emory Managed by Windows
Non-PAE Systems
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. (For more about PAE, see Physical Address Extensions.)
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.