Carl Tyler's Blog

1 - The problem here is that your probably running in PAE mode. You can see this from the system properties. This is automatically turned on if your using SP2 and cannot be turned off.

In PAE mode memory at the upper end of the system is not used by Windows and is mapped to your PCI/PCIe devices, the drivers can see the PAE area and can work faster but the downshot is that Windows only reports 2.something Gb Ram. The 'something' depends on now many PCI/PCIe devices you have in the system.

the memory is itill there and is being used by the hardware.

Anyway, WinXP has a 2Gb user ram limit anyway, the rest is for the kernal.

XP64 and Vista will show the full amount of ram on some boards.

2 - Dec's got it. See microsoft technotes:

http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx

http://support.microsoft.com/kb/875352

-rich

3 - no no no! I am running xp64 beta and have 4 GB installed, but xp64 is only seeing 3 GB!! Microsoft did it again!!!

4 - This is a common complaint on systems will 3GB or more of RAM. The explamation is quite simple but the solution is not.

Ignore the explanation by Declan Lynch above. It is confused and largely incorrect.

All 32bit Windows client operating systems are limited to 4GB RAM. Unfortunately, this address space has to be shared with memory mapped hardware devices which are in the upper address range. With 4GB of RAM there will inevitably be a conflict, leaving a portion of RAM that cannot be addressed. The exact amount varies with the system hardware and is usually between 2.8 and 3.5GB.

PAE is irrelevant to this issue.
XP does not have a 2GB RAM limit. There is a 2GB limit but this is virtual address space and is per process. More than 3GB of RAM can be used for applications, if the system can make it available.

There are two solutions:
1. Use a 64 bit OS.
2. Use a server OS. With compatible hardware RAM can be mapped above 4GB to avoid the conflict with hardware. Unfortunately, many popular hardware devices an drivers are incompatible with such an environment so it is not supported on client systems.

Larry Miller
Microsoft MCSA