In a 32-bit operating system... COPY PASTE!
Does 32-bit Really Have a Memory Limit?
Full size
In any 32-bit operating system, you are limited to 4096 MB of RAM simply because the size of a 32-bit value will not allow any more. On a 32-bit system, each process is given 4 GB of virtual memory to play with, which is separated into 2 GB of user space that the application can actually use at a time.
Savvy readers might point out that modern chips support PAE, a processor technology that allows the operating system to use a little bit more memory?up to 64 GB, but it also requires special application support that most applications don't have or necessarily need.
A common misconception is that this is a Windows-specific problem, when in fact 32-bit Linux and Mac OS X have the same limitations and the same workarounds. 32-bit Linux uses a mapping table to allow access to the extra memory, and OS X Snow Leopard actually ships by default with a 32-bit kernel that can't access all the memory on older systems, even though most of the rest of the OS runs 64-bit processes.
The 4 GB limit for Windows, at least, is also a factor of licensing?the home versions of 32-bit Windows, while technically being able to support PAE, have a hard limit of 4 GB for licensing and driver compatibility reasons.
More Problems with 32-Bit
Not only does 32-bit have a hard limit for the amount of memory it can address, there's also another problem: your devices, like your video card and motherboard BIOS take up room in that same 4 GB space, which means the underlying operating system gets access to even less of your RAM.
Windows expert Mark Russinovich found that a desktop running 32-bit Windows with 4 GB of RAM and two 1 GB video cards only had 2.2 GB of RAM available for the operating system?so the bigger and better your video cards get, the less of that 4 GB will be accessible on a 32-bit system.
What's Different About 64-Bit?
While 32 bits of information can only access 4 GB of RAM, a 64-bit machine can access 17.2 BILLION gigabytes of system memory, banishing any limits far into the future. This also means that your video cards and other devices will not be stealing usable memory space from the operating system. Windows 64-bit Home editions are still limited to 16 GB of RAM for licensing reasons, but the Professional and Ultimate versions can use up to 192 GB of RAM, so keep that in mind when building that killer system.
The per-process limit is also greatly increased?on 64-bit Windows, instead of a 2 GB limit, each application has access to 8 TB of virtual memory without any special API, a huge factor when you consider applications like video editing or virtual machines that may need to use enormous amounts of RAM.
On Windows, the 64-bit versions also come with a technology to prevent hijacking the kernel, support for hardware-enabled data execution protection, and mandatory digitally signed 64-bit device drivers. You also won't be able to use your 16-bit apps anymore, which hardly seems like a loss.
Got it here if you want to know more
http://lifehacker.com/5431284/the-lifehacker-guide-to-64+bit-vs-32+bit-operating-systems
