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<concept id="GUID-B35A70D2-1BC8-51DE-95BF-F315DB394582" xml:lang="en"><title>Demand

Paging Overview</title><shortdesc>Demand paging is a technique where content appears to be present

in RAM, but may in fact be stored on some external media and is transparently

loaded when needed.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-733D5AEF-6488-5F11-ABA7-AB4E5B45F9D0"><title>Purpose</title> <p>Demand

paging trades off increased available RAM against increased data latency,

increased media wear and increased power usage. Overall performance may be

increased as there can be more RAM available to each application as it runs. </p> <p>Demand

paging is used to reduce the amount of RAM that needs to be shipped with a

device and so reduces the device cost. </p> </section>

<section id="GUID-BC958271-5F6B-59D4-A60D-21AB336377D9"><title>Description</title> <p>Demand

paging relies on the fact that most memory access is likely to occur in a

small region of memory and not spread over the entire memory map. This means

that if only that small area of memory is available to a process (or thread)

at any one time, then the amount of RAM required can be reduced. The small

area of memory is known as a page. The working RAM is broken down into pages,

some of which are used directly by the processor and the rest are spare. Since

Symbian platform is a multi-tasking operating system, there is more than one

page in the working RAM at any one time. There is one page per thread. </p> <p>There

are three types of demand paging: ROM paging, code paging and writable data

paging. All of these types of demand paging have some features in common: </p> <ul>

<li id="GUID-82F77F93-433B-59E8-B44E-0EC9F183B5C0"><p>A free RAM pool </p> </li>

<li id="GUID-7318C5DE-4342-51AB-8A2F-86F3B2DB6083"><p>Working RAM </p> </li>

<li id="GUID-296241B1-9896-5673-A9F3-E87F5404E66E"><p>Paging Fault Handler </p> </li>

<li id="GUID-D3C9BC6B-9F1F-56E9-9C31-ABFC172CD7D8"><p>A source of code and/or

data that the processor needs to access. </p> </li>

</ul> <p>With demand paging, the processor requires content to be present

in the working RAM. If the content is not present, then a 'paging fault' occurs

and the required information is loaded into a page of the working RAM. This

is known as "paged-in". When the contents of this page are no longer required,

then the page returns to the pool of free RAM. This is known as "paging-out". </p> <p>The

difference between the types of demand paging is the source that is to be

used: </p> <ul>

<li id="GUID-A6CAEFFD-729A-52C6-8576-15D069A287FB"><p>For ROM paging, it is

code and/or data stored under the <xref href="GUID-1B9EFA73-D949-5E8C-BD7C-17D37079FBC4.dita">ROM

file system</xref>  </p> </li>

<li id="GUID-BC64B7AB-2A33-573A-8F0B-C2EB10A49B51"><p>For code paging, it

is code and/or data stored using <xref href="GUID-02126A62-B2FA-5DAB-AC4D-536A3BCD7BA1.dita">ROFS

file system</xref>  </p> </li>

<li id="GUID-B511A99E-187F-5962-A0E8-2783B53834C7"><p>For writable data paging,

it is the writable data stored in RAM, for example user stacks and heaps.

In this case, the data is moved to a backing store. </p> </li>

</ul> <fig id="GUID-FDD6829A-DAF9-5752-9C3F-2338625054FA">

</fig> <p>The diagram above shows the basic operations involved in demand

paging: </p> <ol id="GUID-598BA3E8-7BB9-585D-913B-5453D8500CBF">

<li id="GUID-E65CC155-049D-5351-9A01-5FCDA3263FAB"><p>The processor wants

to access content which is not available in the working RAM, causing a 'paging

fault'. </p> </li>

<li id="GUID-DF626329-9D08-5242-881A-15D8837BAA02"><p>The paging fault handler

starts the process for copying a page of the source into a page of RAM. </p> </li>

<li id="GUID-BB88798C-1D62-540B-A0C6-9C026E1D1DAB"><p>A page is selected. </p> </li>

<li id="GUID-CD8265F9-D42A-57B7-8A33-712FA4B9923F"><p>The contents of the

source is loaded into the page and this becomes part of the working memory. </p> </li>

<li id="GUID-721F338C-108F-58C9-B430-56730F26B66B"><p>When the contents of

the page are no longer required, the page is returned to the free RAM pool. </p> </li>

</ol> </section>

<section id="GUID-C2E6EE31-1AD6-4580-9296-E8E29E8C76F1"><title>Demand Paging features</title> <p>Demand Paging provides the

following features: </p> <ul>

<li id="GUID-75F892EA-D389-50D2-98AF-F66D16CF89E4"><p>Reduced amount of RAM

required </p> </li>

<li id="GUID-4356F1F0-1DB1-5DBE-AB30-B4AB4883144C"><p>Improved performance

in applications that involve loading a large amount of code into RAM, since

the number of memory access operations required has been reduced </p> </li>

<li id="GUID-21B9223E-2218-5AAA-8611-4DFAB63224BC"><p>Improved stability in

Out Of Memory (OOM) conditions. </p> </li>

</ul> </section>

<section id="GUID-21427657-A93E-465D-AD6E-F26ED83D2734"><title>Demand Paging limitations</title> <p>The following are known

limitations of Demand Paging: </p> <ul>

<li id="GUID-B41C667E-37DF-59B4-A115-E9232F0E293B"><p>The access time cannot

be guaranteed. </p> <p>There is an orders of magnitude difference in the access

time between an access where no page fault occurs and one where a page fault

occurs. If a page fault does not occur, then the time taken for a memory access

is in the tens to hundreds of nanosecond range. If a page fault does occur,

then the time taken for a memory access could be in the millisecond range </p> </li>

<li id="GUID-63D8300F-D884-530A-9394-A51183A399A5"><p>Device drivers have

to be written to allow for data latency when a page fault occurs. </p> </li>

</ul> </section>

</conbody><related-links>

<link href="GUID-6F82A35E-9F11-591D-AA5C-8F60734A2827.dita"><linktext>Demand Paging

Guides</linktext></link>

<link href="GUID-795B8649-B6C3-5540-B52A-9B460F35A5B5.dita"><linktext>ROM paging</linktext>

</link>

<link href="GUID-CE9EA167-0594-5E61-9640-6B2B63A92EA7.dita"><linktext>Code Paging</linktext>

</link>

<link href="GUID-2B7D04D9-98DE-5284-836D-01DB4FA8949D.dita"><linktext>Writable

Data Paging</linktext></link>

</related-links></concept>