4GB RAM reported as 2GB in XP SP2 (32bit)

[qsrk]

So it's AMDs fault. What do you expect with Mickey Mouse CPUs?
Get the real thing. Get an Intel.

Mordy in "Pentium D 830 Dual-Core Heaven" Mode.

Actually, I think Microsoft introduced DEP, and it is implemented in software as well. You should find it if you right-click My Computer, then go to Properties, Advanced tab, Performance Settings, Data Execution Prevention tab.

Also, Intel now has chips with hardware-enforce DEP, using something called "Execute Disable Bit".

[1117]

Well, I decided to move to the 64-bit scene and ordered XP X64, and with it, two more 1GB sticks, for a total of 4GB. We'll see what happens when they get here...

Anyway, I'd like to reply to whoever said that AMD CPU's are Mickey Mouse... well, the fact is that statement is completely FALSE. AMD CPU's weren't the greatest thing in the world when they came out, THAT much I accept, but their current (and I emphasize, CURRENT) line of processors is up to par and even outperforms Intels in MANY respects. Search Anandtech, PCMag, or any other respectable PC magazine and all will say the same: AMD is the way to go. I couldn't be happier with my AMD 64 X2 3800+ CPU right now, the thing flies and is an absolute wonder. Like I said, I'd agree that AMD CPU's were Mickey Mouse when they first came out, but AMD has made great strides since then. Intel is 2nd place right now, that's the reality of things.

[qsrk]

Quick Summary: This a Memory Hole problem, but it's made worse with an AMD dual core CPU and an AGP video card with a lot of memory.

Here is my understanding. I may be off, so please correct me if I am wrong:

AMD CPU: One of the selling features with many of their new CPUs is hardware-enforced Data Execution Prevention (DEP), which prevents some malicious code from executing from system memory. See here.

Windows XP SP2: When you have hardware-enforced DEP, Windows MUST run in PAE mode (Physical Address Extension), so it is automatically loaded, even without the /PAE switch. PAE mode is meant to take advantage of large amounts of RAM, but to improve driver compatibility, it now has a lower limit on RAM address space. So if you have a new AMD CPU, you may have access to less memory, possibly 3.5GB (EDIT: possibly 3.3GB), not 4GB. See here, and here, and let me know if that makes sense.

Memory Hole: The BIOS on your motherboard automatically reserves address space for 32-bit devices and drivers, further reducing the 3.5GB available. That reserved space is called a memory hole. See here.

Video RAM: As Kallex pointed out in an AMD forum, Video RAM seems to be the biggest culprit. It may increase the memory hole by 2 x Video RAM with a dual-core processor (I think it may only be 1 x Video RAM with a single processor). If you have 256 MB video RAM, system address space will be reduced by 512MB. So 3.5GB - 0.5GB = 3.0GB available.

AGP Aperture Size: This is the amount of system RAM that you allow the video card to use. It does not need to be the same as your video RAM size. Whatever amount you select here (between 32MB and 256MB) will increase the size of the Memory Hole by the same amount. With a 256MB Aperture, increase the memory hole by 256MB to 768MB. So 3.5GB - .75GB = 2.75GB available.

Dual-Channel Memory: As suggested by Kallex (again!), the memory that you lose above will be doubled if you use Dual-Channel Memory. So with Dual-Channel memory, you lose 2 x .75GB = 1.5GB! Big Memory Hole. Now you have 3.5GB - 1.5GB = 2.00GB RAM! Is all this doubling of address space really needed?

The information above is simplified, since other devices eat up address space too.

ASRock BIOS: It is missing the option to disable / remap the Memory Hole. Until that is resolved, you will be stuck at 2.00GB (2.50GB if you reduce the AGP Aperture to 32MB). Now the big news. I have had the opportunity to test a beta BIOS that allows me to disable the Memory Hole, and I now see 2.48GB with a 256MB AGP Aperture size, and 2.92GB with a 32MB Aperture . Hopefully you guys will see one soon. Please don't get upset, but ASRock helped me out with the beta, so I think it is fair to let them release it to the public when they are ready (please don't ask me for a copy ). BTW, thanks ASRock! I hope that ASRock tweaks the BIOS more so that entire Memory Hole issue disappears, so we can see the entire 3.5GB that Windows allows. Apparently some motherboard BIOS' have a feature to set "memory hole porting to software", allowing Windows to see all 4GB (3.5GB in our case). I think Tyan K8WE is one, and maybe DFI is another.

I doubt that the remaining 0.5GB will ever return, since Microsoft is now moving on to 64-bit Windows.

Phew!

I took a second look at some memory numbers, and with Memory Hole disabled, Windows is reporting EXACTLY the same amount of RAM as the BIOS. Since the BIOS does not know what OS will be loaded, I don't think Windows PAE mode is a factor here, yet. The BIOS would have to report more than 3.3GB for PAE mode to start showing an effect.

The maximum that I can get the BIOS to show is 2992MB, and Windows reports 2.92GB (2992 / 1024 = 2.92). So that means that the BIOS is reserving 1104MB (1.08GB) of address space on its own. I tried disabling onboard LAN and Audio, thinking that the BIOS is reserving space for them, but no change. Surely the BIOS does not need to reserve that much space!

This means that the theoretical math in my first explanation needs to be revised. I noticed that 3 x 1GB installed is also reported as 2992MB, with the only difference that the motherboard automatically switches to Single-channel memory mode. So right now, the only benefit of an extra 1GB DIMM is to be able to run in Dual-channel mode!

[Churchill]

Further to my previous post - 2x512MB and 2x256MB reporting as 1.25 Gig...

It just so happens that one of my 256MB SIMMs wasn't inserted fully...

Ooops, just glad my motherboard didn't go pop.

Sorry about the red herring.

[sparklelard]

Question - running CPUID shows the RAM as 2GB (dual channel).

If you dual 2gb, doesn't that make 4gb?

[cephia]

I really hate to bump this ... but this is the only place i've found so far where people were having the same problem... anyone find any other workarounds to the 2g out of 4g windows problem.. i've disabled/enabled the memory hole, and just about everything suggested sofar, with no luck

3g installed and my system recognises 2.75... 2 or 4g installed and the system recognises 2g