Microsoft KB Archive/291988

= A description of the 4 GB RAM Tuning feature and the Physical Address Extension switch =

Article ID: 291988

Article Last Modified on 10/26/2007

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APPLIES TO


 * Microsoft Windows Server 2003, Enterprise Edition
 * Microsoft Windows Server 2003, Datacenter Edition (32-bit x86)
 * Microsoft Windows 2000 Datacenter Server
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Professional Edition
 * Microsoft Windows XP Professional
 * Microsoft Windows Server 2003, Standard Edition (32-bit x86)

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This article was previously published under Q291988



SUMMARY
This article describes the 4 gigabyte (GB) random access memory (RAM) Tuning feature and the Physical Address Extension (PAE) switch.



MORE INFORMATION
The /3GB and /PAE switches in the Boot.ini file are to be used with the following products:
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Datacenter Server
 * Microsoft Windows Server 2003, Enterprise Edition
 * Microsoft Windows Server 2003, Datacenter Edition

When the /3GB switch is used with Windows 2000 Professional or with Windows 2000 Server, the kernel components are loaded into the memory space by using the 4 GB RAM Tuning feature in the same way as they load in Windows 2000 Advanced Server and in Windows 2000 Datacenter Server. This functionality lets device-driver developers test their drivers in this configuration without having to install Windows 2000 Advanced Server or Windows 2000 Datacenter Server. The user-mode memory space is still limited to 2 GB.

When the /3GB switch is used with Windows XP Professional, with Windows Server 2003, Datacenter Edition, with Windows Server 2003, Enterprise Edition, the /3GB switch works identically across versions. This functionality lets device-driver developers test their drivers in this configuration without having to install one of the Windows Server 2003 products just listed. The user-mode memory space is now limited to 3 GB.

Caution Microsoft supports using the /3GB switch in Windows Server 2003, Standard Edition in a production environment for use by Active Directory. For other applications, Microsoft supports using the /3GB switch in Windows Server 2003, Standard Edition only in a production environment if the application vendor has tested in this environment and if the vendor is willing to support the customer who is using this functionality. Microsoft Exchange Server 2003 and Microsoft SQL Server 2000 are supported in production using this functionality. Contact your application vendor regarding their application. The /3GB switch can cause some applications to have problems that are related to address dependencies or to a reduction in kernel space. Except in the cases described here, the /3GB switch in Windows Server 2003, Standard Edition is only for development and testing purposes.

Note The /3GB switch can lead to resource starvation of Active Directory when you enable it on an Exchange server.

The /PAE switch lets developers perform similar testing of device drivers by forwarding 64-bit addresses to kernel-mode components. This feature is known as Physical Address Extension (PAE), and it may not work on all chip sets. Any addresses that are over 32 bits are guaranteed to work by using the /nolowmem switch from the Boot.ini file that discards the lower 4 GB of memory.

IMPORTANT These configurations are not supported on Windows 2000 Professional and Windows 2000 Server. These configurations are only made available for testing purposes. Do not use these switches in a production environment unless you are using one of the products in the bulleted list at the top of this section.

In these products, the 4 GB RAM Tuning feature enables a 3 GB area of user-mode memory for programs to use. This feature can expand the virtual address range for user-mode memory from 0x0000000 through 0xBFFFFFF (the user-mode address range is typically from 0x0000000 through 0x7FFFFFFF). The range of memory that is available for kernel-mode components shrinks from 0x80000000-0xFFFFFFFF to 0xC0000000-0xFFFFFFFF. We do not recommend using this feature in Windows Server 2003, Standard Edition in a production environment.

Using the /3GB switch from the Boot.ini file with Windows 2000 Professional or with Windows 2000 Server can give the appearance of a 3 GB range of user-mode memory. However, the memory from 0x80000000 through 0xBFFFFFFF is not usable. Because kernel-mode components are now limited to using from 0xC0000000 through 0xFFFFFFF memory range, developers can test kernel-mode components.

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

815372 How to optimize memory usage in Exchange Server 2003

Keywords: kbproductlink kbappdev kbinfo kbpae kbperformance KB291988

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