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.
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.