To further explain how borked-up things can get with address space, I should explain the reason why I wound up finding this thread in the first place, and what the disappointing outcome has been....
I have a Toshiba laptop, Satellite P105-S9337, that I have been prepping to sell to a friend. It has a discrete graphics module, GEForce Go 7900 GS, on a daughterboard with 256MB of dedicated VRAM. It originally shipped with Vista x86, and I later acquired Toshiba's official Windows 7 upgrade and installed it. Even though the media in that upgrade claimed to include both x86 and x64 installations, and the Core 2 Duo in this laptop supports x64 operating systems, the process was deliberately constrained to only install the same "bittedness" of Windows 7 as the previous instance of Vista. The Windows 7 on the laptop thus wound up being the 32-bit version.
Further, I had aggressively upgraded every facet of this laptop (including some not sanctioned by Toshiba), and that included increasing the RAM to its limit of 4GB. The 32-bit version of Windows 7 would report in the System window that 4GB of RAM was present but that only 3GB was "usable". (Note that the reduction here is actually four times larger than the size of the dedicated VRAM.) This of course is exactly the same symptom reported by the originator of this thread. There's that address space and mapping issue again, right? Installing a 64-bit operating system will remove that ceiling by allowing those things to be remapped 12GB higher in the address space, right?
Not so fast....
When I installed a pristine instance of Windows 7, it was still 32-bit and I was reminded of the above limitation, which I really wanted to remove before I sold it. I tried forcing the Toshiba media to install a 64-bit version, assuming that would eliminate this problem, but it refused to cooperate and I eventually wound up using my own personal 64-bit media to install Windows 7 but still using the product key from the Toshiba package (which worked and activated).
Now here's the kicker: even with Windows 7 x64 installed, the system STILL reports only 3GB as usable. In this instance there is a hardware and/or firmware limit being imposed that still exists even when the theoretical address space is expanded beyond the 4GB ceiling by using a 64-bit operating system. I have yet to get Toshiba to explain exactly what has caused the ceiling to remain, but it's clear that it was by design and was what motivated Toshiba to ship only a 32-bit operating system with it and label it as "incompatible" with 64-bit versions. The BIOS and/or memory controller have imposed a 4GB ceiling on addressability that even a 64-bit operating system can't sidestep.
Edited by VulcanTourist - 9/5/12 at 3:38pm