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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-7287F1D4-023E-5C90-91E1-6D8DF78D8241" xml:lang="en"><title>Virtual

    13 machine model</title><shortdesc>The Kernel provides a 'virtual machine' environment to user processes

    14 where each process accesses its data in the same virtual address range.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    15 <p>The Kernel provides a 'virtual machine' environment to user processes.

    16 Each process accesses its data in the same virtual address range, called the

    17 data section, which ranges from:</p>

    18 <codeblock id="GUID-1FE73FDC-5058-5500-BD9D-C41C41DF2481" xml:space="preserve">0x00400000 to 0x3FFFFFFF</codeblock>

    19 <p>but note that the static data always appears at:</p>

    20 <codeblock id="GUID-10A112F3-28EF-589F-BE5C-0B53ED736596" xml:space="preserve">0x00400000</codeblock>

    21 <p>The code chunk for RAM loaded processes always appears at:</p>

    22 <codeblock id="GUID-18064043-B6FA-5791-81BC-A8D7859540E3" xml:space="preserve">0x20000000</codeblock>

    23 <p>This allows multiple processes to run, each executing the same code (from

    24 the same code chunk) for each of the processes. For example, multiple word

    25 documents open at the same time, each in a separate instance of the word application.

    26 This reduces RAM usage.</p>

    27 <p>In effect, each user process has the same kind of view. Code instructions

    28 address data using the virtual address; the Memory Management Unit (MMU) is

    29 responsible for the translation of the virtual address to the physical RAM

    30 address.</p>

    31 <p>Only one chunk can occupy a given virtual address range at a time, so a

    32 context switch between different processes involves re-mapping the chunks.

    33 The process chunks of the old process are re-mapped to their home addresses.

    34 These are in the home section, which is the virtual address range from:</p>

    35 <codeblock id="GUID-27EFC279-550A-5970-98F5-959647314B44" xml:space="preserve">0x80000000 to 0xFFFFFFFF</codeblock>

    36 <p>ROM code is normally mapped into the address range:</p>

    37 <codeblock id="GUID-D75CC9C3-2AD3-5BA0-A8B6-F486CF1D0232" xml:space="preserve">0x50000000 to 0x5FFFFFFF</codeblock>

    38 <p>The process chunks of the new process are mapped from their home addresses

    39 back to the data section. Chunks which are not accessible by the current user

    40 process reside in the home section, and they have supervisor-mode-only access

    41 permissions, so that only the kernel can access them.</p>

    42 <p>The Kernel's data and stack/heap chunks also reside in the home section.

    43 These are never visible to user processes. Code chunks for RAM-loaded libraries

    44 reside at the top end of the home section and have user read-only access,

    45 so that all user processes can execute code from a loaded library.</p>

    46 <p>A context switch between processes therefore involves:</p>

    47 <ul>

    48 <li id="GUID-F83EAD6D-626D-53B2-9DEB-310617FC03EE"><p>moving the old process

    49 chunks to the home section, and changing their access permissions to supervisor-only</p> </li>

    50 <li id="GUID-7BEBDFDE-DFFB-50F9-A136-5C0018CBD78F"><p>moving the new process

    51 chunks to the data section and changing their access permissions back to user-accessible.</p> </li>

    52 </ul>

    53 <p>This is best seen graphically. In the first diagram shown below, user-process

    54 1 is running and can 'see' all chunks in the clear boxes. Those boxes with

    55 dark background represent chunks which are not visible to user-process 1.</p>

    56 <p>When user-process 2 is running, the context switch re-maps the user-process

    57 2 data to the data section and user process 1 data is re-mapped to the home

    58 section as the second diagram shows.</p>

    59 <fig id="GUID-A8C12C7E-7779-5D66-908B-51A1C36E1B63">

    60 <title>User process 1's view.</title>

    61 <image href="GUID-09044578-9ADB-540F-A854-A818EA3970B7_d0e244111_href.png" placement="inline"/>

    62 </fig>

    63 <fig id="GUID-5F15432A-0CCD-5195-921F-506679F023CC">

    64 <title>User process 2's view.</title>

    65 <image href="GUID-1A7183EC-B55E-5B61-8965-89C939ECB395_d0e244119_href.png" placement="inline"/>

    66 </fig>

    67 </conbody></concept>