Large memory support in Windows 2000/2003

PAE is the added ability of the IA32 processor to address more than 4 GB of physical memory. The following operating systems can use PAE to take advantage of physical memory beyond 4 GB:

• Microsoft Windows 2000 Advanced Server
• Microsoft Windows 2000 Datacenter Server
• Microsoft Windows Server 2003, Enterprise Edition
• Microsoft Windows Server 2003, Datacenter Edition

To enable PAE, use the /PAE switch in the Boot.ini file.

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. This behavior is caused by kernel virtual memory space considerations. Thus, 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 the physical memory.

Reference: http://support.microsoft.com/kb/283037