Windows automatically enables PAE if DEP is enabled on a computer that supports hardware-enabled DEP, or if the computer is configured for hot-add memory devices in memory ranges beyond 4 GB. If the computer does not support hardware-enabled DEP or is not configured for hot-add memory devices in memory ranges beyond 4 GB, PAE must be explicitly enabled.

To explicitly enable PAE, use the following BCDEdit /set command to set the pae boot entry option:

bcdedit /set [{ID}] pae ForceEnable

IF DEP is enabled, PAE cannot be disabled. Use the following BCDEdit /set commands to disable both DEP and PAE:

bcdedit /set [{ID}] nx AlwaysOff

bcdedit /set [{ID}] pae ForceDisable

Windows Server 2003 and Windows XP/2000:  To enable PAE, use the /PAE switch in the Boot.ini file. To disable PAE, use the /NOPAE switch. To disable DEP, use the /EXECUTE switch.

Comparing PAE and other Large Memory Support

PAE, 4-gigabyte tuning (4GT), and Address Windowing Extensions (AWE) serve different purposes and can be used independently of each other:

• PAE allows the operating system to access and use more than 4 GB of physical memory.
• 4GT increases the portion of the virtual address space that is available to a process from 2 GB to up to 3 GB.
• AWE is a set of APIs that allows a process to allocate nonpaged physical memory and then dynamically map portions of this memory into the virtual address space of the process.

When neither 4GT nor AWE are being used, the amount of physical memory that a single 32-bit process can use is limited by the size of its address space (2 GB). In this case, a PAE-enabled system can still make use of more than 4 GB of RAM to run multiple processes at the same time or to cache file data in memory.

4GT can be used with or without PAE. However, some versions of Windows limit the maximum amount of physical memory that can be supported when 4GT is used. On such systems, booting with 4GT enabled causes the operating system to ignore any memory in excess of the limit.

AWE does not require PAE or 4GT but is often used together with PAE to allocate more than 4 GB of physical memory from a single 32-bit process.