%2C+R.&rft_id=https%3A%2F%2Fold.hotchips.org%2Fwp-content%2Fuploads%2Fhc_archives%2Fhc17%2F1_Sun%2FHC17.T1P2.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-44"><span class="mw-cite-backlink"><b><a href="#cite_ref-44">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBensoussanClingenDaley1972" class="citation journal cs1">Bensoussan, André; Clingen, Charles T.; Daley, Robert C. 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Retrieved <span class="nowrap">9 October</span> 2016</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Multicians.org&rft.atitle=Multics+Execution+Environment&rft_id=http%3A%2F%2Fwww.multicians.org%2Fexec-env.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-46"><span class="mw-cite-backlink"><b><a href="#cite_ref-46">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFOrganick1972" class="citation book cs1">Organick, Elliott I. (1972). <span class="id-lock-registration" title="Free registration required"><a rel="nofollow" class="external text" href="https://archive.org/details/multicssystemex00orga"><i>The Multics System: An Examination of Its Structure</i></a></span>. MIT Press. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-262-15012-5" title="Special:BookSources/978-0-262-15012-5"><bdi>978-0-262-15012-5</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=The+Multics+System%3A+An+Examination+of+Its+Structure&rft.pub=MIT+Press&rft.date=1972&rft.isbn=978-0-262-15012-5&rft.aulast=Organick&rft.aufirst=Elliott+I.&rft_id=https%3A%2F%2Farchive.org%2Fdetails%2Fmulticssystemex00orga&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> <li id="cite_note-47"><span class="mw-cite-backlink"><b><a href="#cite_ref-47">^</a></b></span> <span class="reference-text">The most important requirement is that the program be APF authorized.</span> </li> <li id="cite_note-48"><span class="mw-cite-backlink"><b><a href="#cite_ref-48">^</a></b></span> <span class="reference-text">e.g., requesting use of preferred memory</span> </li> <li id="cite_note-49"><span class="mw-cite-backlink"><b><a href="#cite_ref-49">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://pic.dhe.ibm.com/infocenter/zos/v2r1/index.jsp?topic=%2Fcom.ibm.zos.v2r1.ieaa400%2Fswap.htm">"Control swapping (DONTSWAP, OKSWAP, TRANSWAP)"</a>. <i>IBM Knowledge Center</i>. z/OS MVS Programming: Authorized Assembler Services Reference SET-WTO SA23-1375-00. 1990–2014<span class="reference-accessdate">. Retrieved <span class="nowrap">9 October</span> 2016</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=IBM+Knowledge+Center&rft.atitle=Control+swapping+%28DONTSWAP%2C+OKSWAP%2C+TRANSWAP%29&rft.date=1990%2F2014&rft_id=http%3A%2F%2Fpic.dhe.ibm.com%2Finfocenter%2Fzos%2Fv2r1%2Findex.jsp%3Ftopic%3D%252Fcom.ibm.zos.v2r1.ieaa400%252Fswap.htm&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span></span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=15" title="Edit section: Further reading"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li>Hennessy, John L.; and Patterson, David A.; <i>Computer Architecture, A Quantitative Approach</i> (<link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/1-55860-724-2" title="Special:BookSources/1-55860-724-2">1-55860-724-2</a>)</li></ul> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Virtual_memory&action=edit&section=16" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1235681985">.mw-parser-output .side-box{margin:4px 0;box-sizing:border-box;border:1px solid #aaa;font-size:88%;line-height:1.25em;background-color:var(--background-color-interactive-subtle,#f8f9fa);display:flow-root}.mw-parser-output .side-box-abovebelow,.mw-parser-output .side-box-text{padding:0.25em 0.9em}.mw-parser-output .side-box-image{padding:2px 0 2px 0.9em;text-align:center}.mw-parser-output .side-box-imageright{padding:2px 0.9em 2px 0;text-align:center}@media(min-width:500px){.mw-parser-output .side-box-flex{display:flex;align-items:center}.mw-parser-output .side-box-text{flex:1;min-width:0}}@media(min-width:720px){.mw-parser-output .side-box{width:238px}.mw-parser-output .side-box-right{clear:right;float:right;margin-left:1em}.mw-parser-output .side-box-left{margin-right:1em}}</style><style data-mw-deduplicate="TemplateStyles:r1237033735">@media print{body.ns-0 .mw-parser-output .sistersitebox{display:none!important}}@media screen{html.skin-theme-clientpref-night .