Process Address Space
All 32-bit applications have a 4-gigabyte (GB) process address space (32-bit addresses can map a maximum of 4 GB of memory). Microsoft Windows operating systems provide applications with access to 2 GB of process address space, specifically known as user mode virtual address space. All threads owned by an application share the same user mode virtual address space. The remaining 2 GB are reserved for the operating system (also known as kernel mode address space). All operating system editions starting with Windows 2000 Server, including Windows Server 2003, have a boot.ini switch that can provide applications with access to 3 GB of process address space, limiting the kernel mode address space to 1 GB.
This feature will be removed in the next version of Microsoft SQL Server. Do not use this feature in new development work, and modify applications that currently use this feature as soon as possible.
Address Windowing Extensions (AWE) extend the capabilities of 32-bit applications by allowing access to as much physical memory as the operating system supports. AWE accomplishes this by mapping a subset of up to 64 GB into the user address space. Mapping between the application buffer pool and AWE-mapped memory is handled through manipulation of the Windows virtual memory tables.
To enable support for 3 GB of user mode process space, you must add the /3gb parameter to the boot.ini file and reboot the computer, allowing the /3gb parameter to take effect. Setting this parameter allows user application threads to address 3 GB of process address space, and reserves 1 GB of process address space for the operating system.
If there is more than 16 GB of physical memory available on a computer, the operating system needs 2 GB of process address space for system purposes and therefore can support only a 2 GB user mode address space. In order for AWE to use the memory range above 16 GB, be sure that the /3gb parameter is not in the boot.ini file. If it is, the operating system cannot address any memory above 16 GB.