64-bit operating systems are a logical step in the evolution of he PC, why requires some background information. Often the maximum size of the physical memory on a computer is less than the amount needed for all running programs. This is especially true when multiple processes or applications execute simultaneously. The solution: programs store data on the hard drive and copy it back and forth to physical memory as needed.
This solution is often referred to as virtual memory, in which the computer simulates having large amounts of contiguous physical memory. Virtual memory can simulate physical memory of nearly any size. However, there is a practical limit imposed on the memory size by the width of the memory addresses. The total number of addresses in the virtual memory, the total amount of data the computer can keep in its working area for applications, is determined by the width of the registers on the computer processor.
Until recently, almost all consumer PCs used a 32-bit processor, which has registers with an address space 32-bit wide. A 32-bit processor provides 2^(32) bytes, or approximately 4 gigabytes (GB), of the virtual memory satisfied the needs of most programs. However, today Windows computers increasingly confront the 4GB virtual memory ceiling as the result of memory-intensive applications and the need to store multiple applications in memory simultaneously. In response, companies like Intel and AMD developed consumer-grade 64-bit processors.
64-bit processors are theoretically capable of accessing 2^(64) locations in memory, or 16 exabytes of virtual memory, over 4 billion times that of a 32-bit processors. While 64-bit processors obviously remove the virtual memory ceiling inherent to 32-bit processors, to take advantage of the larger memory addresses in 64-bit processors you must have a 64-bit operating system like Windows Vista x64. Windows Vista x64 imposes a more practical 16TB limit on address space as it is extremely unlikely that you will ever need the full 16 exabyte capacity.
