More than 2GB of RAM under Windows?


macrumors member
Original poster
Sep 15, 2006
Is anyone seeing more than 2GB of RAM under Windows?

Does anyone know what is causing this restriction?
Windows (even a normal 32bit XP system) can see up to 4GB of RAM. I have about 8 PCs here all running happily with 3GB installed so I don't understand why the Mac Pro would only see 2GB.

This issue is the only thing holding me back from getting a Mac Pro right now as a Windows system with only 2GB of RAM is next to useless for me.

Any ideas???


macrumors 6502
Jan 7, 2002
Jumping on this thread...

I installed 2x1gb FB-dimms on my Mac Pro in the lower riser last night. Added to the stock 1gb from Apple on the top riser, this gives me 3gb under OS X, but under Windows it says only 1.98gb.

Thanks for any insight...


macrumors member
Aug 13, 2006
i cant remember exactly what i read but it's got something to do with
"2gb switch" or was it "3gb switch"???

google might turn something up


macrumors 68000
Jun 22, 2004
Kushiel's Scion
That's addressing (using) more than 2GB by a single process, not the inability to see it. I think those are two different issues.

I read somewhere (Apple Support Forums?) that the current guess is that its related to the layout of RAM on the two riser cards. WinXP only sees matched (equal) pairs across riser slots. One person changed the layout of his RAM and got WinXP to see all of his installed RAM.

Should be easy enough to test by moving RAM around.


macrumors regular
Aug 11, 2006
Nantes, France
Same probleme here. 3 Gigs installed. Only 2 are recognized by XP ... and obviously, 2 pairs on slot A and one pair on slot B ...



macrumors member
Jul 16, 2006
My uncle who has this dell that has 3gigs of ram and he says that windows only was taking about 2gig or somthing like that. Don't completly know how much he said but he is having a ram problem with windows also. I belive it is the dell that apple compared the mac pro to hehe. (its his work computer)


macrumors member
Aug 13, 2006
what are the allocations on your riser boards?
stock comes with 2x512 sticks.
so say you picked up 4x1 gig sticks...
do you have the 4 sticks on board 1 and the other 2x512 on board 2?

ive seen on a few ram suppliers a note about buying them in 4s.
im not sure where i seen this but i found it odd as they want them installed in pairs.


macrumors regular
Aug 29, 2006
I like the theory about matched pairs on risers. I have 4gb 8x512 and windows is giving me no love despite having altered my boot.ini file to activate the '3gb' swtich everything I have read on the web suggests that this is a knwon bug and those wishing to use more than 2gb need ot go with win64 or wait for vista

This issue is the only thing holding me back from getting a Mac Pro right now as a Windows system with only 2GB of RAM is next to useless for me.
As someone using Extremely high demmand applications on WinXp with 2gb of ram for years now I find this to be a very stupid comment. The actual performce differnce is not that great. Especially since you are in a 32bit envrionment, most apps won't take advantage of the larger ram anyway !