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

The
maximum amount of memory that your system can use is actually limited
in two ways — 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 only
by high-end users), your system will see (and utilize) only 3GB or
3.5GB. Is the problem bad memory?

Relax, there isn’t a problem with the memory. Windows allows for
4GB of memory to be addressed, but this isn’t 100 percent the same as
having 4GB of physical memory.

What happens is that some of the addressable memory (regardless of
how much you have physically installed) is reserved for use by page
files or by some of the devices that you are using, such as a graphics
card, PCI card, integrated network connections, etc., so it’s
unavailable for use as normal main memory.

The amount of memory needed for these devices is calculated by your
system at startup; if you haven’t maxed out the memory in your system,
it’s invisible to you, and all your physical memory (the RAM that’s
installed) is available for use. However if you’ve maxed 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.

Memory maximums for current Microsoft® Windows OSs include:

  • Windows XP Home: 4GB
  • Windows XP Professional: 4GB
  • Windows XP 32-bit: 4GB
  • Windows XP 64-bit: 128GB
  • Windows Vista Home Basic: 4GB
  • Windows Vista Home Basic 64-bit: 8GB
  • Windows Vista Home Premium: 4GB
  • Windows Vista Home Premium 64-bit: 16GB
  • Windows Vista Ultimate: 4GB
  • Windows Vista Ultimate 64-bit: 128GB+
  • Windows Vista 32-bit: 4GB
  • Windows Vista 64-bit: 128GB+

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