Windows XP 64-bit shows less RAM than the amount installed

Hello

This is the system in question:

- 64-bit CPU (Intel i5 750)
- 64-bit Motherboard (Asus P7P55M)
- 64-bit Windows XP
- 64-bit Ubuntu 9.10
- 4 GB RAM installed
- separate Video Card (ATI Radeon HD 5770 - 1 GB Video RAM)


The BIOS, Memtest86+, and Ubuntu, all show 4.0 GB of RAM in the system

Windows XP x64, however, shows 3.5 GB of RAM installed.

Question: Why is it that only Windows x64 show less RAM, while everyone else shows correct amount?

I also HAD Ubuntu 32-bit, and it showed 2.9 GB, so Ubuntu would show the 32-bit limitation.
Then I installed Ubuntu 64-bit, and I can see 3.9 GB (for some reason it doesn't show 4.0, but that's minor)

Windows XP x64 (YES I am sure it is the 64-bit version) shows 3.5 GB of RAM

Memory remapping in BIOS is turned ON, but I don't remember if I turned it on before or after I installed Windows. It was definitely ON before I installed Ubuntu 64-bit.

Now if I toggle memory remap ON/OFF, I see that everyone detects only 3.5 GB with memory remap OFF, but with memory remap ON only XP detects 3.5 GB.

Note: There is no on-board video on this motherboard, so the argument that memory is reserved for the video-card is invalid.
Any ideas?
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More about windows shows amount installed
  1. What service pack do you have? Don't know if you've check this one already.

    http://en.wikipedia.org/wiki/Physical_Address_Extension#Microsoft_Windows
  2. I have Service Pack 1.

    I don't know what you wanted me to see on that link.

    The only relevant information is that it says that Windows XP 64-bit (which I have) supports up to 128 GB of RAM.

    Maybe I'll try to upgrade to a newer service pack (if it exists)
  3. I gave you the wrong link, anyway there was a discussion on one forum that one person manage to see the RAM corectly by updating to XP SP2. just wondering if you did so already?

    http://www.microsoft.com/downloads/en/details.aspx?FamilyId=893FD6C0-6283-44C3-BB84-B2F0315B2AE6&displaylang=en
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