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        and, finally, the leaf level is called PTL3. All architectures using
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        and, finally, the leaf level is called PTL3. All architectures using
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        this mechanism are required to use PTL0 and PTL3. However, the middle
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        this mechanism are required to use PTL0 and PTL3. However, the middle
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        levels can be left out, depending on the hardware hierachy or
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        levels can be left out, depending on the hardware hierachy or
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        structure of software-only page tables. The genericity is achieved
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        structure of software-only page tables. The genericity is achieved
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        through a set of macros that define transitions from one level to
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        through a set of macros that define transitions from one level to
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        another. Unused levels are optimised out by the compiler.</para>
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        another. Unused levels are optimised out by the compiler.
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    <figure float="1">
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          <mediaobject id="mm_pt">
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            <imageobject role="pdf">
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              <imagedata fileref="images/mm_pt.pdf" format="PDF" />
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            </imageobject>
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            <imageobject role="html">
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              <imagedata fileref="images/mm_pt.png" format="PNG" />
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            </imageobject>
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            <imageobject role="fop">
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              <imagedata fileref="images/mm_pt.svg" format="SVG" />
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            </imageobject>
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          </mediaobject>
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          <title>Hierarchical 4-level page tables.</title>
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        </figure>
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    </para>
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      </section>
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      </section>
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      <section>
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      <section>
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        <indexterm>
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        <indexterm>
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          <primary>page tables</primary>
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          <primary>page tables</primary>
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        key for the hash table. One of the major differences between the
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        key for the hash table. One of the major differences between the
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        global page hash table and hierarchical 4-level page tables is that
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        global page hash table and hierarchical 4-level page tables is that
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        there is only a single global page hash table in the system while
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        there is only a single global page hash table in the system while
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        hierarchical page tables exist per address space. Thus, the global
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        hierarchical page tables exist per address space. Thus, the global
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        page hash table contains information about mappings of all address
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        page hash table contains information about mappings of all address
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        spaces in the system.</para>
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        spaces in the system.
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        <figure float="1">
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          <mediaobject id="mm_hash">
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            <imageobject role="pdf">
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              <imagedata fileref="images/mm_hash.pdf" format="PDF" />
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            </imageobject>
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            <imageobject role="html">
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              <imagedata fileref="images/mm_hash.png" format="PNG" />
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            </imageobject>
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            <imageobject role="fop">
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              <imagedata fileref="images/mm_hash.svg" format="SVG" />
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            </imageobject>
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          </mediaobject>
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          <title>Global page hash table.</title>
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        </figure>
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</para>
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        <para>The global page hash table mechanism uses the generic hash table
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        <para>The global page hash table mechanism uses the generic hash table
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        type as described in the chapter dedicated to <link
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        type as described in the chapter dedicated to <link
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        linkend="hashtables">data structures</link> earlier in this
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        linkend="hashtables">data structures</link> earlier in this
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        book.</para>
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        book.</para>