This article was previously published under Q292934
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When you start Windows 2000 Datacenter Server, you may not observe any memory greater than 16 GB in size even though you have more memory in the server.
This behavior can occur if Windows 2000 Datacenter Server has more than 16 GB of physical memory and is restarted with both the /3gb and /pae switches which are set in the Boot.ini file.
This behavior is by design.
When a computer is started with the /3gb switch, the kernel space is limited to 1 GB of memory and properly compiled programs are able to allocate up to 3 GB of memory rather than the 2 GB of memory that can normally be allocated by a program.
When a computer is started with the /pae switch, the size of the Page Table Entry (PTE) increases from 4 KB to 8 KB. The number of PTEs that are required when you translate memory is increased as the amount of memory in a computer increases.
When the /3gb switch is used, the kernel mode space is limited to 1 GB rather than the usual 2 GB amount. When a computer contains large amounts of memory, the combination of reduced kernel space, increased PTE size, and increased PTE utilization can cause a server to be unable to index any additional memory and can cause the server to stop responding.
When a computer is started and the presence of the /pae and /3gb switches are detected, the operating system does not recognize any memory above 16 GB. If you start the computer without the /3gb switch, the computer is enabled to locate all available memory up to 32 GB. This behavior protects the stability of the operating system from known conditions and the limitations of providing support for large memory configurations on a 32-bit platform.
For additional information, click the article number below to view the article in the Microsoft Knowledge Base:
283037 Large Memory Support Is Available in Windows 2000
266650 Information About BackOffice Program Support on Windows 2000 Datacenter Server-Based Computers
189327 HOWTO: Map Adapter RAM into Process Address Space
More information is available at the following Microsoft Web sites: