Microsoft has delivered its initial Release Candidate (RC1) of Windows XP Professional X64 Edition. The 64bit version of XP is scheduled for release in the middle of next year. The appearance of this prototype does much to suggest this time Microsoft will stick to its promise.

Microsoft has produced 64bit operating systems before, but XP X64 is the first Windows OS designed for PCs based on 64bit CPUs that can also run today's 32bit applications – most notably AMD's 64bit Athlons, which have been on the market since 2003.

Commercial 64bit native applications are not yet available, so we put RC1 through its paces with several popular current (32bit) applications. In general we were impressed.

Only a few technologies from today's XP are absent from the new OS. These include the 16bit subsystem that enables DOS and 16bit Windows applications to run, and legacy network protocols such as AppleTalk and NetBEUI. "This is an opportunity to clean house on some of those items," says Brian Marr, senior product manager for Microsoft's Windows Client Group.

In use, XP X64 feels quite similar to 32bit versions of Windows, with a few noteworthy exceptions. The Start menu, for example, has two renditions of Internet Explorer: a new 64bit version and the same 32bit version that Microsoft shipped in XP SP2. Why? Since 32bit plug-ins for IE won't run in the 64bit version, Microsoft had to retain the 32bit version for people who want to keep using legacy IE add-ons, such as QuickTime or Google Toolbar.

From a performance perspective, XP X64 appears to work as well as 32bit Windows XP Professional on the same hardware. Results were generally close to those achieved by the same desktop running XP Pro: In some cases XP Pro was a bit faster, and in others XP X64 was quicker.

Of course, the biggest benefits of 64bit computing can be realised only when applications capable of taking advantage of its huge memory resources appear. Several companies have offered vague promises about developing native 64bit programs, but concrete plans are rarely available.

Hardware device drivers are another problem. Today's 32bit drivers won't install on XP X64; and even though RC1 includes a wide range of drivers for popular devices, the absence of third-party drivers will torment many users of the new OS.

Generally, XP X64 shapes up as a solid addition to the XP product line. But given the problems that many users are likely to encounter when attempting to install applications and drivers on XP X64, we advise caution to owners of 64bit PCs who are thinking of getting the new OS. As the adage goes, if you need it, you probably already know that you need it.

Owners of 64bit PCs can download RC1 from Microsoft's website.