On a computer that has 4 gigabytes (GB) of random access memory (RAM), the System Properties dialog box and the System Information dialog box may report less memory than you expect.
This problem occurs in Windows Vista, in Windows Server 2003, and in Windows Server 2008.
Important Windows Vista Service Pack 1 (SP1) made changes to the way components of the user interface (UI) report memory. For example, some components of the Windows Vista SP1 UI will now report when there is 4 GB or more of total physical memory that is installed on the computer. For more information about how memory is reported in Windows Vista Service Pack 1, click the following article number to view the article in the Microsoft Knowledge Base:
946003 Windows Vista Service Pack 1 will report 4 GB of system memory (RAM) on systems that have 4 GB of memory installed
This problem occurs because the address space is limited to 4 GB in a 32-bit hardware environment. Memory may be relocated to make room for addresses that the basic input/output system (BIOS) reserves for hardware. However, because of this limitation, Windows Vista, Windows Server 2003, and Windows Server 2008 cannot access memory that is relocated above the 4 GB boundary.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
A 32-bit operating system can address memory that is relocated above the 4 GB boundary if the following conditions are true:
The computer is in Physical Address Extension (PAE) mode.
The computer has 4 GB of RAM.
In this case, the operating system correctly reports how much memory is installed.
Additionally, some x64-based operating systems can address up to 2 terabytes (TB) of RAM. For more information, visit the following Microsoft Web page:
To enable PAE mode, you have to add PAE to the boot entry in the BCD file. Open an elevated command prompt. Type BCDEDIT /SET PAE ForceEnable.
Note There is no Boot.ini file in Windows Vista. The Boot.ini file is used for legacy Windows versions when dual-booting only
For more information, click the following article numbers to view the articles in the Microsoft Knowledge Base:
268230Scaling out versus scaling up with Intel Physical Addressing Extensions (PAE)
283037 Large memory support is available in Windows Server 2003 and in Windows 2000
If you add more memory to the system, the BIOS might recognize all the physical RAM even though Windows recognizes only a part of the RAM. If the computer uses a redundant memory feature or a memory mirroring feature, the full complement of memory may be invisible 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. You can enable or disable these features by using the BIOS. You cannot enable or disable these features by using Windows. To modify the settings for these features, refer to the computer's user manual or to the BIOS manufacturer's Web site. Or, contact the manufacturer.
For example, if the computer has 4 GB of RAM installed, and you add 4 GB of additional RAM, Windows may recognize only 4 or 6 GB of RAM 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 resemble the symptoms that occur when you do not add the /PAE switch to the Boot.ini file.
For more information about the memory limits for Windows operating systems, visit the following Microsoft Web site: