<question id="pae">

          <para>Why is &os; finding the wrong amount of memory?</para>

        </question>

        <answer>

          <para>The reason is the difference between physical memory

            addresses and virtual addresses.</para>

          <para>The convention for most PC hardware is to use memory

            between 3.5G and 4G for special use (usually for PCI).

            This is physical memory address space that is used to access

            the PCI hardware.  As a result real memory (RAM) can not

            exist there.</para>

          <para>What happens to the memory that should appear in that

            location is dependent on your hardware.  Unfortunately,

            some hardware does nothing and the ability to use that

            last 500M of RAM is lost.</para>

          <para>Luckily, most hardware remaps the memory to a higher

            location so that it can still be used.  However, this can

            cause some confusion if you watch the boot messages.</para>

          <para>On a 32 bit version of &os;, the memory appears lost,

            since it will be remapped above 4G, which a 32 bit kernel

            is unable to access.  In this case, the solution is to

            build a PAE enabled kernel.  See

            <link linkend="memory-limits">this FAQ entry</link> for

            more information.</para>

          <para>On a 64 bit version of &os;, or when running a PAE-enabled

            kernel, &os; will correctly detect and remap the memory so

            it is usable.  During boot, however, it may seem as if &os;

            is detecting more memory than the system really has.  This

            is normal and the available memory will be corrected as the

            boot process completes.<para>

        </answer>

      </qandaentry>

      <qandaentry>