PAE(4)               FreeBSD/i386 Kernel Interfaces Manual              PAE(4)

NNAAMMEE
     PPAAEE

SSYYNNOOPPSSIISS
     ooppttiioonnss PPAAEE

DDEESSCCRRIIPPTTIIOONN
     The PPAAEE option provides support for the physical address extensions capa-
     bility 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 PPAAEE 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 for a process or
     the kernel to access more memory than they would otherwise be able to
     access, through a sliding window or otherwise.

SSEEEE AALLSSOO
     smp(4) tuning(7)

HHIISSTTOORRYY
     The PPAAEE option first appeared in FreeBSD 5.1.

AAUUTTHHOORRSS
     Jake Burkholder <jake@FreeBSD.org>

BBUUGGSS
     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 PPAAEE 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 PPAAEE option is used, these drivers must use a facility for remap-
     ping or substituting physical memory which is not accessible to the
     device.  One such facility is provided by the bbuussddmmaa interface.  Device
     drivers which do not make affordances for such devices will not work
     reliably in a system with more than 4 gigabytes of memory when the PPAAEE
     option is used, and may cause data corruption.  The PPAAEE kernel configura-
     tion file includes the PPAAEE option, and explicitly excludes all device
     drivers which are known to not work or have not been tested in a system
     with the PPAAEE 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 paramters 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 avail-
     able 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 KKVVAA__PPAAGGEESS option may be used to increase the kernel virtual address
     space, and the kkeerrnn..mmaaxxvvnnooddeess sysctl 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.

     Do not taunt Happy Fun PPAAEE.

FreeBSD 5.0                      April 8, 2003                     FreeBSD 5.0