PAE (Physical Address Extension)
PAE is the added ability of the IA32 processor to address more than 4GB of physical memory. The following OS's can use PAE to take advantage of physical memory beyond 4GB:
To enable PAE, use the /PAE switch in your server's boot.ini file.
N.B. In Windows Server 2003, PAE is enabled automatically if your server is using hot-add memory devices.In all other cases, you must add the /PAE switch to boot.ini
Typically, a process running under Windows 2000 or Windows 2003 can address up to 2GB of memory address space (3GB if the /3GB switch is used in boot.ini) with some of the memory being physical memory and some being virtual. The more programs (and therefore, more processes) you run, the more memory you commit up to the full 2GB of address space. When this occurs, the paging process increases dramatically and performance may be negatively affected.
Windows 2000 and 2003 memory managers use PAE to provide more physical memory to a program. This reduces the need to swap the memory of the page file and results in increased performance. The program itself is not awareof the memory size, the memory management and allocation of the PAE memory is handled by the memory manager independently of the programs that run.
When the /3GB switch is used is conjunction with the /PAE switch, the OS does not use any memory in excess of 16GB. This is caused by kernel virtual memory space considerations. If the system restarts with the /3GB switch in boot.ini, and the system has more than 16GB available (physical memory), the additional RAM is not used by the OS. Restarting without the /3GB switch enables use of all the physical memory.
AWE (Address Windowing Extensions)
AWE is a set of API's to the memory manager's functionsthat enables programs to address more memory than is available through standard 32-bit addressing. It enables programs to reserve physical memory as non-paged memory to the program's working set of memory. Enables memory intensive programs, such as RDBMS, to reserve large amounts of physical memory for data without having to be paged in and out of a paging for for usage. Data is swapped in and out of of the working set and reserved memory is in excess of the 4GB range. Additionally, the range of memory in excess of 4GB is exposed to the memory manager and the AWE functions by PAE. Without PAE, AWE cannot reserve memory in excess of 4GB.
Memory Limits for Windows ReleasesOperating Systems and PAE support
please take a look at my article on SQL Server Configuration Settings will complete links to MS KB...