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I recently upgraded my system by adding 2GB of RAM in my motherboard. Now I am having 4GB of RAM but the problem is that Windows doesn't recognize the full 4GB, instead it shows only 3GB. I know the motherboard recognizes all four because it displays all 4GB during POST. Then why won't Windows recognize all 4GB?
This is normal for 32bit XP. The Maximum RAM memory that can be handled by 32-bit OS is 4 GB. However the entire 4 GB may not be available to you. The rest of the memory is being reserved by pci devices.
The maximum amount of memory that your system can use is actually limited in two ways — a maximum amount of memory that your computer motherboard can accept, and also a maximum amount of memory that your operating system (OS) can accept.
When you install 4GB of memory in a 32-bit Windows system, your system will see (and utilize) only 3GB or 3.5GB. The reason for this is that some of the addressable memory 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 (i.e. RAM) is available for use. However if you've maxed out the DRAM in your system, this amount will be deducted from your physical memory.
Well, there's no point to install 4GB in WinXP 32-bit system simply because Windows allocates memory to PCI/PCI-E/AGP devices in the 4GB range. In other words, you can install 4GB, but you may see anything from 2.5-3.5GB even though the BIOS says 4096MB. Windows reserves parts of that upper memory register for other devices.
XP doesn't recognize 4 gigs of ram. It only recognizes about 3. However you may be able to tweak some settings in you bios to get it to show more in Windows but it depends on your hardware.
I was thinking to install Windows XP 32 bit on my new build but then I read this thread and it says that it only supports up to 3gb or so. So should I go for 64 bit system in order to get my full 4gb of ram?
Yeah that's true, 32bit XP will not give you whole 4GB. You should use 64 bit OS. According to my experience, I have tried XP 64 bit and it has great stability but very little driver support. On the contrary, Vista 64bit is great and have a good driver support.