A lot of people have been upgrading to 4GB with their 32-bit operating systems only to find that the full amount isn't being recognized. The amount varies from system to system depending on hardware, but it usually falls between 3.0 - 3.5GB, sometimes even less than 3.0GB:
Until recently, almost all consumer PCs used 32-bit processors.
The bit size of a processor refers to the size of the address space it can reference. A 32-bit processor can reference 2^32 bytes, or 4 GB of memory. These 32-bit processors were standard at a time when 4 GB was thought to be more than enough memory space for software applications on Windows.
In 2003, AMD released the first widely accepted 64-bit processor aimed at consumers, the Athlon 64, and coined AMD64 as the name for the new instruction set.
The 64-bit processors are theoretically capable of referencing 2^64 locations in memory, or 16 exabytes, which is more than 4 billion times the number of memory locations 32-bit processors can reference. However, all 64-bit versions of Microsoft operating systems currently impose a 16 TB limit on address space and allow no more than 128 GB of physical memory due to the impracticality of having 16 TB of RAM.
How to find out if your cpu (processor) or your operating system is 32 or 64 bit in linux.
links
zone.ni.com
computing.net
