Missing 512MB RAM... Conundrum

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I upgraded my computer's memory today and for some reason, I though I would try to add some together. I bought 4GB (2x2GB DDR2) and that adds up to 4GB nicely. I then wanted to see if my 1GB will work along side them. When I booted into Windows, I had only 4.5GB where it should have been 5GB... Weird... So, I threw caution to the wind and installed the other 1GB I had (matching pair) and I have a successful complete 6GB in dual channel.

Before my upgrade I had 2GB in single channel mode as dual channel wasn't going too well. Both modules were 100% identical and both 667MHz and 1GB each, so where did the 512MB RAM go?

Anyone else find a similar problem? I am using 64bit version of Windows, but the BIOS would also show 2.5GB. My own theory is that in single channel 512MB was reserved.

Any thoughts, please feel free. I am not looking for fixes, so please, no "help in trying to get 5GB" I was experimenting with the configuration.

It might be down to which slots you used for which memory - as I understand it the motherboard can look at a particular slot, pick the settings from that and apply it to all the slots, which may end up confusing it. If you have four slots in two dual channels, AB and CD, did you try variations like A=2 B=2 C=1 D=0, A=2 B=2 C=0 D=1, A=1 B=0 C=2 D=2 and so on?

Are you getting the memory totals from BIOS or the Operating System? Some older operating systems can't see beyond 4 gig...tho that's not as apparent here.

If your motherboard doesn't support that much memory, then you can get funky behavior like that (though you'd also be lucky for it to boot at that point)

Often especially in cheaper motherboards they tend to be built on the cheap with the addressing, so the address lines may be shared between slots. For example, it may be important what order the sticks are in the slots (i.e. biggest memory stick needs to go in a specific slot, and down from there)

Another theory is that there is an address space issue. This arises sometimes when running 32-bit Vista with 4 GB of memory, in almost exactly the same fashion as your problem - with 4 GB installed, Vista can only use 3.3 or 3.5 GB worth - because the 32-bit address space only gives 4 GB worth of addresses, and typically that 512MB video card must be shoved in that address space somewhere as well. However, this issue is very specific to 32-bit vista. 64-bit vista should have no difficulties on motherboards that support that amount of RAM

My board supports up to 8GB RAM. The totals were from both Windows Vista, Windows XP, and BIOS. The memory was set up as dual channel for the 2x2GB and single channel for the single 1GB. This result caused a stability issue to withdraw from the dual channel and stick with single channel.

On a 64bit processor, I would be wasting my time with a 32bit operating system. Anyone want to stick a beetle engine in a Porsche?

All the operating systems I install are 64bit to breake the 4GB limit that truncates to 3.5GB or was it 3GB? I don't know, will never encounter it on my computers.

the actual limit with 32-bit vista varies depending on the system. if you've got one of those new fangled video cards with 1 GB of memory, it has to map that all 1 GB from the video card into the 4 GB of address space for DMA to work, meaning you're left even even less (3 GB) of memory