PAE (Physical Address Extension) switch is used to address more than 4 GB of physical memory, to enable PAE, you have to use the /PAE switch in the Boot.ini file.
PAE switch to provide more physical memory to a program which reduces the need to swap memory of the page file and results in increased performance. The program 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 programs that run.
PAE switch is supported only on 32-bit versions of the Windows operating system. 64-bit versions of Windows do not support PAE
64-bit programs use a 16-terabyte tuning model, from 16-terabyte 8 terabytes for User and 8 terabytes for Kernel, 64-bit versions of Windows do not support the use of the /3GB switch in the boot options
PAE is disabled by default. You must use the /PAE boot parameter to enable PAE.
However, Windows automatically enables PAE switch when the computer is configured for hot-add memory devices in memory ranges beyond the 4 GB region