For the past week, I've been running Windows XP 64 Bit. This version is still a beta version of the OS. In other words, should you try this for yourself, don’t expect full functionality. What this 64-bit operating system does allow is the running of 64-bit programs. Unfortunately, at this point I have none of those. (If you have one, and can spare a license...)

Let's begin with the hardware: the Athlon64/FX/Opteron has the ability to run in three modes. The first is "Legacy," which almost everyone is using right now. That's where you have a 32-bit O/S, and 32-bit programs. You get none of the 64-bit advantages that I'll explain later.

The next two modes you only can get with a 64-bit operating system: the "compatibility" and "64-bit" modes. Those two are pretty self explanatory, with "compatibility" mode to run older 32-bit code. That's the mode that all of these tests today were run in.

As a user, you do not get to select the mode--it's done by the hardware, which looks at specific bits of each instruction at runtime. What Windows XP 64 Bit Edition allows is that once you do have 64 bit programs, you can run them in the 64-bit mode, allowing for optimizations to be made. You have to remember that both Intel and AMD have been pushing programmers to make use of the SSE/SSE2 and now SSE3 extensions for floating point calculations. That means that they are being routed through an 80-bit interface (with 64-bit precision internally). So nothing is going to come from moving to 64-bit that way, despite what MS claims on their XP 64-Bit Preview page. Very little new FP code goes through the x87 execution units these days, for performance, precision, and ease of use reasons, and this is not supported in 64-bit mode.

Integer, however, is a bit different. Even though most integer calculations really don't need the size of 64 bits, it's now there, and someone will find a way to use it, much like what happened with the move from 16 to 32 bits. The really big advantage is the availability of double the number of general registers available to the programmer and compiler. This can help reduce the number of load/store commands in the code. I'll stop before your head explodes, but I warn that I'll be going more into depth later, going over how all these factors affect things. For now, let's just say that this is the main performance difference in moving to x86-64 for consumer level software.