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1.\" 2.\" Copyright (c) 2003 Networks Associates Technology, Inc. 3.\" All rights reserved. 4.\" 5.\" This software was developed for the FreeBSD Project by Jake Burkholder, 6.\" Safeport Network Services, and Network Associates Laboratories, the 7.\" Security Research Division of Network Associates, Inc. under 8.\" DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the DARPA 9.\" CHATS research program. 10.\" 11.\" Redistribution and use in source and binary forms, with or without 12.\" modification, are permitted provided that the following conditions 13.\" are met: 14.\" 1. Redistributions of source code must retain the above copyright 15.\" notice, this list of conditions and the following disclaimer. 16.\" 2. Redistributions in binary form must reproduce the above copyright 17.\" notice, this list of conditions and the following disclaimer in the 18.\" documentation and/or other materials provided with the distribution. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" $FreeBSD$ 33.\" 34.Dd April 8, 2003 35.Dt PAE 4 i386 36.Os 37.Sh NAME 38.Nm PAE 39.Nd Physical Address Extensions 40.Sh SYNOPSIS 41.Cd "options PAE" 42.Sh DESCRIPTION 43The 44.Dv PAE 45option provides support for the physical address extensions capability 46of the 47.Tn Intel 48.Tn Pentium Pro 49and above CPUs, 50and allows for up to 64 gigabytes of memory to be used in systems capable 51of supporting it. 52With the 53.Dv PAE 54option, memory above 4 gigabytes is simply added to the general page pool. 55The system makes no distinction between memory above or below 4 gigabytes, 56and no specific facility is provided for a process or the kernel to access 57more memory than they would otherwise be able to access, through a sliding 58window or otherwise. 59.Sh SEE ALSO 60.Xr smp 4 , 61.Xr tuning 7 , 62.Xr config 8 , 63.Xr bus_dma 9 64.Sh HISTORY 65The 66.Dv PAE 67option first appeared in 68.Fx 4.9 69and 70.Fx 5.1 . 71.Sh AUTHORS 72.An Jake Burkholder Aq jake@FreeBSD.org 73.Sh BUGS 74Since KLD modules are not compiled with the same options headers that 75the kernel is compiled with, 76they must not be loaded into a kernel compiled with the 77.Dv PAE 78option. 79.Pp 80Many devices or their device drivers are not capable of direct memory access 81to physical addresses above 4 gigabytes. 82In order to make use of direct memory access IO in a system with more than 834 gigabytes of memory when the 84.Dv PAE 85option is used, 86these drivers must use a facility for remapping or substituting physical 87memory which is not accessible to the device. 88One such facility is provided by the 89.Nm busdma 90interface. 91Device drivers which do not account for such devices will not work reliably 92in a system with more than 4 gigabytes of memory when the 93.Dv PAE 94option is used, 95and may cause data corruption. 96The 97.Pa PAE 98kernel configuration file includes the 99.Dv PAE 100option, and explicitly excludes all device drivers which are known to not work 101or have not been tested in a system with the 102.Dv PAE 103option and more than 4 gigabytes of memory. 104.Pp 105Many parameters which determine how memory is used in the kernel are based on 106the amount of physical memory. 107The formulas used to determine the values of these parameters for specific 108memory configurations may not take into account the fact there may be more 109than 4 gigabytes of memory, and may not scale well to these memory 110configurations. 111In particular, 112it may be necessary to increase the amount of virtual address space available 113to the kernel, 114or to reduce the amount of a specific resource that is heavily used, 115in order to avoid running out of virtual address space. 116The 117.Dv KVA_PAGES 118option may be used to increase the kernel virtual address space, 119and the 120.Va kern.maxvnodes 121.Xr sysctl 8 122may be used to decrease the number of vnodes allowed, 123an example of a resource that the kernel is likely to overallocate in 124large memory configurations. 125For optimal performance and stability it may be necessary to consult the 126.Xr tuning 7 127manual page, and make adjustments to the parameters documented there.