Uchiro's Tavelogue

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.

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

Editing Physical Address Extension settings

To cosmetically correct the issue where Windows XP only shows 3.5GB of memory or less when 4GB of physical memory installed, follow the step by step guide below:

1. Click 'Start' then 'Control Panel'.
2. Once the control panel has opened, double click 'System' or 'Performance and Maintenance' then 'System'.
3. In the System Properties window, click the Advanced tab.
4. Toward the bottom of the Advanced window, look for the Startup and Recovery section.
5. Click the Settings button.
6. The Startup and Recovery window opens to the System Startup section.
7. Click the Edit button to open the Boot.Ini file in Notepad.
8. Place the cursor after /fastdetect, and then press the spacebar.
9. Type /PAE. (PAE is short for Physical Address Extension.)

   The Boot.ini file should resemble this:

   [boot loader]
   timeout=30
   default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
   [operating systems]
   multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect /PAE
   

10. In the File menu, click Save.
11. Exit Notepad, Start Up and Recovery and System Properties.
12. Restart your machine. WindowsXP should now recognize REPORT 4GB of memory.

If Windows does not detect the full amount of memory after C, please follow the trouble shooting steps below:

• Check if the full amount of memory is registered in the BIOS.
• If the full amount of memory is registered by the BIOS, the problem lies with the operating system. Please contact Microsoft for further assistance. You may have to upgrade to a 64-bit operating system and processor to enable the full 4GB of memory.
• If the full amount of memory is not detected by the BIOS, please try each module individually in the system. For example, if you have 4 x 1GB modules, try them in pairs. If both pairs of modules register 2GB in the BIOS and within windows, it shows the memory is working correctly. Try installing all 4GB of memory again and check the BIOS. If the BIOS still does not register the full amount, the problem lies with the motherboard. Please contact the system/motherboard manufacturer for further assistance.
• Some systems/motherboards reserve a certain amount of memory for system usage, i.e. PCI slots and therefore the actual displayed memory amount is less than the physical installed amount. Please consult your system/motherboard manual for further information.