Windows NT / Windows 2000 Server currently only supports up to 4GB of RAM. However, in most cases, it is not even possible to use the full 4GB of RAM because Windows can only address up to 4GB of contiguous RAM and most system devices use some of the address space for internal functions.

For Example:
A LAN Adapter may use 1MB of Address Space starting at the 3712MB boundary which means Windows will only be able to use 3712MB even if RAM is mapped to Address Space after the LAN Adapter Address Space.

Also, If 3712MB is not specified to Windows (using the boot.ini variable /MAXMEM=3712) then the LAN adapter may not function properly.

Additionally, the PCI Hot Plug function can reserve a considerable amount of Address Space (from 0MB up to 1.5GB) for Hot Plug Resource Padding. The amount of address space used for Resource Padding can be adjusted in CMOS Setup under "PCI Configuration". If you do not plan on using the PCI Hot Plug function, you can disable it to maximize the amount of memory available to Windows.

1. After installing all the PCI Devices in the system, boot Windows and note the amount of contiguous memory available to Windows while loading the Kernel (see example below):
   
   
2. Edit the boot.ini file in the root directory of the drive Windows is installed on Example: "C:\boot.ini"
3. Add the /MAXMEM=xxxx variable to the end of the ARC Path (see example below):
   [boot loader]
   timeout=30
   default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
   [operating systems]
   multi(0)disk(0)rdisk(0)partition(1)\WINNT="Windows NT Server Version 4.00" /MAXMEM=3904
   multi(0)disk(0)rdisk(0)partition(1)\WINNT="Windows NT Server Version 4.00 [VGA mode]" /basevideo /sos
4. Save the changes to the boot.ini and restart the computer.
   

COMMENTS:
Additional problems can arise while attempting to install Windows with 4GB of RAM or more installed. Microsoft suggests installing Windows with only 2GB of RAM installed and then apply SP3 or higher and then insert the remaining 2GB of RAM. See Microsoft Knowledge Base Article # Q160392

The MAXMEM variable uses rounded numbers for KBytes (1K=1024 Bytes).
Therefore specifying MAXMEM=3712 means actual memory is 3,712,000KB X 1,024 = 3,801,088,000 Bytes. This is why Windows will report 3801MB when MAXMEM=3712.

Windows NT 4.0 Server will only allocate a maximum of 2GB of RAM for applications. The other 2GB is reserved for the kernel. Windows NT 4.0 Enterprise will allow you to allocate 3GB to applications, and restrict the kernel to 1GB. This is called "4GT" for "4GB Tuning".

Windows 2000 Advanced Server supports up to 8GB of RAM. However, the user must add the /PAE switch to the end of the ARC path for Win2k AS to see 8GB.

Refer to the Microsoft Knowledge Base for more information.