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Adaptation/GUID-E21E7992-607A-5A49-B022-189ECA9E76D1.dita
author Graeme Price <GRAEME.PRICE@NOKIA.COM>
Fri, 15 Oct 2010 14:32:18 +0100
changeset 15 307f4279f433
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<?xml version="1.0" encoding="utf-8"?>
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<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
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<!-- This component and the accompanying materials are made available under the terms of the License
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"Eclipse Public License v1.0" which accompanies this distribution,
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and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
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<!-- Initial Contributors:
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    Nokia Corporation - initial contribution.
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Contributors:
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-->
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<!DOCTYPE concept
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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="GUID-E21E7992-607A-5A49-B022-189ECA9E76D1" xml:lang="en"><title>Code
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Paging Overview</title><shortdesc>Overview of demand paging when applied to code paging. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
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<section id="GUID-254768D6-B78B-5193-8583-2F8CF57614AC"><title>Purpose</title> <p>Demand
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paging is a technique where memory appears to application programs to be present
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in RAM, but may in fact be stored on some external media and transparently
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loaded into RAM when needed. Demand paging trades off increased available
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RAM against decreased performance, increased media wear and increased power
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usage. More RAM is made available by loading pages only on demand, but a cost
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is incurred every time a page is loaded. </p> <p>Demand paging is used to
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reduce the amount of RAM that needs to be shipped with a device and so reduce
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its cost. </p> <p>For the code paging type of demand paging, the executable
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is stored in a ROM. Since the memory locations that will be pointed to cannot
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be determined ahead of time, the pointers in the executable have to be modified
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after the page-in process. This process is known as 'relocation' and 'fix-up'.
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It is usually done by loader. </p> </section>
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<section id="GUID-217936E7-4F09-5291-B6C5-FF00BCE1B4BD"><title>Description</title> <p>The
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executable is in a ROM and so has to be loaded into RAM before it can be executed.
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When the required part of the executable is not present in RAM, then a paging
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fault is generated which starts the paging in process of specifying which
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part of the ROM is to be paged-in along with which RAM page is to be used. </p> <p>The
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above process (in very simple terms) describes how ROM paging works. With
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code paging, there is the added complication that the executable is not execute
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in place (it is probably stored via the use of an operating system e.g. ROFS)
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and so when it is paged in any pointers in the page will not point to any
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valid location. Hence a new step has to be carried out that modifies the pointers
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in the new page so that will point to meaningful locations once they are RAM.
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This process is known as 'relocation' and 'fix-up'. It is usually done by
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loader. </p> <p> </p> </section>
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<section id="GUID-85164F03-A6B5-5227-A380-5B199C805670"><title>Components</title> <p>These
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are the main components of code demand paging: </p> <ul>
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<li id="GUID-FB01C6A6-A688-5A2C-85ED-9AA81550031B"><p>Rom image - The executable
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is stored in a ROM. </p> </li>
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<li id="GUID-1A2D6E09-4A95-5000-B526-A005AD07A2A5"><p>RAM - Where the executable
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will be executed. </p> </li>
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<li id="GUID-ED582DCB-2018-5E02-8B97-84D41D79F06C"><p>Paging Fault Handler
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- To detect that a page-in process is required and to carry it out. </p> </li>
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<li id="GUID-ECF22038-E5E2-56BC-9DDF-4EB50ADAFBBC"><p>Loader - This usually
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does the 'relocation' and 'fix-up' process after the 'page-in' process. </p> </li>
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</ul> </section>
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<section id="GUID-A0780848-8A13-41F8-9BCD-C93B6CC7E0B8"><title>Using ROM Paging</title> <p>Which
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type of paging is used and for which area of memory is first specified the <codeph>oby</codeph> and <codeph>mmp</codeph> files
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and finally build by using specific parameters in the buildrom utility. </p> </section>
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</conbody><related-links>
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<link href="GUID-BDB847A2-557A-5902-AA6D-C1AE10D8E493.dita"><linktext>Code Paging
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Guide</linktext></link>
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<link href="GUID-795B8649-B6C3-5540-B52A-9B460F35A5B5.dita"><linktext>ROM Paging</linktext>
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</link>
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<link href="GUID-B35A70D2-1BC8-51DE-95BF-F315DB394582.dita"><linktext>Demand Paging
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Overview</linktext></link>
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</related-links></concept>
MCL/sftools/depl/docscontent