4GB of memory installed but only 3GB is showing in Windows.
Does this sound familiar?
Many people have noticed that when they install large amounts of memory in their systems, that Windows will show much lower then the actual physical amount. This is basically a limit of the 32 bit OS. Example: you install 4GB of memory. The system BIOS will show 4 GB of memory but the Windows OS only shows 3.25GB of memory.
Windows XP 32 bit version will accept up to 4GB of memory. However Windows will reserve the address space 3.5GB to 4GB to hardware devices. If these devices need more memory, Windows will allocate more of it to accommodate it. It is rare but not uncommon to see a system with 4GB of memory installed and only 2.75GB that is usable. Another note PCI-E 16x usually will gobble up 256MB of the address space. So if you have an SLI configuration, the 2 PCI-E 16x cards will chew up 512MB on top of the other devices that need the address space. It is possible to tweak the BIOS to reduce the amount of memory these devices use but often the result is negligible.
This is a note taken from an Intel MB manual:
ftp://download.intel.com/support/mot.../sp2gp2tps.pdf
Note: Memory between 4GB and 4GB minus 512MB will not
be accessible for use by the operating system and may be lost
to the user, because this area is reserved for BIOS, APIC
configuration space, PCI adapter interface, and virtual video
memory space. This means that if 4GB of memory is installed,
3.5GB of this memory is usable. The chipset should allow the
remapping of unused memory above the 4GB address, but
this memory may not be accessible to an operating system that
has a 4GB memory limit.
You can have more than 4GB on Windows XP 32 bit by using the PAE or Physical Address Extension switch and it may show in Windows, however the OS is still limited by 4GB and any memory installed above that will not be usable.
One must realize this is not a hardware issue but an operating system issue. These limits are not in the 64 bit versions of XP or Vista. Even the 32 bit version of Vista will have this limitation.
Also note that any application running in Windows XP 32bit is limited to 2GB and will not address anything above 2GB. This is lifted in 64bit, however if you are running a 32bit application in a 64bit OS, you still have the 2GB limit.
More information.
http://dlsvr01.asus.com/pub/ASUS/mb/4GB_Rev1.pdf