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-en-v2.svg"]{background-color:white}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-en-v2.svg"]{background-color:white}}</style><div class="side-box side-box-right plainlinks sistersitebox"><style data-mw-deduplicate="TemplateStyles:r1126788409">.mw-parser-output .plainlist ol,.mw-parser-output .plainlist ul{line-height:inherit;list-style:none;margin:0;padding:0}.mw-parser-output .plainlist ol li,.mw-parser-output .plainlist ul li{margin-bottom:0}</style> <div class="side-box-flex"> <div class="side-box-image"><span class="noviewer" typeof="mw:File"><a href="/wiki/File:Wikisource-logo.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/38px-Wikisource-logo.svg.png" decoding="async" width="38" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/57px-Wikisource-logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/76px-Wikisource-logo.svg.png 2x" data-file-width="410" data-file-height="430" /></a></span></div> <div class="side-box-text plainlist"><a href="/wiki/Wikisource" title="Wikisource">Wikisource</a> has original text related to this article: <div style="margin-left: 10px;"><b><a href="https://en.wikisource.org/wiki/The_Paging_Game" class="extiw" title="wikisource:The Paging Game">The Paging Game</a></b></div></div></div> </div> <ul><li><a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP">Operating Systems: Three Easy Pieces</a>, by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau. Arpaci-Dusseau Books, 2014. Relevant chapters: <a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP/vm-intro.pdf">Address Spaces</a> <a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP/vm-mechanism.pdf">Address Translation</a> <a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP/vm-segmentation.pdf">Segmentation</a> <a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP/vm-paging.pdf">Introduction to Paging</a> <a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP/vm-tlbs.pdf">TLBs</a> <a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP/vm-smalltables.pdf">Advanced Page Tables</a> <a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP/vm-beyondphys.pdf">Swapping: Mechanisms</a> <a rel="nofollow" class="external text" href="http://pages.cs.wisc.edu/~remzi/OSTEP/vm-beyondphys-policy.pdf">Swapping: Policies</a></li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20121101100208/https://1a9f2076-a-62cb3a1a-s-sites.googlegroups.com/site/michiganterminalsystem/documentation/documents/timeSharingSupervisorPrograms-1971.pdf?attachauth=ANoY7cq50xGif66cFkdyUd2V34MN67JzvBSC2d8Bh9YyKRUdJhWIZajTpdyvTUA9GlC6qgTpQc6Vwy1PIj94BwEehXgqir8V9Pa7C1e8gDt_KDW_nCp7wtn34odFoL2BK9pBYRP2KMSY6TpHua2OZkiAAN78JDgbMiidGIzEd-Mqns5h0AMxTB2Oj4glJmdV6GU6yZ1aNWeP876U3BSdlsTv0FrJmI8eG8KQnC7K7dvmR95s7Yi2IAm9iTletQuFbtosE7cSVrfXBj5-Kxn-wQmP4mSJaSjTC0n2-HbyHwQ7bzfHePbE0lQ%3D&attredirects=0">"Time-Sharing Supervisor Programs"</a> <span class="cs1-format">(PDF)</span>. Archived from <a rel="nofollow" class="external text" href="http://archive.michigan-terminal-system.org/documentation/documents/timeSharingSupervisorPrograms-1971.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 1 November 2012.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Time-Sharing+Supervisor+Programs&rft_id=http%3A%2F%2Farchive.michigan-terminal-system.org%2Fdocumentation%2Fdocuments%2FtimeSharingSupervisorPrograms-1971.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AVirtual+memory" class="Z3988"></span> by Michael T. Alexander in <i>Advanced Topics in Systems Programming</i>, University of Michigan Engineering Summer Conference 1970 (revised May 1971), compares the scheduling and resource allocation approaches, including virtual memory and paging, used in four mainframe operating systems: <a href="/wiki/CP-67" title="CP-67">CP-67</a>, <a href="/wiki/TSS/360" class="mw-redirect" title="TSS/360">TSS/360</a>, <a href="/wiki/Michigan_Terminal_System" title="Michigan Terminal System">MTS</a>, and <a href="/wiki/Multics" title="Multics">Multics</a>.</li> <li><a rel="nofollow" class="external text" href="http://linux-mm.org/">LinuxMM: Linux Memory Management</a>.</li> <li><a rel="nofollow" class="external text" href="https://gnulinuxclub.org/birth-of-linux-kernel/">Birth of Linux Kernel</a>, mailing list discussion.</li> <li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20100622062522/http://msdn2.microsoft.com/en-us/library/ms810616.aspx">The Virtual-Memory Manager in Windows NT, Randy Kath, Microsoft Developer Network Technology Group, 12 December 1992</a> at the <a href="/wiki/Wayback_Machine" title="Wayback Machine">Wayback Machine</a> (archived 22 June 2010)</li></ul> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist li{margin:0;display:inline}.mw-parser-output .hlist.inline,.mw-parser-output .hlist.inline dl,.mw-parser-output .hlist.inline ol,.mw-parser-output .hlist.inline ul,.mw-parser-output .hlist dl dl,.mw-parser-output .hlist dl ol,.mw-parser-output .hlist dl ul,.mw-parser-output .hlist ol dl,.mw-parser-output .hlist ol ol,.mw-parser-output .hlist ol ul,.mw-parser-output .hlist ul dl,.mw-parser-output .hlist ul ol,.mw-parser-output .hlist ul ul{display:inline}.mw-parser-output .hlist .mw-empty-li{display:none}.mw-parser-output .hlist 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(IOMMU)</li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a class="mw-selflink selflink">Virtual memory</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Demand_paging" title="Demand paging">Demand paging</a></li> <li><a href="/wiki/Memory_paging" title="Memory paging">Memory paging</a></li> <li><a href="/wiki/Page_table" title="Page table">Page table</a></li> <li><a href="/wiki/Virtual_memory_compression" title="Virtual memory compression">Virtual memory compression</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Memory_segmentation" title="Memory segmentation">Memory segmentation</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Protected_mode" title="Protected mode">Protected mode</a></li> <li><a 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title="Ptmalloc">ptmalloc</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Manual_memory_management" title="Manual memory management">Manual memory management</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Static_variable" title="Static variable">Static memory allocation</a></li> <li><a href="/wiki/C_dynamic_memory_allocation" title="C dynamic memory allocation">C dynamic memory allocation</a></li> <li><a href="/wiki/New_and_delete_(C%2B%2B)" title="New and delete (C++)">new and delete (C++)</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Garbage_collection_(computer_science)" title="Garbage collection (computer science)">Garbage collection</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Automatic_Reference_Counting" title="Automatic Reference Counting">Automatic Reference Counting</a></li> <li><a href="/wiki/Boehm_garbage_collector" title="Boehm garbage collector">Boehm garbage collector</a></li> <li><a href="/wiki/Cheney%27s_algorithm" title="Cheney's algorithm">Cheney's algorithm</a></li> <li><a href="/wiki/Concurrent_mark_sweep_collector" title="Concurrent mark sweep collector">Concurrent mark sweep collector</a></li> <li><a href="/wiki/Finalizer" title="Finalizer">Finalizer</a></li> <li><a href="/wiki/Garbage_(computer_science)" title="Garbage (computer science)">Garbage</a></li> <li><a href="/wiki/Garbage-first_collector" title="Garbage-first collector">Garbage-first collector</a></li> <li><a href="/wiki/Mark%E2%80%93compact_algorithm" title="Mark–compact algorithm">Mark–compact algorithm</a></li> <li><a href="/wiki/Reference_counting" title="Reference counting">Reference counting</a></li> <li><a href="/wiki/Tracing_garbage_collection" title="Tracing garbage collection">Tracing garbage collection</a></li> <li><a href="/wiki/Weak_reference" title="Weak reference">Strong reference</a></li> <li><a href="/wiki/Weak_reference" title="Weak reference">Weak reference</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Memory_safety" title="Memory safety">Memory safety</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Buffer_overflow" title="Buffer overflow">Buffer overflow</a></li> <li><a href="/wiki/Buffer_over-read" title="Buffer over-read">Buffer over-read</a></li> <li><a href="/wiki/Dangling_pointer" title="Dangling pointer">Dangling pointer</a></li> <li><a href="/wiki/Stack_overflow" title="Stack overflow">Stack overflow</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Issues</th><td class="navbox-list-with-group navbox-list navbox-even" 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class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/23px-Symbol_category_class.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/31px-Symbol_category_class.svg.png 2x" data-file-width="180" data-file-height="185" /></span></span> <a href="/wiki/Category:Virtual_memory" title="Category:Virtual memory">Virtual memory</a></li> <li><span class="noviewer" typeof="mw:File"><span title="Category"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/16px-Symbol_category_class.svg.png" decoding="async" width="16" height="16" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/23px-Symbol_category_class.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/31px-Symbol_category_class.svg.png 2x" data-file-width="180" data-file-height="185" /></span></span> <a 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[\"CITEREFLiptay1968\"] = 1,\n [\"CITEREFOrganick1972\"] = 1,\n [\"CITEREFQuintero2013\"] = 1,\n [\"CITEREFSayre1969\"] = 1,\n [\"CITEREFSharma2009\"] = 1,\n [\"CITEREFSmithUhlig2005\"] = 1,\n [\"CITEREFSong_JiangXiaodong_Zhang2005\"] = 1,\n [\"CITEREFWaychoff\"] = 1,\n [\"CITEREFWilner1972\"] = 1,\n [\"CITEREFZhang\"] = 1,\n [\"CITEREFZhouPetrov2008\"] = 1,\n}\ntemplate_list = table#1 {\n [\"About\"] = 1,\n [\"Authority control\"] = 1,\n [\"Citation\"] = 2,\n [\"Citation needed\"] = 1,\n [\"Cite book\"] = 13,\n [\"Cite conference\"] = 1,\n [\"Cite encyclopedia\"] = 1,\n [\"Cite journal\"] = 8,\n [\"Cite web\"] = 17,\n [\"Cmn\"] = 1,\n [\"Code\"] = 2,\n [\"DEFAULTSORT:Virtual Memory\"] = 1,\n [\"Distinguish\"] = 1,\n [\"Dubious\"] = 1,\n [\"Efn\"] = 5,\n [\"ISBN\"] = 1,\n [\"Memory management navbox\"] = 1,\n [\"More citations needed section\"] = 1,\n [\"Notelist\"] = 1,\n [\"R\"] = 1,\n [\"Reflist\"] = 1,\n [\"Rp\"] = 3,\n [\"See also\"] = 1,\n [\"Sfnp\"] = 1,\n [\"Short description\"] = 1,\n [\"Sic\"] = 1,\n [\"Use dmy dates\"] = 1,\n [\"Webarchive\"] = 1,\n [\"Wikisource\"] = 1,\n}\narticle_whitelist = table#1 {\n}\nciteref_patterns = table#1 {\n}\n"},"cachereport":{"origin":"mw-web.codfw.main-65b64b4b74-4jcf2","timestamp":"20250219122700","ttl":2592000,"transientcontent":false}}});});</script> <script type="application/ld+json">{"@context":"https:\/\/schema.org","@type":"Article","name":"Virtual memory","url":"https:\/\/en.wikipedia.org\/wiki\/Virtual_memory","sameAs":"http:\/\/www.wikidata.org\/entity\/Q189401","mainEntity":"http:\/\/www.wikidata.org\/entity\/Q189401","author":{"@type":"Organization","name":"Contributors to Wikimedia projects"},"publisher":{"@type":"Organization","name":"Wikimedia Foundation, 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