NAME

       PAE — Physical Address Extensions

SYNOPSIS

       options PAE

DESCRIPTION

       The  PAE  option provides support for the physical address extensions capability of the Intel Pentium Pro
       and above CPUs, and allows for up to 64 gigabytes of memory to be used in systems capable  of  supporting
       it.   With the PAE option, memory above 4 gigabytes is simply added to the general page pool.  The system
       makes no distinction between memory above or below 4 gigabytes, and no specific facility is provided  for
       a  process  or  the  kernel  to access more memory than they would otherwise be able to access, through a
       sliding window or otherwise.

SEE ALSO

       smp(4), tuning(7), config(8), bus_dma(9)

HISTORY

       The PAE option first appeared in FreeBSD 4.9 and FreeBSD 5.1.

AUTHORS

       Jake Burkholder <jake@FreeBSD.org>

BUGS

       Since KLD modules are not compiled with the same options headers that the kernel is compiled  with,  they
       must not be loaded into a kernel compiled with the PAE option.

       Many  devices or their device drivers are not capable of direct memory access to physical addresses above
       4 gigabytes.  In order to make use of direct memory access IO in a system with more than 4  gigabytes  of
       memory  when  the  PAE  option  is  used, these drivers must use a facility for remapping or substituting
       physical memory which is not accessible to the device.  One such  facility  is  provided  by  the  busdma
       interface.   Device drivers which do not account for such devices will not work reliably in a system with
       more than 4 gigabytes of memory when the PAE option is used, and may  cause  data  corruption.   The  PAE
       kernel  configuration  file includes the PAE option, and explicitly excludes all device drivers which are
       known to not work or have not been tested in a system with the PAE option and more than  4  gigabytes  of
       memory.

       Many  parameters  which  determine  how  memory is used in the kernel are based on the amount of physical
       memory.  The formulas used to determine the values of these parameters for specific memory configurations
       may not take into account the fact there may be more than 4 gigabytes of memory, and may not  scale  well
       to  these  memory  configurations.   In particular, it may be necessary to increase the amount of virtual
       address space available to the kernel, or to reduce the amount of a specific  resource  that  is  heavily
       used,  in  order  to  avoid  running  out  of virtual address space.  The KVA_PAGES option may be used to
       increase the kernel virtual address space, and the kern.maxvnodes sysctl(8) may be used to  decrease  the
       number  of  vnodes  allowed,  an example of a resource that the kernel is likely to overallocate in large
       memory configurations.  For optimal performance  and  stability  it  may  be  necessary  to  consult  the
       tuning(7) manual page, and make adjustments to the parameters documented there.

Debian                                            April 8, 2003                                           PAE(4)