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Windows Operating Systems and Physical RAM in excess of 3GB


With many Windows users installing now more than 3GB of physical RAM, this is increasingly becoming an issue members have to deal with and come to terms with. We tested the Windows operating systems indicated below.


Test System Specifications

Intel Core 2 Duo E6600 2.40 GHz 4 MB L2 Cache
ASUS P5B Deluxe WiFi/AP
2 X 1024 MB OCZ GOLD PC2-6400 DDR2 800 MHz (Non-ECC)
2 X 1024 MB Transcend PC2-6400 DDR2 800 MHz (Non-ECC)
NVIDIA GeForce 7900 GS 512 MB DDR3


Windows XP Professional X86 (32-Bit)

Even with SP2 Windows XP just does not have the capacity to use more than 3GB or physical RAM. There are boot.ini and application switches which permit certain applications to do so. In terms of the operating system itself though, this limitation prevails no matter what boot.ini switch one uses.

With Memory Remapping enabled in the BIOS (allows full identification of more than 3GB by the BIOS), the operating system is unable to use beyond 2GB of RAM




Disabling Memory Remapping (has the effect of limiting the BIOS identification of RAM to 3GB) lets the OS use more than 2GB of RAM, but limited to a maximum of 3GB




Windows Server 2003 Enterprise SP1 X86

Windows Server 2003 Enterprise SP1 X86 with RAM remapping set to Enabled in the BIOS it has no issues at all reading the full 4GB.

With remapping Disabled, it reverts back to 3GB simply because the BIOS only reflects 3GB with that setting. The 512 MB RAM GPU causes no issues at all and is read with its full RAM available.




Windows XP Professional X64 Edition


Windows XP Professional X64 Edition also has no issues. This screenshot is before any driver installation but the same applied after all drivers were installed. I mention the drivers because I've been reading on other forums that drivers can use address space and thereby reduce the amount of RAM identified, but this seems not to have impacted on Windows XP Professional X64.




Windows Vista Ultimate Edition X86 (32-Bit)

Windows Vista Ultimate RTM X86 with memory remapping enabled, in other words with exactly the same hardware and settings as were used for Windows Server 2003 Enterprise X86 and Windows XP Professional X64 does NOT have the ability to read more than 2GB of RAM




When memory remapping is disabled though, 3GB of RAM show up




The fact that Windows Server 2003 Enterprise X86 and XP Pro X64 with the same settings (Memory Remapping Enabled), were able to show and utilize all 4GB of RAM, indicates to me that this is a Software/Operating System limitation in Window Vista Ultimate X86.


Windows Vista Ultimate Edition X64 (64-Bit)

Here's Windows Vista Ultimate X64. As with Windows Server 2003 Enterprise X86 and Windows XP Professional X64 Edition, it has no problems running 4GB of physical RAM:




Other Windows Vista x64 Editions

Large memory support for other editions of Windows Vista X64 was not tested. HERE is information provided by Microsoft about them.




Hardware and BIOS Limitations

Despite claims by some motherboard manufacturers that the chipset used supports 4GB or more of physical memory, it has been found that is not always the case. All memory controller chipsets don't have the ability to map more than 3GB. Motherboards with a BIOS which does not provide an option to "Remap memory", generally do not have that ability.

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See Have 4 GB on my Win2003 Enterprise server - task manager shows only 3 GB!

It would appear in that instance that using the /PAE switch together with both hardware and an operating system which supports the use of more than 3GB physical RAM worked without having to perform a reinstall of Windows Server 2003 Enterprise.

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Other RAM related discussions of interest are:

Adding /3gb switch to Vista Home Premium

PAE enabled but Photoshop still can't see third gig of ram