Microsoft has been exploring 64-bit Windows versions since the release of Digital Equipment's Alpha processor. But by late 2004, you'll have a variety of 64-bit Windows options for both workstations and servers that will run on numerous Intel and AMD hardware platforms. Here's what you need to know about 64-bit Windows.

Understanding the Platforms
The first 64-bit microprocessing platform that Microsoft supported with Windows Server 2003 and Windows XP was the Intel Itanium processor, which targets high-end server systems. Because Intel designed the Itanium family from scratch and the product isn't a technical successor to Intel's popular 32-bit x86 line of processors (which includes the Pentium 4 and Xeon families), Itanium processors can't natively run 32-bit Windows applications. However, Intel designed a 32-bit Windows on Windows (WOW) subsystem that lets users run 32-bit applications on Itanium in a special compatibility environment. Despite recent improvements, however, 32-bit application performance on the Itanium is still slow and is one of the product's weak points.

To leverage the massive installed base of 32-bit Windows applications, Intel's chief competitor, AMD, announced two new families of 64-bit processors in 2003—the AMD Opteron and the AMD Athlon 64. However, these processors are based on the x86 design and therefore maintain full backward-compatibility with AMD's Intel-compatible chips and can leverage their inherent performance benefits. AMD's 64-bit systems were an immediate hit with customers, especially the Athlon 64 products, which target desktop users (the Opteron is aimed at servers, a market that has historically eluded AMD).

Sensing the inevitable, Intel began a skunk works project to add Athlon 64 compatibility to its 32-bit microprocessors. The company also announced that it would add 64-bit extensions to its Pentium 4 and Xeon chips in early 2004.

It's unclear how this work will affect the company's Itanium family of processors, which has never sold well, despite yearly improvements. But for customers who want a range of 64-bit platforms from which to choose, Intel's announcement is good news.

Understanding the Windows Versions
Understanding which Windows versions go with which 64-bit platforms can be confusing. On the server end, Microsoft markets Windows 2003 64-Bit Edition for Itanium systems, and the company will release Windows 2003 for 64-Bit Extended Systems, targeting the Opteron and Intel 64-bit x86 platforms. Windows 2003 for 64-Bit Extended Systems is still in beta but should ship by late 2004, according to Microsoft.

On the client side, Microsoft offers its XP 64-Bit Edition for Itanium-based workstations and 64-Bit Edition for 64-Bit Extended Systems for the Athlon 64 and Intel 64-bit x86 platforms. Like its server brethren, this XP version is still in beta testing but will ship by late 2004. But unlike the Itanium versions of Windows, the 64-bit Extended Systems versions will run 32-bit x86 code at full speed, so today's applications will run as usual on these PCs.

Although the increased memory capacity of 64-bit systems will be a boon in certain scientific, analytical, rendering, and database scenarios, most mainstream business users will see little benefit in moving to a 64-bit platform in the near future. Compounding this issue is compatibility: The Itanium platform doesn't offer the performance that users will need to run 32-bit applications effectively. To provide the best of both worlds on the client side, however, either Intel's 64-bit x86 systems or AMD's Athlon 64 will offer leading-edge performance for both 32-bit and 64-bit systems, although you'll need to run XP 64-Bit Edition for 64-Bit Extended Systems to take advantage of 64-bit applications.

Both the 64-bit x86 and Athlon 64 systems are quite a bargain: You can purchase them today for roughly the same price as an equivalent 32-bit PC, but they can grow in the future to accommodate 64-bit applications as well. If you think you'll need 64-bit compatibility on the desktop going forward, I recommend choosing one of these options over the Itanium for the foreseeable future.