This is a limitation of a 32-bit operating system. In Windows, the Windows memory manager is limited to a 4 GB physical address space. Most of that address space is filled with RAM, but not all of it. Memory-mapped devices (such as your video card) will use some of that physical address space, as will the BIOS ROMs. After all the non-memory devices have had their say, there will be less than 4GB of address space available for RAM below the 4GB physical address boundary. MacOS X Tiger and Leopard are both 64-bit operating systems and will not experience this problem. Neither will 64-bit versions of Windows XP or Vista or Windows 7.
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