With Windows XP, Microsoft created specific Windows product editions, or SKUs, that targeted 64-bit processors from Intel and AMD. One of these product editions, Windows XP Professional x64 Edition (see my preview and mini-review), is the most interesting of these because it provides functional parity with the mainstream 32-bit version of XP Professional (with a few key but understandable exceptions including 16-bit support). XP x64 is a surprisingly stable and well-designed product, but it was doomed to failure because of hardware and software incompatibilities: Unlike with 32-bit XP versions, many hardware devices will not work on XP x64 because of a dearth of 64-bit drivers (32-bit drivers will not work in a native 64-bit OS). Likewise, many software applications will not install or run because of various issues, including a surprising amount of 16-bit application installers and poorly-designed version detection.

The x64 versions of Windows Vista adopt virtually none of the negative quirks of their XP x64 predecessor, bringing new levels of hardware and software compatibility to the mix. Like XP x64, the various Vista x64 versions support x64-compatible PCs based on the AMD-64 (Athlon-64, Opteron, Turion processors) and Intel EMT-64 (Pentium D, Xeon, Core Duo, and Core 2 Duo) platforms. But unlike XP x64, these 64-bit Vista versions are now viable for mainstream use.

Tip: Windows Vista Ultimate comes with both 32-bit (x86) and 64-bit (x64) versions in the box, on separate DVDs; purchasers of other retail 32-bit Vista versions can order the 64-bit version from Microsoft for a nominal fee.

64-bit versions of Windows Vista are functionally identical to their 32-bit counterparts, though the 64-bit versions support a few unique features as outlined in this article. So the 32-bit version of Windows Vista Home Premium, for example, offers essentially identical functionality to Windows Vista Home Premium x64.

For the most part, the x64 support in each Vista x64 edition is identical. One exception is that they each support different amounts of RAM. Vista Home Basic (and Home Basic N) support up to 8 GB of RAM, compared to 4 GB for all 32-bit versions of Vista. Home Premium, meanwhile, supports 16 GB. And Business (and Business N), Enterprise, and Ultimate all support 128 GB or more of memory. (The "or more" bit refers to the fact that there are currently no PCs available yet that suport over 128 GB of RAM; when that happens, these Vista versions will support it.)

Security features

One of the primary benefits of using an x64-based version of Windows Vista is that these versions provide somewhat improved security functionality when compared to their 32-bit counterparts. Working in tandem with the No Execute (NX) technologies in modern x64 microprocessors from both AMD and Intel, Windows Vista x64 versions, like XP x64, also provide support for hardware-backed Data Execution Protection (DEP), which helps to prevent the buffer overflows that are commonly used in electronic attacks. (32-bit Vista versions utilize a less effective, software-based version of DEP.)

Another unique x64 feature, Kernel Patch Protection (sometimes called PatchGuard), prevents malicious software from patching the Windows Vista kernel. PatchGuard, Microsoft says, works by preventing kernel-mode drivers from extending or replacing other kernel services and preventing third-party software from patching any part of the kernel.

Before Windows Vista was finalized, Kernel Patch Protection came under fire from security software vendors who complained that this feature prevented them from patching the Vista kernel and thus protecting users from kernel-based attacked. After some deliberation, Microsoft agreed to let these companies patch the Vista kernel. This change became available as part of Windows Vista Service Pack 1 (SP1).

Additionally, Windows Vista x64 versions require that all drivers be digitally signed by the developer. If you've ever installed a driver in Windows XP, you'll likely be familiar with the unsigned driver dialog, which offers a "Continue Anyway" option when a setup application attempts to load an unsigned driver. In the x64 versions of Windows Vista, this will not be possible. Microsoft says that it is requiring signed drivers so that it can ensure that drivers are of the highest possible quality; poorly-written drivers are, today, still the leading causes of blue screens and other system instability issues.

When taken together, these features ensure that the x64 versions of Windows Vista are the most secure and reliable desktop versions of Windows ever created. Likewise, they are more secure and reliable than the 32-bit Vista versions.

Other unique x64 features

The x64 Vista versions include a few esoteric and unique features that will likely be of interest only to the corporate market. For example, a service called Subsystem for UNIX-based Applications (SUA, previously called Windows Services for UNIX) is included with Vista Enterprise x64 and Ultimate x64 and provides a native 64-bit POSIX (Portable Operating System Interface) subsystem on x64 versions of Windows Vista, the first time such technology has been made available by Microsoft.

Fun fact: Microsoft originally intended to provide another compatibility feature called Virtual PC Express in the x64 versions of Windows Vista Enteprise and Ultimate. However, this feature was removed when the company decided instead to make Virtual PC free to all Windows users. There is a native x64 version of Virtual PC available for Vista x64 users.

Compatibility issues and other limitations

With the initial shipping (RTM) version of Windows Vista, the various benefits of the x64 Vista versions were somewhat counterbalanced by a number of limitations, the most important of which were compatibility issues. 16-bit applications are not supported, which is less problematic than it was a few years ago, but still an issue for some applications that use legacy application installers. 32-bit device drivers are not supported, so you can't use any of the existing hardware drivers out there, but must instead use native x64-based drivers. This situation improved dramatically over the first 12 to 18 months that Vista was on the market, however. So by the time that SP1 shipped, most compatibility issues were gone. Today, there is no reason to opt out of Vista x64 because of comaptibility.

Those hoping to upgrade should be aware of a few remaining issues, however. 32-bit versions of XP can only be upgraded to 32-bit versions of Windows Vista. And Windows XP Professional x64 Edition can only be upgraded to 64-bit versions of Windows Vista (Business and above). There is no way to do an in-place upgrade of 32-bit versions of Windows Vista to any 64-bit version; you'll need to backup data and install from scratch.

Finally, it's worth noting that while Microsoft is proudly trumpeting the fact that Vista's new modular architecture will allow corporations to rollout Vista to multiple desktop types using only a single Windows Imaging Format (WIM)-based installation image, the truth is a bit more complicated. Companies that plan to rollout both 32-bit and 64-bit Vista versions will need to maintain separate install images for both 32-bit and x64 Vista versions.

Final thoughts

Unlike with Windows XP Professional x64 Edition, the various Windows Vista x64 versions no longer represent a compromise of sorts: You can now safely choose a 64-bit version of Windows Vista and take advantage of over 4 GB of RAM, if needed, as well as that system's improved security and reliability.

Think of Vista as the "line in the sand" for the x64 platform on the client: Post-Vista, x64 Windows versions will outsell their 32-bit brethren, as any remaining compatibility issues will be resolved or rendered moot by new hardware and software versions that are more x64-savvy. Hopefully with Windows 7, the successor to Windows Vista, Microsoft will allow for migrating 32-bit systems to x64 versions. In the meantime, Vista x64 is here now and works surprisingly well.