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I've added 4gb of RAM and Windows XP just doesn't care....

So, you threw caution (and lots of money) to the wind and upgraded your system to to 4gb of RAM.  Thinking of how cool it will look, you right click MY COMPUTER, select PROPERTIES... and there before your startled eyes appears... 3gb of RAM.

(I picked the nice, round looking number of 3gb.  The actual number may vary anywhere from 2.98 to 3.5 depending on your system manufacturer and motherboard.)

What's the deal?  Microsoft, a little thing called Service Pack 2, and PCI-Express.

According to Microsoft, Windows XP will support 4gb of RAM, but only with PAE (Physical Address Extensions) enabled.  If your system is pre-Service Pack 2, you might see 3.8gb or so.  

You can enable PAE by opening up BOOT.INI and adding the /PAE switch to the ARC path.  It will look something like this:

multi(0)disk(0)rdisk(0)partition(2) \WINNT="Windows XP" /PAE

Once PAE is enabled, it will show up on the MY COMPUTER --> PROPERTIES status page.  Sometimes, Windows will automatically enable PAE.  It's best to check the system properties first.  It will be located just under the processor and RAM information on the General tab.

The missing .2gb or so is being gobbled up by various resources that are allocating memory.  PCI Express is a big culprit.  Although I haven't personally tested it, supposedly a system running SLI can't see more than 2gb total.

To add additional "stability" to Windows XP, Service Pack 2 allocated additional resources now taking up even more memory, resulting in a final product of 3.0gb.

No one seems real keen on documenting this, at least not thoroughly.

Here is the link where Microsoft tells you that XP supports 4gb of RAM:
"Memory Support and Windows Operating Systems"

And here is the link where Microsoft then tells you "not quite....":
"The amount of RAM reported by the System Properties dialog box and the System Information tool is less than you expect after you install Windows XP Service Pack 2"  

And lastly (and linked in the first article) a rundown of PAE:
"Operating Systems and PAE Support"

How to fix this?  There isn't a lot you can do, other than upgrade your system to Windows 2003 server, which will access the full amount (Don't forget to enable PAE though).  If you have RAM that will work in that 64-bit motherboard you've been thinking about, that is another option as XP-64 will support up to 32gb of memory.  You can also wait on Windows Vista, which  lacks that restriction.

UPDATE: The Dell GX280 does not see more than 2.99gb regardless of what OS you're running.  So far I have not been able to pin down whether it is a Dell problem, or a problem with all systems based on the Grantsdale chipset.  The Workstation 470 does not have a problem seeing 4gb (3.93) under Server 2003.  Looking for the greatest return on your RAM?  The Dell GX620 sees 3.5gb under Windows XP.

So you now have 4 1gb RAM sticks that don't seem to work.  If you're thinking of popping one out and selling it to your buddy... consider this....  Most motherboards prefer pairs of DDR RAM, but will operate with an odd set of  RAM chips. (In other words, it would like 2 sticks or 4 sticks of RAM, but it will operate with 3 sticks installed).  When operating with an odd number, the system will be running in "Asymmetrical mode"... which is computer speak  for "Not so good".  You will take a noticeable performance hit with an asymmetrical 3gb machine running slower in all applications than a 2gb (dual channel) machine or a 4gb system (with its "missing gig") also running in dual channel mode.  The good news is most systems will also support mis-matched pairs, allowing you to use 2 x 1gb DIMMS, and 2 x 512mb DIMMS (4 sticks)  for a total of 3gb, without a loss in  performance.  (In my testing, I've found that this configuration also runs a little faster than the 4gb setup.)

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