--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Symbian3/SDK/Source/GUID-7287F1D4-023E-5C90-91E1-6D8DF78D8241.dita	Thu Jan 21 18:18:20 2010 +0000
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+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
+<!-- This component and the accompanying materials are made available under the terms of the License 
+"Eclipse Public License v1.0" which accompanies this distribution, 
+and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
+<!-- Initial Contributors:
+    Nokia Corporation - initial contribution.
+Contributors: 
+-->
+<!DOCTYPE concept
+  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
+<concept id="GUID-7287F1D4-023E-5C90-91E1-6D8DF78D8241" xml:lang="en"><title>Virtual
+machine model</title><shortdesc>The Kernel provides a 'virtual machine' environment to user processes
+where each process accesses its data in the same virtual address range.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
+<p>The Kernel provides a 'virtual machine' environment to user processes.
+Each process accesses its data in the same virtual address range, called the
+data section, which ranges from:</p>
+<codeblock id="GUID-1FE73FDC-5058-5500-BD9D-C41C41DF2481" xml:space="preserve">0x00400000 to 0x3FFFFFFF</codeblock>
+<p>but note that the static data always appears at:</p>
+<codeblock id="GUID-10A112F3-28EF-589F-BE5C-0B53ED736596" xml:space="preserve">0x00400000</codeblock>
+<p>The code chunk for RAM loaded processes always appears at:</p>
+<codeblock id="GUID-18064043-B6FA-5791-81BC-A8D7859540E3" xml:space="preserve">0x20000000</codeblock>
+<p>This allows multiple processes to run, each executing the same code (from
+the same code chunk) for each of the processes. For example, multiple word
+documents open at the same time, each in a separate instance of the word application.
+This reduces RAM usage.</p>
+<p>In effect, each user process has the same kind of view. Code instructions
+address data using the virtual address; the Memory Management Unit (MMU) is
+responsible for the translation of the virtual address to the physical RAM
+address.</p>
+<p>Only one chunk can occupy a given virtual address range at a time, so a
+context switch between different processes involves re-mapping the chunks.
+The process chunks of the old process are re-mapped to their home addresses.
+These are in the home section, which is the virtual address range from:</p>
+<codeblock id="GUID-27EFC279-550A-5970-98F5-959647314B44" xml:space="preserve">0x80000000 to 0xFFFFFFFF</codeblock>
+<p>ROM code is normally mapped into the address range:</p>
+<codeblock id="GUID-D75CC9C3-2AD3-5BA0-A8B6-F486CF1D0232" xml:space="preserve">0x50000000 to 0x5FFFFFFF</codeblock>
+<p>The process chunks of the new process are mapped from their home addresses
+back to the data section. Chunks which are not accessible by the current user
+process reside in the home section, and they have supervisor-mode-only access
+permissions, so that only the kernel can access them.</p>
+<p>The Kernel's data and stack/heap chunks also reside in the home section.
+These are never visible to user processes. Code chunks for RAM-loaded libraries
+reside at the top end of the home section and have user read-only access,
+so that all user processes can execute code from a loaded library.</p>
+<p>A context switch between processes therefore involves:</p>
+<ul>
+<li id="GUID-F83EAD6D-626D-53B2-9DEB-310617FC03EE"><p>moving the old process
+chunks to the home section, and changing their access permissions to supervisor-only</p> </li>
+<li id="GUID-7BEBDFDE-DFFB-50F9-A136-5C0018CBD78F"><p>moving the new process
+chunks to the data section and changing their access permissions back to user-accessible.</p> </li>
+</ul>
+<p>This is best seen graphically. In the first diagram shown below, user-process
+1 is running and can 'see' all chunks in the clear boxes. Those boxes with
+dark background represent chunks which are not visible to user-process 1.</p>
+<p>When user-process 2 is running, the context switch re-maps the user-process
+2 data to the data section and user process 1 data is re-mapped to the home
+section as the second diagram shows.</p>
+<fig id="GUID-A8C12C7E-7779-5D66-908B-51A1C36E1B63">
+<title>User process 1's view.</title>
+<image href="GUID-09044578-9ADB-540F-A854-A818EA3970B7_d0e215495_href.png" placement="inline"/>
+</fig>
+<fig id="GUID-5F15432A-0CCD-5195-921F-506679F023CC">
+<title>User process 2's view.</title>
+<image href="GUID-1A7183EC-B55E-5B61-8965-89C939ECB395_d0e215503_href.png" placement="inline"/>
+</fig>
+</conbody></concept>
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