From: Ken Blake, MVP on
On Fri, 19 Feb 2010 12:49:01 -0800, Damon
<Damon(a)> wrote:

> Hi,
> I just added two 1-GB sticks of SDRAM into my Dell computer to bring 2.5 GB
> up to 4 GB, but it's only showing up under System as 3.25. Which is
> interesting.

All 32-bit client versions of Windows (not just Vista/XP/7) have a 4GB
address space (64-bit versions can use much more). That's the
theoretical upper limit beyond which you can not go.

But you can't use the entire 4GB of address space. Even though you
have a 4GB address space, you can only use *around* 3.1GB of RAM.
That's because some of that space is used by hardware and is not
available to the operating system and applications. The amount you can
use varies, depending on what hardware you have installed, but can
range from as little as 2GB to as much as 3.5GB. It's usually around

Note that the hardware is using the address *space*, not the actual
RAM itself. If you have a greater amount of RAM, the rest of the RAM
goes unused because there is no address space to map it to.

> My tower has four RAM slots. When I bought the computer it had four 256 MB
> sticks, two of which I replaced with two 1 GB sticks a while back (2.5) and
> today I replaced the remaining two 256 MB sticks with two 1 GB sticks to
> bring it up to 4 GB.
> When I turned my computer back on, I was notified that my computer's memory
> had changed, giving me the choice to continue or enter setup. I chose
> "continue".
> Everything seems to be running smoothly but it only shows up as 3.25 GB in
> System. Is this because I accidentally skipped setup when prompted? Did I
> perhaps install wrong? I am aware that RAM but be installed in a certain way
> and as far as I can tell I made no mistakes. 3.25 is a strange number.
> Any advice?
> My specs:
> Dell Dimension E510, Pentium 4 3.0 ghz, Windows XP Professional, and I
> bought the new RAM off of the Dell website as per my system specs, don't know
> the precise name. Any help would be appreciated.

Ken Blake, Microsoft MVP - Windows Desktop Experience
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