32-bit vs. 64-bit CPUs Explained

Reader Tim Ficke writes: What is the difference between 32- and 64-bit processors? My computer has an Athlon 64 processor and is 3 years old and I think it is great. But I can’t seem to figure out what the 64-bit does. Help me out!

In a nutshell, "XX-bit" refers to the addressable memory space of a CPU. In the old days, we had 8-bit computers, which could handle only a few KB of RAM. 32-bit processors (like most Pentiums) can scale up to 4GB of both RAM and virtual memory, but 64-bit chips can handly up to 16 terabytes of virtual memory and 128GB of RAM. 64-bit computing is also theoretically faster thanks to its ability to handle twice as much data with each clock tick.

So how do you take advantage of these 64-bit features? You need both a 64-bit CPU and a 64-bit operating system.

First let's consider the CPU: The AMD Athlon 64 line, the Intel Core 2 Duo, and the Intel Pentium D and Xeon are all, most of the time, capable of running a 64-bit OS. But not all Intels in these lines are 64-bit chips. You'll need to check your specific chip on Intel's processor pages to see if it's "EM64T" compatible. If it is, it will run 64-bit apps. If not, forget it. (Click anything marked "specifications" on these pages to check.)

Now let's talk about the OS side, and here we're talking about the 64-bit version of Windows XP. The so-called Windows XP Professional x64 Edition looks and feels just like the standard version of XP that you know and love (er, use every day), but it only runs on 64-bit CPUs.

Now here's the catch: 64-bit OSes prefer to run 64-bit applications. The good news is that XP x64 will also run 32-bit applications in a special 32-bit subsystem. And since there are very few 64-bit applications, you'll be spending most of your time in this special mode. Here, performance gains will be limited by the design of the 32-bit applications. So there goes that benefit.

There's a bigger problem, though, and that's with software that has to integrate directly with the OS. Namely this means hardware drivers, few of which have been written for XP x64, and kernel-accessing software like antivirus applications. Numerous other problems exist with 32-bit software that has special functions, like right-clicking a file in Windows Explorer and being able to access custom menus. A final issue: ancient 16-bit software, supported under regular XP, doesn't work under XP x64 at all.

All of these drawbacks add up to the main reason why virtually no one uses XP x64, even those who've had 64-bit CPUs for years. Want to give it a try yourself? You can try x64 for free by downloading it here or ordering a CD from Microsoft. You'll receive a demo version of x64 good for 120 days. I'd be surprised if you leave it installed for a more than a week.

Naturally, Vista will cause more headaches in this arena, but it too will offer both 32-bit and 64-bit versions (though I'm not sure how exactly this will play out with new installations). Expect similar dilemmas, and years before drivers and software are widely available for the 64-bit version of Vista.