XP - I have found where the missing 2GB has gone for 4GB machines.

Question

Hi Guys,
I have been looking into why XP can only see 2GB of my 4GB ram and it looks like the MacPro is reserving roughly 2GB of addressable memory for the PCI bus!
If you view resources by connection in Device Manager there are two areas of memory reserved for the PCI bus:
0x000A0000 to 000BFFFF ( less than 1 MB )
0x80000000 to 0xFE000000 ( 2016 MB )
I have no idea why such a large chunk of addressable memory is being reserved for the PCI bus but it is and thats why you cannot see you full 4GB.
If I actually count up how much of the address space is actually used by devices I get :
0x80000000 to 0x90C04BFF ( 268 MB )
there is also an area reserved for motherboard resources but not used
0xD0000000 to 0xDFFFFFFF ( 256 MB )
So we get an area missing here
0x90C04BFF to 0xD0000000 ( 1012 MB )
And here
0xDFFFFFFF to 0xFE000000 ( 480 MB )
This is a total of 1492 MB of wasted address space!
Apple please can we have our ram back?
Cheers
Andy

Answer 1

i think its highly unlikely that its actually got
anything to do with the pci bus, its just doze
confusing itself. xp can only utilise 3 gigs of ram,
and if you have it managing virtual memory then it
may be counting 1gig of drivespace over the 1gig of
physical ram. you should be okay to turn off virtual
memory in xp with that much physical ram (cant
remember how you do it, its in the performance
settings somewhere), provided you dont use multiple
very ram heavy apps at once. that might enable xp to
see 3 gigs of your physical ram and may speed things
up a bit.
On my MacBook Pro C2D with 3GB I see all 3GB booted to Windows XP, on my Mac Pro with 6GB I see only 2GB. So I don't think it is "Windoze".
Steve