https://gunkies.org/w/index.php?title=Virtual_memory&feed=atom&action=historyVirtual memory - Revision history2024-03-28T11:09:05ZRevision history for this page on the wikiMediaWiki 1.30.0https://gunkies.org/w/index.php?title=Virtual_memory&diff=33074&oldid=prevJnc: /* Further reading */ typo2024-01-22T18:01:29Z<p><span dir="auto"><span class="autocomment">Further reading: </span> typo</span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>==Further reading=</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>==Further reading<ins class="diffchange diffchange-inline">=</ins>=</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Peter J. Denning, [ ''Virtual Memory''], Computing Surveys, Vol. 2, No. 3, September 1977 - includes some sections which have fairly detailed mathematical models</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Peter J. Denning, [ ''Virtual Memory''], Computing Surveys, Vol. 2, No. 3, September 1977 - includes some sections which have fairly detailed mathematical models</div></td></tr>
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</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=33073&oldid=prevJnc: /* Further reading */ +Denning, 'Virtual Memory'2024-01-22T18:01:02Z<p><span dir="auto"><span class="autocomment">Further reading: </span> +Denning, 'Virtual Memory'</span></p>
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<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
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<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">* Peter J. Denning, [ ''Virtual Memory''], Computing Surveys, Vol. 2, No. 3, September 1977 - includes some sections which have fairly detailed mathematical models</ins></div></td></tr>
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</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=29922&oldid=prevJnc: /* Details */ Also called 'relocation'2023-06-08T15:02:54Z<p><span dir="auto"><span class="autocomment">Details: </span> Also called 'relocation'</span></p>
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<td colspan="2" style="background-color: white; color:black; text-align: center;">Revision as of 15:02, 8 June 2023</td>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>There are usually three related things going on when virtual memory is in use:</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>There are usually three related things going on when virtual memory is in use:</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''[[virtual address]]ing'', which means that the addresses seen by a [[user]]'s computation, e.g. a [[process]], is not the actual address of the main memory location that the [[hardware]] knows for that memory location, but rather a 'virtual' address private to that user/process. (Early [[memory management]] systems such as [[base and bounds]] [[register]]s provided this feature alone.)</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''[[virtual address]]ing'' <ins class="diffchange diffchange-inline">(also called '''relocation''')</ins>, which means that the addresses seen by a [[user]]'s computation, e.g. a [[process]], is not the actual address of the main memory location that the [[hardware]] knows for that memory location, but rather a 'virtual' address private to that user/process. (Early [[memory management]] systems such as [[base and bounds]] [[register]]s provided this feature alone.)</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''non-residence'', or '''demand loading''' - i.e. a computation can run without ''all'' of its virtual memory being present in physical memory, and missing items are automatically brought in by the operating system when they are needed. To do this, the user's memory has to be divided into blocks somehow, either through [[segment]]ation, or via the third:</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''non-residence'', or '''demand loading''' - i.e. a computation can run without ''all'' of its virtual memory being present in physical memory, and missing items are automatically brought in by the operating system when they are needed. To do this, the user's memory has to be divided into blocks somehow, either through [[segment]]ation, or via the third:</div></td></tr>
</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=29919&oldid=prevJnc: /* Implementation */ History2023-06-08T14:58:56Z<p><span dir="auto"><span class="autocomment">Implementation: </span> History</span></p>
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<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>In addition, the use of fixed-size pages, and the usual practise of allowing any page of the address space to be allocated to any page frame, means there is no 'breakage' of main memory, and no need to move data around in main memory to coalesce free space (which was usually necessary on systems which did not use pages).</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>In addition, the use of fixed-size pages, and the usual practise of allowing any page of the address space to be allocated to any page frame, means there is no 'breakage' of main memory, and no need to move data around in main memory to coalesce free space (which was usually necessary on systems which did not use pages).</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">==Implementation==</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">In many of the first machines which implemented virtual memory (such as the [[KA10]]), an additional piece of hardware, the '''pager''' (sometimes called a '''paging box''') was inserted in between the CPU and main memory, to perform the relocation function. The pager sometimes needed direct access to the internals of the CPU, to properly perform the demand loading function, which meant adding a pager was not necessarily a simple 'drop in' operation. (The [[KT11-B Paging Option]] for the [[PDP-11/20]] is a example of this.) </ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==See also==</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>==See also==</div></td></tr>
