Update PAE section. Added one sentence to the original submission

describing the versions of FreeBSD that support PAE.

Submitted by:	hmp
Reviewed by:	simon
Discussed with:	simon, murray

en_US.ISO8859-1/books/handbook/kernelconfig/chapter.sgml

@@ -1329,32 +1329,100 @@ pseudo-device   bpf           # Berkeley packet filter</programlisting>
       <filename>/usr/src/sys/i386/conf/LINT</filename>.</para>
 
       <sect2>
-        <title>Defeating Memory Limitations</title>
+        <title>Large Memory Configurations (<acronym>PAE</acronym>)</title>
+	<indexterm>
+	  <primary>
+	    Physical Address Extensions (<acronym>PAE</acronym>)
+	  </primary>
+	  <secondary>
+	    Large Memory Configurations
+	  </secondary>
+	</indexterm>
 
-	<para>In the past, &os; never made use of memory beyond four
-	  gigabytes.  This was termed the four gigabytes limit and
-	  was a hassle to those who owned machines which supported a
-	  memory size larger than four gigabytes.  For those users, the
-	  &man.pae.4; driver was written.  The <literal>PAE</literal>
-	  driver provides for memory address extensions ultimately
-	  permitting up to sixty-four gigabytes.  To make use of this
-	  feature, just add:</para>
+	<para>Large memory configuration machines require access to
+	  more than the 4 gigabyte limit on User+Kernel Virtual
+	  Address (<acronym>KVA</acronym>) space.  Due to this
+	  limitation, Intel added support for 36-bit physical address
+	  space access in the &pentium; Pro and later line of CPUs.</para>
+	
+	<para>The Physical Address Extension (<acronym>PAE</acronym>)
+	  capability of the &intel; &pentium; Pro and later CPUs
+	  allows memory configurations of up to 64 gigabytes.
+	  &os; provides support for this capability via the
+	  <option>PAE</option> kernel configuration option, available
+	  in the 4.X series of &os; beginning with 4.9-RELEASE and
+	  in the 5.X series of &os; beginning with 5.1-RELEASE.  Due to
+	  the limitations of the Intel memory architecture, no distinction
+	  is made for memory above or below 4 gigabytes.  Memory allocated
+	  above 4 gigabytes is simply added to the pool of available
+	  memory.</para>
+
+	<para>To enable <acronym>PAE</acronym> support in the kernel,
+	  simply add the following line to your kernel configuration
+	  file:</para>
 	
 	<programlisting>options		    PAE</programlisting>
 
-      <para>to the kernel configuration file as explained above.</para>
-
 	<note>
-	<para>This option is slightly experimental, and could cause
-	  minor problems.  For instance, the kernel's virtual address
-	  space may need to be increased.  Add the
-	  <option>KVA_PAGES</option> from <filename>NOTES</filename> to
-	  the kernel configuration file.  The default number,
-	  <literal>260</literal>, may need to be increased and the
-	  <option>kern.maxvnodes</option> may need to be decreased by
-	  using the <command>sysctl</command> utility.  Reading the
-	  &man.tuning.7; manual page is certainly advised.</para>
+	  <para>The <acronym>PAE</acronym> support in &os; is only
+	    available for &intel; IA-32 processors.  It should also be
+	    noted, that the <acronym>PAE</acronym> support in &os; has
+	    not received wide testing, and should be considered beta
+	    quality compared to other stable features of &os;.</para>
 	</note>
+
+	<para>PAE support in &os; has a few limitations:</para>
+	
+	<itemizedlist>
+	  <listitem>
+	    <para>A process is not able to access more than 4
+	      gigabytes of VM space.</para>
+	  </listitem>
+
+	  <listitem>
+	    <para><acronym>KLD</acronym> modules cannot be loaded into
+	      a <acronym>PAE</acronym> enabled kernel, due to the
+	      differences in the build framework of a module and the
+	      kernel.</para>
+	  </listitem>
+
+	  <listitem>
+	    <para>Device drivers that do not use the &man.bus.dma.9;
+	      interface will cause data corruption in a
+	      <acronym>PAE</acronym> enabled kernel and are not
+	      recommended for use.  For this reason, the
+	      <filename>PAE</filename> kernel
+	      configuration file is provided in &os; 5.X, which
+	      excludes all drivers not known to work in a PAE enabled
+	      kernel.</para>
+	  </listitem>
+
+	  <listitem>
+	    <para>Some system tunables determine memory resource usage
+	      by the amount of available physical memory.  Such
+	      tunables can unnecessarily over-allocate due to the
+	      large memory nature of a <acronym>PAE</acronym> system.
+	      One such example is the <option>kern.maxvnodes</option>
+	      sysctl, which controls the maximum number of vnodes allowed
+	      in the kernel.  It is advised to adjust this and other
+	      such tunables to a reasonable value.</para>
+	  </listitem>
+
+	  <listitem>
+	    <para>It might be necessary to increase the kernel virtual
+	      address (<acronym>KVA</acronym>) space or to reduce the
+	      amount of specific kernel resource that is heavily used
+	      (see above) in order to avoid <acronym>KVA</acronym>
+	      exhaustion.  The <option>KVA_PAGES</option> kernel option
+	      can be used for increasing the
+	      <acronym>KVA</acronym> space.</para>
+	  </listitem>
+	</itemizedlist>
+
+	<para>For performance and stability concerns, it is advised to
+	  consult the &man.tuning.7; manual page.  The &man.pae.4;
+	  manual page contains up-to-date information on &os;'s
+	  <acronym>PAE</acronym> support.</para>
     </sect2>
   </sect1>