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

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Article ID: 291988 - View products that this article applies to.
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SUMMARY

This article lists resources that will help you use the 4-gigabyte (GB) random access memory (RAM) Tuning feature, formerly known as 4GT RAM Tuning, and the Physical Address Extension (PAE) parameter.

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Virtual memory in Windows 32-bit operating systems

Before you try to understand how the 4-GB random access memory (RAM) Tuning feature and the Physical Address Extension (PAE) parameter change memory allocation, it is helpful to review how virtual memory works in Windows 32-bit operating systems. For a detailed description about how virtual memory works in Windows 32-bit operating systems, visit the following Microsoft Web site:
Memory Management 101
http://blogs.technet.com/askperf/archive/2007/02/23/memory-management-101.aspx

The /3GB parameter

On 32-bit versions of Windows, the /3GB parameter enables 4-gigabyte (GB) random access memory (RAM) Tuning, a feature that enlarges the user-mode virtual address space to 3 GB and restricts the kernel-mode components to the remaining 1 GB.

For a detailed description of the /3GB parameter, visit the following Microsoft Web site:
Memory Management - Demystifying /3GB
http://blogs.technet.com/askperf/archive/2007/03/23/memory-management-demystifying-3gb.aspx
For a description of the /3GB parameter, subparameters, and operating system support, visit the following Microsoft Web site:
Windows Driver Kit: Driver Development Tools: /3GB
http://msdn.microsoft.com/en-us/library/ms791558.aspx
Note On Windows Vista and on later versions of Windows, use the IncreaseUserVA element in BCDEdit. For more information about BCD boot options in Windows Vista and in later versions of Windows, visit the following Microsoft Web site:
http://msdn.microsoft.com/en-us/library/aa906217.aspx

The /PAE parameter

The /PAE parameter enables Physical Address Extension (PAE). This parameter directs the system to load the PAE version of the Windows kernel. PAE is an addressing strategy that uses a page-translation hierarchy to enable systems that have 32-bit addressing to address more than 4 GB of physical memory.

For a description of the /PAE parameter, subparameters, and operating system support, visit the following Microsoft Web sites:
Windows Driver Kit: Driver Development Tools: /PAE
http://msdn.microsoft.com/en-us/library/ms791485.aspx
Physical Address Extension - PAE Memory and Windows
http://www.microsoft.com/whdc/system/platform/server/PAE/PAEdrv.mspx
Operating Systems and PAE Support
http://www.microsoft.com/whdc/system/platform/server/PAE/pae_os.mspx
Caution Microsoft supports the use of the /3GB parameter in Windows Server 2003, Standard Edition in a production environment for use by Active Directory directory service. For other applications, Microsoft supports the use of the /3GB parameter 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 and the /3GB functionality are supported in a production environment. Contact your application vendor for more information about their application. The /3GB parameter can cause some applications to have problems that are related to address dependencies or to a reduction in kernel space. Except in the cases that are described here, the /3GB parameter in Windows Server 2003, Standard Edition is only for development and for testing.

Note The /3GB parameter could lead to resource starvation of Active Directory when you enable the parameter on an Exchange Server.

The /PAE parameter 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 parameter from the Boot.ini file that discards the lower 4 GB of memory.

Important These configurations are not supported on Microsoft Windows 2000 Professional and on Windows 2000 Server. These configurations are only made available for testing. Do not use these parameters in a production environment if you are using Windows 2000 Professional or Windows 2000 Server.

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 that you use this feature in Windows Server 2003, Standard Edition in a production environment.

To use the /3GB parameter from the Boot.ini file together 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 the memory range 0xC0000000 through 0xFFFFFFF, developers can test kernel-mode components.

MORE INFORMATION

For more information about the /3GB and /PAE parameters and about how to optimize memory allocation, visit the following Microsoft Web sites:
Exchange Server 2003: How to Set the /3GB Startup Switch in Windows
http://technet.microsoft.com/en-us/library/bb124810.aspx
4-Gigabyte Tuning
http://msdn.microsoft.com/en-us/library/bb613473(VS.85).aspx
For more information, click the following article numbers to view the articles in the Microsoft Knowledge Base:
815372 How to optimize memory usage in Exchange Server 2003
283037 Large memory support is available in Windows Server 2003 and in Windows 2000
171793 Information on application use of 4GT RAM Tuning

Properties

Article ID: 291988 - Last Review: May 21, 2008 - Revision: 16.0
APPLIES TO
  • Microsoft Windows Server 2003, Enterprise Edition (32-bit x86)
  • 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)
Keywords: 
kbexpertiseadvanced kbresolve kbproductlink kbappdev kbinfo kbpae kbperformance KB291988

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