I wrote about this somewhere on this forum a couple of months ago ... Please excuse me, as large parts of this have been lifted from the original posting ![tongue.gif](http://asustech.15.<a href="http://www.forumer.com">forumer</a>.com/html/emoticons/tongue.gif)
The issue now affecting you, was actually decided upon when finalising the x86 (32bit) architecture. x86 architecture has a memory address space, of which a maximum of 4GB can be assigned. However, of this 4GB memory address space, the system also has to allocate memory addresses for your other hardware. This starts at around 16mb, but is generally any value from 256MB right upto a maximum of 1GB. If a quote I found while trawling google for information is correct, certain Intel chipsets actually reserve a maximum of 1.5GB for memory addresses.
By far the biggest chunk of address space, is taken up by GPU's, which, by current G80 GTX specs, a maximum of 768MB can be allocated. Then, every piece of hardware in your system is allocated SOME memory space. According to this (Virtual address space - Wikipedia) article, the memory space (or Virtual Address Space) can also be taken up by your pagefile usage and files loaded into memory by the OS.
Because XP Pro is based around x86 architecture (as is Vista 32bit for that matter), the 4GB limit can only be marginally worked around by fooling the OS with a PAE (Physical Address Extension) switch. However, this is only effective as it swaps and changes address space not being used to attempt to fool the OS into using more physical memory than it can actually see. What it does not do, is allow the system to utilize more than (in your case) 2.5GB.
That is the reason ... here are the differences (lifted directly from "Overview of Windows XP Professional x64 Edition" - http://www.microsoft.com/windowsxp/64bit/overview.mspx)
The issue now affecting you, was actually decided upon when finalising the x86 (32bit) architecture. x86 architecture has a memory address space, of which a maximum of 4GB can be assigned. However, of this 4GB memory address space, the system also has to allocate memory addresses for your other hardware. This starts at around 16mb, but is generally any value from 256MB right upto a maximum of 1GB. If a quote I found while trawling google for information is correct, certain Intel chipsets actually reserve a maximum of 1.5GB for memory addresses.
By far the biggest chunk of address space, is taken up by GPU's, which, by current G80 GTX specs, a maximum of 768MB can be allocated. Then, every piece of hardware in your system is allocated SOME memory space. According to this (Virtual address space - Wikipedia) article, the memory space (or Virtual Address Space) can also be taken up by your pagefile usage and files loaded into memory by the OS.
Because XP Pro is based around x86 architecture (as is Vista 32bit for that matter), the 4GB limit can only be marginally worked around by fooling the OS with a PAE (Physical Address Extension) switch. However, this is only effective as it swaps and changes address space not being used to attempt to fool the OS into using more physical memory than it can actually see. What it does not do, is allow the system to utilize more than (in your case) 2.5GB.
That is the reason ... here are the differences (lifted directly from "Overview of Windows XP Professional x64 Edition" - http://www.microsoft.com/windowsxp/64bit/overview.mspx)
| QUOTE |
Currently, 32-bit editions of Windows are capable of supporting up to 4 gigabytes (GB) of system memory, with up to 2 GB of dedicated memory per process. Windows Professional XP Edition x64 currently supports up to 128 GB of RAM, with the potential to support up to 16 terabytes of virtual memory as hardware capabilities and memory sizes improve. |
As a comparison, the versions of 2003 server have differing limits for RAM usage, but they all follow the same trend for x64 applications - Installed RAM over and above 4GB is not limited by VAS/memory space, and does not require PAE switches.
Information from Technet "Windows Server 2003 System Requirements" (http://technet.microsoft.com/en-gb/windows...r/bb430827.aspx);
Microsoft Windows Server 2003 R2 Standard Edition
Memory - 128 MB of RAM required; 256 MB or more recommended; 4 GB maximum
-------
Microsoft Windows Server 2003 R2 Enterprise Edition
Memory - 128 MB of RAM minimum required; 256 MB or more recommended; 64 GB maximum for x86-based computers; 2 TB maximum for x64 and ia64 computers
-------
Microsoft Windows Server 2003 R2 Datacenter Edition
Memory - 512 MB of RAM required; 1 GB of RAM recommended; 128 GB maximum for x86-based computers; 2 TB maximum for x64 and ia64 computers