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</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=19586&oldid=prevJnc: +cat2018-12-15T19:36:38Z<p>+cat</p>
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<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
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</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=16922&oldid=prevJnc: /* Details */ rm dup link2018-06-15T12:26:32Z<p><span dir="auto"><span class="autocomment">Details: </span> rm dup link</span></p>
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<td colspan="2" style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: white; color:black; text-align: center;">Revision as of 12:26, 15 June 2018</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l13" >Line 13:</td>
<td colspan="2" class="diff-lineno">Line 13:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. When demand loading is in use, only a selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones are brought into free page frames as they are needed.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. When demand loading is in use, only a selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones are brought into free page frames as they are needed.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>In modern systems, generally all three are going on, but this is not necessarily mandatory. For example, one can have the first two without having the third; e.g. the [[Burroughs B5000]] supported demand loading of <del class="diffchange diffchange-inline">[[segmentation|</del>segments<del class="diffchange diffchange-inline">]]</del>, but did not have paging. It is also possible to have a sytem which has paging, but not demand loading - the paging would only be used for memory allocation (see the bottom of this page for why paging is advantageous for this).</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>In modern systems, generally all three are going on, but this is not necessarily mandatory. For example, one can have the first two without having the third; e.g. the [[Burroughs B5000]] supported demand loading of segments, but did not have paging. It is also possible to have a sytem which has paging, but not demand loading - the paging would only be used for memory allocation (see the bottom of this page for why paging is advantageous for this).</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Some systems (e.g. [[Multics]]) had/have support for multiple page sizes. The distinction between segments (which usually can be of varying size) and multiple page sizes is that segments are generally visible to to user as first-class objects, are supported in the [[Central Processing Unit|CPU]] (in the [[instruction]]s), etc; whereas pages are generally invisible to the user.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Some systems (e.g. [[Multics]]) had/have support for multiple page sizes. The distinction between segments (which usually can be of varying size) and multiple page sizes is that segments are generally visible to to user as first-class objects, are supported in the [[Central Processing Unit|CPU]] (in the [[instruction]]s), etc; whereas pages are generally invisible to the user.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>(But not always; some OS's allow processes to share pages, or to map [[file]] pages into process address spaces, etc. Which can make it complex to separate the two...)</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>(But not always; some OS's allow processes to share pages, or to map [[file]] pages into process address spaces, etc. Which can make it complex to separate the two <ins class="diffchange diffchange-inline">concepts</ins>...)</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>Segments also usually have a much larger range of sizes (the segment size, measured in small units, is often stored in a <del class="diffchange diffchange-inline">[[word]], </del>as part of the data which describes the segment to the CPU's hardware), whereas with variable page sizes, only a few are supported.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Segments also usually have a much larger range of sizes (the segment size, measured in small units, is often stored in a <ins class="diffchange diffchange-inline">field </ins>as part of the data which describes the segment to the CPU's hardware), whereas with variable page sizes, only a few are supported.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Systems which implement a [[single-level store]] often use both segmentation ''and'' paging to do so; the segments are visible to the users, and are invisibly broken up into pages for making them resident in main memory. (The segment sizes are often given in pages in such systems.)</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Systems which implement a [[single-level store]] often use both segmentation ''and'' paging to do so; the segments are visible to the users, and are invisibly broken up into pages for making them resident in main memory. (The segment sizes are often given in pages in such systems.)</div></td></tr>
</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=16920&oldid=prevJnc: Avoid redir2018-06-15T12:21:05Z<p>Avoid redir</p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class="diff-marker" />
<col class="diff-content" />
<col class="diff-marker" />
<col class="diff-content" />
<tr style="vertical-align: top;" lang="en">
<td colspan="2" style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: white; color:black; text-align: center;">Revision as of 12:21, 15 June 2018</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l9" >Line 9:</td>
