View Full Version : DEP and physical address extension

11-28-05, 01:11 PM
Having finally got around to installing XP SP2, I notice these are new elements of the OS. While I've done a bit of research on them, and understand what DEP does, what is the purpose of PAE? Should/could it be disabled? (A64 3500+ Newcastle on a Asus A8N-E). :confused:

11-28-05, 02:03 PM

Physical Address Extension
The Physical Address Extension (PAE) enables applications to address more than 4 GB of physical memory. It is supported by Intel processors. The following systems can use PAE to take advantage of physical memory beyond 4 GB:

Windows Server 2003 Enterprise Edition
Windows Advanced Server, Limited Edition
Windows 2000 Datacenter Server
Windows 2000 Advanced Server
To enable PAE, you must use the /PAE switch in the Boot.ini file.

With PAE enabled, the operating system moves from two-level linear address translation to three-level address translation. The extra layer of translation provides access to physical memory beyond 4 GB. Instead of a linear address being split into three separate fields for indexing into memory tables, it is split into four separate fields; a 2-bit field, two 9-bit fields, and a 12-bit field that corresponds to the page size implemented by Intel Architecture (4 KB).

Typically, a process can access up to 2 GB of memory address space (assuming the /3GB switch was not used), with some of the memory being physical memory and some being virtual memory. The memory manager uses PAE to provide more physical memory to an application, which reduces the need to swap memory to the page file and increases performance. The application itself is not aware of the actual memory size. All of the memory management and allocation of the PAE memory is handled by the memory manager independently of the application.

Applications that are 4GT-aware are likely to remain in physical memory rather than be paged out, which increases their performance. The exception is when the /3GB switch is used in conjunction with the /PAE switch. In this case, the operating system does not use any memory in excess of 16 GB. Therefore, if the system restarts with the /3GB entry in the Boot.ini file, and the system has more than 16 GB of physical memory, the additional physical random access memory (RAM) is not used by the operating system. Restarting the computer without the /3GB switch enables the use of all of the physical memory.

Address Windowing Extensions (AWE) enables applications to address more than 4 GB. AWE enables an application to reserve physical memory as nonpaged memory, then dynamically map portions of the nonpaged memory to its working set. This enables memory-intensive programs to reserve large amounts of physical memory for data without swapping to disk. Instead, the data is swapped between the working set and reserved memory above the 4 GB range. The memory above 4 GB is exposed to the memory manager and the AWE functions by PAE. Without PAE, AWE is unable to reserve memory in excess of 4 GB.

11-28-05, 02:05 PM
to answer your question about disabling PAE - just remove the /PAE switch from your boot.ini

(I would'nt mess with that file though if I was you).

11-28-05, 02:16 PM
If DEP is enabled, then PAE is enabled.

11-28-05, 02:51 PM
to answer your question about disabling PAE - just remove the /PAE switch from your boot.ini

(I would'nt mess with that file though if I was you).

Sounds like good advice! Thanks to all.