Not only is there a maximum amount of memory that your system's 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 Microsoft® Windows® OS, your system may report only 3GB or 3.5GB of available memory. If this occurs, don't worry. The memory you purchased and installed is fine. The problem revolves around how much memory the OS can address.
While the Windows OS allows for a maximum 4GB of installed physical memory, this does not equate to 4GB of available memory. The reason? A portion of your system's memory (regardless of how much you have installed) is reserved to run devices, such as any graphics cards, PCI cards, integrated network connections, etc., meaning that certain amounts of installed memory may be unavailable for use as available memory.
Upon startup, your system calculates the amount of memory needed to run devices. If you've maxed out the amount of physically installed memory in your system, the amount of memory necessary to run devices will be deducted from your system's available memory, meaning that you won't be able to use all of your physical memory. However, if you haven't maxed out your physical memory, all of it will be available for use.
The maximum amount of memory that an OS will recognize varies by type and version of the operating system. In terms of differentiating between OS types, 32-bit operating systems are typically the lower-end consumer versions, while 64-bit operating systems are designed for high-use consumers and business users. Here's how to determine whether a computer is running a 32-bit version or 64-bit version of the Windows operating system.
Physical memory limits for recent Windows versions are as follows.
Note to Windows Server Users: Certain 32-bit Microsoft server operating systems can support over 4GB of memory via Physical Address Extension (PAE). Please refer to a Microsoft knowledge base article located here for more information.
Note to Windows Vista users: Microsoft addressed the installed memory limitation problem in 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.
Note to Windows XP Users: You can cosmetically correct the installed memory limitation problem by editing the Physical Address Extension settings.