Microsoft KB Archive/117373

= Windows NT recognizes only up to 64 MB RAM on some computers =

Article ID: 117373

Article Last Modified on 2/20/2007

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APPLIES TO


 * Microsoft Windows 2000 Server
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Professional Edition
 * Microsoft Windows NT Advanced Server 3.1
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition
 * Microsoft Windows NT Workstation 3.1
 * Microsoft Windows NT Workstation 3.5
 * Microsoft Windows NT Workstation 3.51
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Advanced Server 3.1

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This article was previously published under Q117373





SUMMARY
Windows NT detects up to 4 gigabytes (GB) of RAM, but if you are using a computer with more than 64 megabytes (MB) of RAM, Windows NT may detect only up to 64 MB. This behavior is based on how the BIOS returns the amount of memory to Windows NT. Even though the BIOS may correctly display memory above 64 MB, it may not pass this information correctly to Windows NT.

Note The AST Manhattan may display this behavior and may require updating the EISA configuration by running the EISA configuration utility. AST Manhattan machines may also require disabling video memory range in BIOS in order for NT to report the actual amount of memory (RAM) installed.



MORE INFORMATION
Windows NT supports only memory the BIOS reports using INT 15, 88H, E801H, and E820H. Depending on how the BIOS reports memory is how Windows NT will recognize it. If the BIOS is using INT 88H, it can only support up to 64 MB RAM. If the BIOS is using INT E801H can support more, but it can only be in three ranges: 0 - 640K, 1 MB - 16 MB, 16 MB and up. For the BIOS to support INT E801H, INT 88H can only support up to 16 MB. This may be a problem, since it means that some existing applications or products only see 16 MB of memory on computers with more than 64 MB. Also, supporting E801 typically means the manufacturer of the computer ships a TSR that turns off INT E801 and allows INT 88H to return 64 MB.

If the BIOS is using INT E820, it does not have this problem, and it allows for other features such as reporting RAM that is in use by the BIOS. In addition to this just being a feature, it effectively reclaims 256 kilobytes (K) of memory which currently utilizes INT 88H or INT E801H. INT E820 can report addresses that cannot be used by devices and ensure that different drivers don't attempt to use the same address for their devices. INT E820 can return as many different 'runs' of real memory as it wants, so large amounts of memory work on Extended Industry Standard Architecture (EISA) and Industry Standard Architecture (ISA) with INT E801, but there are limitations.

Windows NT only supports the memory returned by the BIOS. For Windows NT to support computers with more than 64 MB RAM, the computer must return this memory through INT E820 or INT E801.

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.

Additional query words: prodnt INTEL PCI

Keywords: kbhardware KB117373

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