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Add a new FAQ entry with an explanation about why some times

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physical memory seems to probe incorrectly, with or without
PAE-enabled kernels.

PR:             docs/94454
Submitted by:   Bill Moran <wmoran@collaborativefusion.com>
This commit is contained in:
Giorgos Keramidas 2006-05-10 19:00:46 +00:00
parent 97253f8b53
commit d6a6d98242
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=27795
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en_US.ISO8859-1/books/faq/book.sgml
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@ -2867,6 +2867,44 @@ device card 1</programlisting>
<title>Troubleshooting</title>
<qandaset>
<qandaentry>
<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 the memory area
between 3.5G and 4G for a special purpose (usually for PCI). This
address space is used to access PCI hardware. As a result real,
physical memory can not appear in that address space.</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 entirely
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>
<question id="awre">
<para>What do I do when I have bad blocks on my hard drive?</para>