<td colspan="2" class="diff-lineno">Line 9:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''[[virtual address]]ing'', which means that the addresses seen by a [[user]]'s computation, e.g. a [[process]], is not the actual address of the main memory location that the [[hardware]] knows for that memory location, but rather a 'virtual' address private to that user/process. (Early [[memory management]] systems such as [[base and bounds]] [[register]]s provided this feature alone.)</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''[[virtual address]]ing'', which means that the addresses seen by a [[user]]'s computation, e.g. a [[process]], is not the actual address of the main memory location that the [[hardware]] knows for that memory location, but rather a 'virtual' address private to that user/process. (Early [[memory management]] systems such as [[base and bounds]] [[register]]s provided this feature alone.)</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''non-residence'', or '''demand loading''' - i.e. a computation can run without ''all'' of its virtual memory being present in physical memory, and missing items are automatically brought in by the operating system when they are needed. To do this, the user's memory has to be divided into blocks somehow, either through [[<del class="diffchange diffchange-inline">segmentation</del>]], or via the third:</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''non-residence'', or '''demand loading''' - i.e. a computation can run without ''all'' of its virtual memory being present in physical memory, and missing items are automatically brought in by the operating system when they are needed. To do this, the user's memory has to be divided into blocks somehow, either through [[<ins class="diffchange diffchange-inline">segment</ins>]]<ins class="diffchange diffchange-inline">ation</ins>, or via the third:</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. When demand loading is in use, only a selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones are brought into free page frames as they are needed.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. When demand loading is in use, only a selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones are brought into free page frames as they are needed.</div></td></tr>
</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=16324&oldid=prevJnc: /* Details */ Non-demand paging2018-05-05T12:46:49Z<p><span dir="auto"><span class="autocomment">Details: </span> Non-demand paging</span></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class="diff-marker" />
<col class="diff-content" />
<col class="diff-marker" />
<col class="diff-content" />
<tr style="vertical-align: top;" lang="en">
<td colspan="2" style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: white; color:black; text-align: center;">Revision as of 12:46, 5 May 2018</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l9" >Line 9:</td>
<td colspan="2" class="diff-lineno">Line 9:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''[[virtual address]]ing'', which means that the addresses seen by a [[user]]'s computation, e.g. a [[process]], is not the actual address of the main memory location that the [[hardware]] knows for that memory location, but rather a 'virtual' address private to that user/process. (Early [[memory management]] systems such as [[base and bounds]] [[register]]s provided this feature alone.)</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''[[virtual address]]ing'', which means that the addresses seen by a [[user]]'s computation, e.g. a [[process]], is not the actual address of the main memory location that the [[hardware]] knows for that memory location, but rather a 'virtual' address private to that user/process. (Early [[memory management]] systems such as [[base and bounds]] [[register]]s provided this feature alone.)</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''non-residence'', or ''demand loading'' - i.e. a computation can run without ''all'' of its virtual memory being present in physical memory, and missing items are automatically brought in by the operating system when they are needed. To do this, the user's memory has to be divided into blocks somehow, either through [[segmentation]], or via the third:</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''non-residence'', or <ins class="diffchange diffchange-inline">'</ins>''demand loading<ins class="diffchange diffchange-inline">'</ins>'' - i.e. a computation can run without ''all'' of its virtual memory being present in physical memory, and missing items are automatically brought in by the operating system when they are needed. To do this, the user's memory has to be divided into blocks somehow, either through [[segmentation]], or via the third:</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. <del class="diffchange diffchange-inline">A </del>selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones <del class="diffchange diffchange-inline">can be </del>brought into free page frames as they are needed.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. <ins class="diffchange diffchange-inline">When demand loading is in use, only a </ins>selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones <ins class="diffchange diffchange-inline">are </ins>brought into free page frames as they are needed.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>In modern systems, generally all three are going on, but this is not necessarily mandatory. For example, one can have the first two without having the third; e.g. the [[Burroughs B5000]] supported demand loading of [[segmentation|segments]], but did not have paging.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>In modern systems, generally all three are going on, but this is not necessarily mandatory. For example, one can have the first two without having the third; e.g. the [[Burroughs B5000]] supported demand loading of [[segmentation|segments]], but did not have paging<ins class="diffchange diffchange-inline">. It is also possible to have a sytem which has paging, but not demand loading - the paging would only be used for memory allocation (see the bottom of this page for why paging is advantageous for this)</ins>.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Some systems (e.g. [[Multics]]) had/have support for multiple page sizes. The distinction between segments (which usually can be of varying size) and multiple page sizes is that segments are generally visible to to user as first-class objects, are supported in the [[Central Processing Unit|CPU]] (in the [[instruction]]s), etc; whereas pages are generally invisible to the user.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Some systems (e.g. [[Multics]]) had/have support for multiple page sizes. The distinction between segments (which usually can be of varying size) and multiple page sizes is that segments are generally visible to to user as first-class objects, are supported in the [[Central Processing Unit|CPU]] (in the [[instruction]]s), etc; whereas pages are generally invisible to the user.</div></td></tr>
