Here’s an interesting tidbit of information that I wasn’t aware of. Apparently your system memory isn’t only limited by your 32-bit operating system. I installed 4GB of RAM into a system the other day and installed Windows 7 64-bit, and found that the System Properties page showed 4GB installed, but only 3GB usable. What gives?
As it turns out, the amount of usable memory can be limited by the chipset, as well as the BIOS. In my case, the Intel 945 chipset only supports a 4GB address space. Since memory on other hardware devices (i.e. video cards) needs to be mapped somewhere in that address space, some of it is reserved, and cannot be used by the operating system.
Now, if your chipset supports more than 4GB of address space, the BIOS still needs to support memory remapping. The memory remapping feature allows for the other hardware devices to use memory addresses above the 4GB limit, thus allowing the operating system to use all 4GB of system memory. I’m assuming that if the chipset supports more than 4GB of address space, then the BIOS should support memory remapping, but you know what happens when you assume.
What’s interesting to note, and makes perfect sense once you think about it, is that if your amount of system memory is equal to the amount of address space supported by your chipset, then your operating system will never be able to use all of the system memory. Again, this is because of the reserved address space for other hardware in the system.
Microsoft’s knowledge base article #929605 has a good explanation of the requirements for making 4GB or more available to the operating system.
Good write up, young sir. It’s surprising that Intel didn’t have the foresight when designing some of their fairly recent chipsets to include proper 64-bit memory addressing. We ran into it with Mandie’s notebook (wouldn’t even boot properly with 4GB of RAM installed) and we bought that thing in 2008 when they should’ve known better.