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The slow but inexorable march to 64-bit computing--the successor to familiar 32-bit x86-based technology, which has dominated the desktop world since the advent of Windows 95 almost a full decade ago--seems poised to speed up. Not only has Microsoft announced its plans to release its first 64-bit operating system for mainstream desktops, Windows XP Professional X64 Edition, by the middle of this year, but also the company has delivered its initial Release Candidate (RC1) of the operating system.

Microsoft has produced 64-bit OSs before, most recently Windows XP 64-Bit Edition for high-end workstations powered by Intel's Itanium CPU, but XP X64 is the first Windows OS designed for PCs based on 64-bit CPUs that can also run today's 32-bit apps--most notably AMD's 64-bit Athlons, which have been on the market since 2003. Only with a 64-bit OS and applications can early adopters enjoy everything 64-bit computing has to offer, including access to up to 16 terabytes of RAM. (RC1 supports only 32GB of RAM--Microsoft says that the shipping OS will support 128GB--as well as 16 terabytes of virtual memory, but that's still vastly greater than the 4GB maximum of today's 32-bit systems.) For users, that translates into greater speed: 64-bit apps won't have to swap large data sets between memory and disk, and will therefore be able to load and process the data faster and more efficiently than 32-bit programs can.

Commercial 64-bit native applications are not yet available, so we put RC1 through its paces with several popular current (32-bit) apps--and in general we were impressed. (Owners of 64-bit PCs can download RC1 from Microsoft's Web site.)

Originally conceived as a stripped-down version of Windows XP (an early beta looked more like Windows 2000), XP X64 was recast early last year after Microsoft concluded that X64-based PCs were likely to become mainstream by the latter part of 2005 (see "Intel, AMD Plan New 64-Bit Desktop CPUs"). Consequently, RC1 is a vast improvement over previous beta releases, offering most of the functionality of today's 32-bit Windows XP Pro with Service Pack 2 (SP2), including the Security Center and pop-up blocking in Internet Explorer. Windows XP X64 RC1 carries the 32-bit version of Windows Media Player 10, too.

Only a few technologies from today's XP are absent from the new OS. These include the 16-bit subsystem that enables DOS and 16-bit Windows applications to run, and legacy network protocols like 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.

Because many 32-bit applications continue to use 16-bit installers, however, they can't be installed on XP X64. (Microsoft says that it's working with application makers to get 32-bit installers completed.)