</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=16323&oldid=prevJnc: /* Details */ LInk this, not obvious it's the same2018-05-05T11:33:28Z<p><span dir="auto"><span class="autocomment">Details: </span> LInk this, not obvious it's the same</span></p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class="diff-marker" />
<col class="diff-content" />
<col class="diff-marker" />
<col class="diff-content" />
<tr style="vertical-align: top;" lang="en">
<td colspan="2" style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: white; color:black; text-align: center;">Revision as of 11:33, 5 May 2018</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l13" >Line 13:</td>
<td colspan="2" class="diff-lineno">Line 13:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. A selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones can be brought into free page frames as they are needed.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. A selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones can be brought into free page frames as they are needed.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>In modern systems, generally all three are going on, but this is not necessarily mandatory. For example, one can have the first two without having the third; e.g. the [[Burroughs B5000]] supported demand loading of segments, but did not have paging.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>In modern systems, generally all three are going on, but this is not necessarily mandatory. For example, one can have the first two without having the third; e.g. the [[Burroughs B5000]] supported demand loading of <ins class="diffchange diffchange-inline">[[segmentation|</ins>segments<ins class="diffchange diffchange-inline">]]</ins>, but did not have paging.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Some systems (e.g. [[Multics]]) had/have support for multiple page sizes. The distinction between segments (which usually can be of varying size) and multiple page sizes is that segments are generally visible to to user as first-class objects, are supported in the [[Central Processing Unit|CPU]] (in the [[instruction]]s), etc; whereas pages are generally invisible to the user.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>Some systems (e.g. [[Multics]]) had/have support for multiple page sizes. The distinction between segments (which usually can be of varying size) and multiple page sizes is that segments are generally visible to to user as first-class objects, are supported in the [[Central Processing Unit|CPU]] (in the [[instruction]]s), etc; whereas pages are generally invisible to the user.</div></td></tr>
</table>Jnchttps://gunkies.org/w/index.php?title=Virtual_memory&diff=16321&oldid=prevJnc: Mention combo of segmentation/paging2018-05-05T11:30:05Z<p>Mention combo of segmentation/paging</p>
<table class="diff diff-contentalign-left" data-mw="interface">
<col class="diff-marker" />
<col class="diff-content" />
<col class="diff-marker" />
<col class="diff-content" />
<tr style="vertical-align: top;" lang="en">
<td colspan="2" style="background-color: white; color:black; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: white; color:black; text-align: center;">Revision as of 11:30, 5 May 2018</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l7" >Line 7:</td>
<td colspan="2" class="diff-lineno">Line 7:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>There are usually three related things going on when virtual memory is in use:</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>There are usually three related things going on when virtual memory is in use:</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''[[virtual address]]ing'', which means that the addresses seen by a [[user]]'s computation, e.g. a [[process]], is not the actual address of the main memory location that the [[hardware]] knows for that memory location, but rather a 'virtual' address private to that user/process. (Early [[memory management]] systems such as [[base and bounds]] [[register]]s provided this alone.)</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''[[virtual address]]ing'', which means that the addresses seen by a [[user]]'s computation, e.g. a [[process]], is not the actual address of the main memory location that the [[hardware]] knows for that memory location, but rather a 'virtual' address private to that user/process. (Early [[memory management]] systems such as [[base and bounds]] [[register]]s provided this <ins class="diffchange diffchange-inline">feature </ins>alone.)</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''non-residence'', or ''demand loading'' - i.e. a computation can run without ''all'' of its virtual memory being present in physical memory, and missing items are automatically brought in by the operating system when they are needed. To do this, the user's memory has to be divided into blocks somehow, either through [[segmentation]], or via the third:</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* What we can call ''non-residence'', or ''demand loading'' - i.e. a computation can run without ''all'' of its virtual memory being present in physical memory, and missing items are automatically brought in by the operating system when they are needed. To do this, the user's memory has to be divided into blocks somehow, either through [[segmentation]], or via the third:</div></td></tr>
<tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l13" >Line 13:</td>
<td colspan="2" class="diff-lineno">Line 13:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. A selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones can be brought into free page frames as they are needed.</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>* Dividing the address space into '''pages''', identically-sized blocks of address space; similarly, the physical main memory is also divided into similarly-sized '''page frames'''. A selected subset of the pages of a given computation's address space are assigned to page frames at any given time, and missing ones can be brought into free page frames as they are needed.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>In modern systems, generally all three are <del class="diffchange diffchange-inline">happening</del>, but this is not necessarily mandatory. For example, one can have the first two without having the third; e.g. the [[Burroughs B5000]] supported demand loading of segments, but did not have paging.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>In modern systems, generally all three are <ins class="diffchange diffchange-inline">going on</ins>, but this is not necessarily mandatory. For example, one can have the first two without having the third; e.g. the [[Burroughs B5000]] supported demand loading of segments, but did not have paging.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>Some systems (e.g. [[Multics]]) had/have support for multiple page sizes. The distinction between segments (which usually can be of varying size) and multiple page sizes is that segments are generally visible to to user as first-class objects, are supported in the CPU (in the [[instruction]]s), etc; whereas pages are generally invisible to the user.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Some systems (e.g. [[Multics]]) had/have support for multiple page sizes. The distinction between segments (which usually can be of varying size) and multiple page sizes is that segments are generally visible to to user as first-class objects, are supported in the <ins class="diffchange diffchange-inline">[[Central Processing Unit|</ins>CPU<ins class="diffchange diffchange-inline">]] </ins>(in the [[instruction]]s), etc; whereas pages are generally invisible to the user.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>(But not always; some OS's allow processes to share pages, or to map [[file]] pages into process address spaces, etc. Which can make it complex to separate the two...)</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>(But not always; some OS's allow processes to share pages, or to map [[file]] pages into process address spaces, etc. Which can make it complex to separate the two...)</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'>−</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>Segments also usually have a much larger range of sizes (the segment size is often stored in a [[word]]), whereas with variable page sizes, only a few are supported.</div></td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>Segments also usually have a much larger range of sizes (the segment size<ins class="diffchange diffchange-inline">, measured in small units, </ins>is often stored in a [[word]]<ins class="diffchange diffchange-inline">, as part of the data which describes the segment to the CPU's hardware</ins>), whereas with variable page sizes, only a few are supported.</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div> </div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">Systems which implement a [[single-level store]] often use both segmentation ''and'' paging to do so; the segments are visible to the users, and are invisibly broken up into pages for making them resident in main memory. (The segment sizes are often given in pages in such systems.)</ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>There is no requirement that a virtual memory system have only two levels of storage; some systems had more (e.g. Multics, which used a three-level virtual memory system, with [[core memory|core]], [[drum]] and disk memories; later modified to fast [[dynamic RAM|semi-conductor]] main memory, slower bulk core, and disk).</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>There is no requirement that a virtual memory system have only two levels of storage; some systems had more (e.g. Multics, which used a three-level virtual memory system, with [[core memory|core]], [[drum]] and disk memories; later modified to fast [[dynamic RAM|semi-conductor]] main memory, slower bulk core, and disk).</div></td></tr>
<tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l31" >Line 31:</td>
<td colspan="2" class="diff-lineno">Line 33:</td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>In addition, the use of fixed-size pages, and the usual practise of allowing any page of the address space to be allocated to any page frame, means there is no 'breakage' of main memory, and no need to move data around in main memory to coalesce free space (which was usually necessary on systems which did not use pages).</div></td><td class='diff-marker'> </td><td style="background-color: #f9f9f9; color: #333333; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #e6e6e6; vertical-align: top; white-space: pre-wrap;"><div>In addition, the use of fixed-size pages, and the usual practise of allowing any page of the address space to be allocated to any page frame, means there is no 'breakage' of main memory, and no need to move data around in main memory to coalesce free space (which was usually necessary on systems which did not use pages).</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">==See also==</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color:black; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">* [[Overlay]]</ins></div></td></tr>
</table>Jnc