To access an area in the computer's physical memory (RAM) to store or retrieve data, the processor needs the address of that location, which is an integer number representing one byte of memory storage.

Suddenly, having 64-bit registers makes sense as, while a 32-bit processor can access up to 4.3 billion memory addresses (232) for a total of about 4GB of physical memory, a 64-bit processor could conceivably access over 18 petabytes of physical memory. This is the one area that clearly shows why 64-bit processors are the future of computing, as demanding applications such as databases have long been scraping on the 4GB memory ceiling, and although Microsoft and Intel have combined to enable servers using the 32-bit Xeon processor and certain versions of Windows 2003 Server to utilize more than 4GB of memory, the amount that can be accessed per-application is still less than 3GB.

If you are a business with a database of a terabyte or more of information, 64-bit processors look pretty good right now.

Formerly known as X86-64, the AMD64 architecture is AMD's method of implementing 64-bit processors.

AMD64 is massively different from Intel's approach to 64-bit processors as seen in their Itanium line. While Intel used a completely different architecture for the Itanium chips, forcing software developers to relearn in order to program for them, or use emulation which slowed down performance, AMD decided to simply extend the existing x86 architecture (the foundation of all PC's since Intel developed the 8086 processor in 1978) to accommodate 64-bit registers as mentioned above.

There are several advantages to this. First, obviously, reworking code for AMD 64-bit processors should be considerably easier, since the basis is the same. Secondly, the AMD64 based Opteron and Athlon 64, are fully compatible with 32-bit applications.

A system based on either of these processors can use a 32-bit operating system and software without a hitch, providing a stress free upgrade path for businesses and opening up the desktop market to 64-bit processors, and more specifically, AMD's Athlon 64.

AMD accomplishes this by enabling the AMD64 processors to run in one of two modes, Legacy mode and Long mode. Legacy mode removes all 64-bit support and enables the processor to run strictly in 32-bit mode, necessary for running most current operating systems, including Windows. Long mode is comprised of two submodes, Compatibility mode and 64-bit mode.

Compatibility mode is designed for a 64-bit operating system such as Microsoft's impending 64-bit versions of XP and Server 2003, due late this year or early in the next, but running 32-bit software such as current databases. The advantage of this is that each 32-bit application, though still limited by the 4GB memory limit, can have all of that 4GB to itself with no overhead for the operating system, since that will use 64-bit addressing and can thus access additional memory space.

This provides some improved performance for demanding 32-bit apps before they are ported over to 64-bit. 64-bit mode is intended for a pure 64-bit environment, operating system and software, and offers one huge advantage.....