Why Windows XP see 3 gig not 4 gig of RAM
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Windows XP Professional 32-bit is limited to 4GB of address space. If you have 4GB of RAM installed, it will not show 4GB within Windows. What you'll probably get is the size of your pagefile subtracted from the 4GB.
Some of this address space is reserved for hardware and the OS... so you'll never have access to the full 4GB anyway. If you want to access 4GB+ of RAM... your best solution is to pick up XP Pro 64-bit... but that will only work if your P4 CPU has EMT64.
The 32-bit versions of the Windows 2000 Advanced Server and Windows NT Server 4.0, Enterprise Edition, operating systems were the first versions of Windows to provide applications with a 3-GB flat virtual address space, with the kernel and executive components using only 1 GB