pae(4) [debian man page]

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

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 com-
     piled 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 docu-
     mented there.

BSD
								   April 8, 2003							       BSD

Check Out this Related Man Page

mem(7D) 							      Devices								   mem(7D)

NAME

       mem, kmem, allkmem - physical or virtual memory access

SYNOPSIS

       /dev/mem

       /dev/kmem

       /dev/allkmem

DESCRIPTION

       The file /dev/mem is a special file that provides access to the physical memory of the computer.

       The  file  /dev/kmem  is  a special file that provides access to the virtual address space of the operating system kernel, excluding memory
       that is associated with an I/O device.

       The file /dev/allkmem is a special file that provides access to the virtual address space of the operating system kernel, including  memory
       that is associated with an I/O device.  You can use any of these devices to examine and modify the system.

       Byte  addresses	in  /dev/mem are interpreted as physical memory addresses. Byte addresses in /dev/kmem and /dev/allkmem are interpreted as
       kernel virtual memory addresses.  A reference to a non-existent location returns an error. See ERRORS for more information.

       The file /dev/mem accesses physical memory; the size of the file is equal to the amount of physical memory in the computer.  This size  may
       be  larger  than 4GB on a system running the 32-bit operating environment. In this case, you can access memory beyond 4GB using a series of
       read(2) and write(2) calls, a pread64() or pwrite64() call, or a combination of llseek(2) and read(2) or write(2).

ERRORS

       EFAULT	       Occurs when trying to  write(2) a read-only location (allkmem),	read(2) a write-only location  (allkmem),  or  read(2)	or
		       write(2) a non-existent or unimplemented location (mem, kmem, allkmem).

       EIO	       Occurs  when trying to read(2) or write(2) a memory location that is associated with an I/O device using the /dev/kmem spe-
		       cial file.

       ENXIO	       Results from attempting to mmap(2) a non-existent physical (mem) or virtual (kmem, allkmem) memory address.

FILES

       /dev/mem        Provides access to the computer's physical memory.

       /dev/kmem       Provides access to the virtual address space of the operating system kernel, excluding memory that is  associated  with	an
		       I/O device.

       /dev/allkmem    Provides  access  to  the virtual address space of the operating system kernel, including memory that is associated with an
		       I/O device.

SEE ALSO

       llseek(2), mmap(2), read(2), write(2)

WARNINGS

       Using these devices to modify (that is, write to) the address space of a live running operating system or to modify  the  state	of	 a
       hardware  device  is  extremely	dangerous  and	may  result  in a system panic if kernel data structures are damaged or if device state is
       changed.

SunOS 5.10							    18 Feb 2002 							   mem(7D)

Linux and UNIX Man Pages

pae(4) [debian man page]

Check Out this Related Man Page