256 MB of physical memory appears to be missing when 4 GB of memory is installed

If you are using a computer that has over 4 gigabytes (GB) of memory installed, System properties, Microsoft System Diagnostics (WinMSD) or other system utilities report a memory value that is 256 megabytes (MB) less than the total physical memory that is installed.

This issue can occur if a server is using the Intel Profusion chipset. In the read-only (ROM) memory of the computer, the upper 256-MB memory region is reserved for memory-mapped input/output (I/O) devices. The amount of reserved physical memory may increase depending on the number of I/O devices that are installed on the computer. Typically, a computer that has 4 gigabytes (GB) of actual physical memory looks as if it has 3.84 GB of total physical memory.

This behavior is by design.

The Intel Profusion is an eight-way symmetric multiple-processing chipset that is designed for enterprise-level server programs. It focuses on raw processing power and high-throughput I/O performance that is commonly found in 8-way Dell, Compaq, Hewlett-Packard (HP), and IBM servers. The amount of memory that is consumed can be larger than 256 MB; however, memory increases in 256-MB increments.

For more information, refer to the Intel Pentium III Xeon Processor. To do this, visit the following Intel Web site: For more information about Physical Address Extension (PAE) server design, visit the following Microsoft Web site: For more information about supporting PAE memory in Windows 2000, visit the following Microsoft Web site: For more information about Address Windowing Extensions (AWE), visit the following Microsoft Web site: For more information, click the following article numbers to view the articles in the Microsoft Knowledge Base: If you add more memory to the system, it is possible that the BIOS will recognize the full amount of physical RAM that is installed in the server but that Windows will recognize only a part of the RAM. If the server has a redundant memory feature or a memory mirroring feature that is enabled, the full complement of memory may not be visible to Windows. Redundant memory provides the system with a failover memory bank when a memory bank fails. Memory mirroring splits the memory banks into a mirrored set. Both features are enabled or disabled in the BIOS and cannot be accessed through Windows. To modify the settings for these features, you may have to refer to the system user manual or the OEM Web site. Alternatively, you may have to contact the hardware vendor.

For example, if you are running a system that has 4 GB of RAM installed and you then add 4 GB of additional RAM, Windows may recognize only 4 GB of physical memory or possibly 6 GB instead of the full 8 GB. The redundant memory feature or the memory mirroring feature may be enabled on the new memory banks without your knowledge. These symptoms are similar to the symptoms that occur when you do not add the /PAE switch to the Boot.ini file.

Technical support for x64-based versions of Microsoft Windows

If your hardware came with a Microsoft Windows x64 edition already installed, your hardware manufacturer provides technical support and assistance for the Windows x64 edition. In this case, your hardware manufacturer provides support because a Windows x64 edition was included with your hardware. Your hardware manufacturer might have customized the Windows x64 edition installation by using unique components. Unique components might include specific device drivers or might include optional settings to maximize the performance of the hardware. Microsoft will provide reasonable-effort assistance if you must have technical help with a Windows x64 edition. However, you might have to contact your manufacturer directly. Your manufacturer is best qualified to support the software that your manufacturer installed on the hardware. If you purchased a Windows x64 edition such as a Windows Server 2003 x64 edition separately, contact Microsoft for technical support.

For product information about Windows XP Professional x64 Edition, visit the following Microsoft Web site: For product information about x64-based versions of Windows Server 2003, visit the following Microsoft Web site: