Why doesn't my Windows® PC recognize the whole 4GB of memory I installed?

Not only is there a maximum amount of memory that your computer motherboard can accept, there is also a maximum amount of memory that your operating system (OS) can accept. For instance, when you install 4GB of memory in a 32-bit Windows system (the most common version; 64-bit systems are typically used by high-end users), your system reports only 3GB or 3.5GB.

First , we can assure you, there isn't a problem with the memory. While Windows allows for 4GB of memory to be addressed, it does not equate to 4GB of physical memory.

Some of the memory (regardless of how much you have installed) is reserved for use by the devices that you are using, such as a graphics card, PCI card, integrated network connections, etc, meaning it is unavailable for use as normal main memory.

Upon startup, your system calculates the amount of memory needed for these devices; if you haven't maxxed out the memory in your system, it's invisible to you, and all your physical memory (the installed RAM) is available for use. However if you've maxxed out the DRAM in your system, this amount will be deducted from your physical memory, so you can't use 100% of your DRAM.

The maximum memory limitation varies by operating system, For instance, the 4GB memory limitation doesn't exist in 64-bit versions of Windows.

How to determine whether a computer is running a 32-bit version or 64-bit version of the Windows operating system

Note for Windows Vista users:
Microsoft has addressed this issue for Windows Vista with its Service Pack 1.

If you have a system board that can handle more than 4GB of memory and a processor capable of handling x64 instructions and memory remapping, Vista SP1 can help. Because of Windows and the driver stacks, Windows loaded them into 'high' memory locations to avoid potential driver compatibility issues. (Meanwhile, the 32-bit versions of Windows Vista limit the total available memory to 3.12 GB.) VistaSP1 has other features to enhance your computing, so we recommend you add it, if you haven't done so already.

If you are running Windows XP, you can cosmetically correct the issue by editing the Physical Address Extension settings.

Memory maximums for current Microsoft® Windows OSs include:

Windows Vista (32 bit)

  • Ultimate: 4 GB
  • Enterprise: 4 GB
  • Business: 4 GB
  • Home Premium: 4 GB
  • Home Basic: 4 GB
  • Starter: 1 GB

Windows XP (32 bit)

  • Professional: 4 GB
  • Home: 4 GB
  • Starter Edition: 512 MB

Windows Server 2003 (32 bit)

  • Datacenter SP2: 128 GB
  • Enterprise SP2: 64 GB
  • Standard SP1: 4 GB*
  • Datacenter R2: 128 GB
  • Enterprise R2: 64 GB
  • Standard R2: 4 GB*
  • Web Edition: 4 GB*
  • Small Business Edition: 4 GB*

* Certain Microsoft server operating systems can support over 4GB of memory via Physical Address Extension (PAE). Please refer to Microsoft knowledgebase article located here for more information.

Windows Server 2008 (32 bit)

  • Datacenter: 64 GB
  • Enterprise: 64 GB
  • Standard: 4 GB
  • Web Server: 4 GB

Windows Vista (64 bit)

  • Ultimate: 128 GB
  • Enterprise: 128 GB
  • Business: 128 GB
  • Home Premium: 16 GB
  • Home Basic: 8 GB

Windows XP (64 bit)

  • Professional: 128 GB
  • Windows Server 2003 (64 bit)
  • Datacenter SP2: 2 TB
  • Enterprise SP2: 2 TB
  • Standard SP1: 32 GB
  • Datacenter R2: 1 TB
  • Enterprise R2: 1 TB
  • Standard R2: 32 GB
  • Small Business Edition: 128 GB

Windows Server 2008 (64 bit)

  • Datacenter: 2 TB
  • Enterprise: 2 TB
  • Standard: 32 GB
  • Web Server: 32 GB