Virtual Memory Manager
The 32-bit operating systems such as Windows 2000 and Windows Server 2003 provide access to 4-gigabyte (GB) of virtual address space. The lower 2 GB of virtual memory is private per process and available for application use. The upper 2 GB is reserved for operating system use. All operating system editions, starting with Microsoft Windows XP Professional and later, including Windows Server 2003, include a boot.ini switch that can provide applications with access to 3 GB of virtual memory, limiting the operating system to 1 GB. See your Windows documentation for more information on using the /3GB switch memory configuration.
Note: In Windows Server 2003, PAE is automatically enabled only if the server is using hot-add memory devices. In this case, you do not have to use the /PAE switch on a system that is configured to use hot-add memory devices. In all other cases, you must use the /PAE switch in the Boot.ini file to take advantage of memory over 4GB.
The 4-GB address space is mapped to the available physical memory by the Windows Virtual Memory Manager (VMM). The amount of physical memory supported has increased with the introduction of the Windows Server 2003 operating systems. The physical memory accessible by AWE therefore depends on which operating system you are using:
Windows Server 2003, Standard Edition supports physical memory up to 4 GB.
Windows Server 2003, Enterprise Edition supports physical memory up to 32 GB.
Windows Server 2003, Datacenter Edition supports physical memory up to 64 GB.
A Win32 application such as SQL Server perceives only virtual or logical addresses, not physical addresses. How much physical memory an application uses at a given time (the working set) is determined by available physical memory and the VMM. The application cannot control memory residency directly.
Windows 2000 Advanced Server and Windows 2000 Datacenter Server as well as Windows Server 2003, Enterprise Server and Windows Server 2003, Datacenter Server 32-bit editions have the capability to exceed the 4 GB limit with the use of Intel Physical Addressing Extensions (PAE). Use of the /PAE switch in the Windows boot.ini configuration file provides access to physical memory above the 4 GB limit. This is a requirement for AWE memory access above 4 GB. For more information on using this memory configuration for Windows 2000 and Windows Server 2003, see the Windows documentation.
Virtual address systems such as Windows 2000 or Windows Server 2003 allow the over-commitment of physical memory, so that the ratio of virtual to physical memory can exceed 1:1. As a result, larger programs can run on computers with a variety of physical memory configurations. However, using significantly more virtual memory than the combined average working sets of all the processes results in poor performance.