Why doesn't Windows show all 4gigabytes of memory I have installed?

Right click on My Computer, click on Properties, and in Windows Vista you'll get a window that includes, among other things, this information:

That's a snapshot taken from my laptop. The laptop with 4 gigabytes of RAM installed but showing only 3326 Megabytes of RAM available.

That window also includes another important clue: "32-bit Operating System". Ultimately therein lies the dilemma.

(A quick over-simplification and aside for some: a "bit" is simply a single digit that can contain either a 0 or a 1. Thus when we talk about a "32 bit" operating system or processor, we're talking about systems that operate natively on 32 bits at a time.)

If you look at all possible arrangements of a collection of 32 1's and 0's, you'll find that there are 4,294,967,296 possible combinations. 4 Giga-combinations.

Computer memory is arrange in bytes, so when you order 4 gigabytes of RAM, you're actually getting 4,294,967,296 bytes of memory. And yes, each byte of memory is assigned it's own unique number or "address" - that's how the processor tells the memory hardware which bytes of RAM it wants to operate on.

All's well and good, and you would expect that while a 32 bit operating system would be able to address at most 4 gigabytes of RAM, it seems like it should be able to address all 4 gigabytes of RAM.

Unfortunately, not so.

Enter the concept of "memory mapped" hardware.

The best, and often the largest example, will be your video card. It typically includes video memory of its own. That memory is "mapped into" or made visible within your PC's 4 gigabyte address space. Say I have a 512 megabyte video card, the memory layout might look something like this:

What you'll notice is that since the video card must place its 512Meg video memory somewhere into the 4 gigabyte address range that your computer can access, it "gets in the way of" 512Meg of your system RAM. That 512Meg of system RAM becomes inaccessible.

Windows works very hard to minimize the impact, and on any system that has less than 4Gig of RAM you'd never notice, since Windows will make sure to put the video and other memory mapped hardware in places that don't conflict with physical RAM. But as soon as you put in 4Gig of RAM that's the maximum a 32 bit system can address and as a result there's no place the memory mapped hardware can hide. It will have to obscure some of that RAM.

There is one solution, but you probably won't like it. At least not yet.

64bit Windows.

There's a very good chance you actually have a 64 bit processor in your newer machine. The free Securable utility from grc.com will tell you. If you do, you could switch to the 64 bit version of Windows Vista. By switching the processor to use a 64 bit architecture, the maximum amount of addressable memory changes from 4 gigabytes to 17,179,869,184 gigabytes - plenty of room to find a spare 512Meg for some video memory and still leave all the installed RAM visible.

Why won't you like it? Mostly because not all hardware is supported yet. For example you might not be able to find drivers for your video card or some of the other hardware or accessories installed on your computer. This is something that'll get better over time as more manufacturer's release 64 bit drivers, but as I write this it's not really there yet for the average computer user.