Windows operating systems come in two basic editions: 32-bit and 64-bit. Logically, 32-bit editions of Windows run on 32-bit processors, many of which, along with their chipset, implement 32-bit memory and I/O address buses. These non-PAE machines can address a maximum of 4,294,967,296 bytes (4 gigabytes, GB), per address space.

On non-PAE machines, the 4 GB memory address space is not devoted exclusively to main memory. Video, disk, network, and other devices also require memory address space. Because of this, only a reduced amount of the 4 GB address space is actually available to main memory.

In order to eliminate a host of problems associated with the device drivers that require memory address space, starting with Windows XP SP2 Microsoft has limited the amount of main memory accessible in the 4 GB address space to 3.25 GB. Therefore, even if a non-PAE machine has more than 3.25 GB of main memory installed, only 3.25 GB of it will be accessible to Windows. The 0.75 GB of address space is reserved for memory-mapped devices.

On machines with PAE hardware, all main memory can be access -- at least theoretically. For Windows to access the memory, it must also be running in PAE mode. All 32-bit Windows operating systems from Windows 2000 forward support PAE, including 32-bit Windows XP and Vista. To determine if Windows is running in PAE mode, open the System Properties dialog. To enable PAE you may have to add "/pae" to the boot.ini line for the active Windows installation. (This applies only to Windows editions before Vista. Windows Vista and Windows Server 2008 always install PAE-aware kernels.)

For machines that do not have PAE hardware, this access limit to main memory cannot be overcome. It is a hardware limitation.

On PAE machines, Windows versions from XP SP2 forward still dedicate the upper 0.75 GB of the first 4 GB of memory address space to memory-mapped devices. However, if Windows is running with PAE mode enabled, then it is capable of accessing whatever amount main memory is installed after the first 3.25 GB. This memory simply will be mapped to start above the first 4 GB of address space. In other words, assuming the machine has more than 3.25 GB of main memory, the first 3.25 GB of memory will be accessible in the lower 3.25 GB of memory address space, while the remaining main memory will be accessible at addresses starting at and above 4.0 GB.