Microsoft released an updated version for Windows Server 2003 (NT 5.2) called . It was stable and powerful, but the ecosystem was dead. AMD saw the opening and struck. The Game Changer: AMD64 and Windows XP x64 Edition In 2003, AMD released the Opteron and then Athlon 64, introducing AMD64 (later called x86-64). This brilliant design extended the classic x86 instruction set to 64 bits while preserving full, fast, native 32-bit compatibility . Intel, embarrassed, was forced to adopt it under the name Intel 64. Microsoft, having burned its hands on Itanium, pivoted quickly.

Microsoft is now facing the next frontier: and possibly 128-bit computing. While a 128-bit Windows seems distant (memory capacities would need to exceed 16 exabytes), the lessons learned from the Itanium disaster—never break backward compatibility, always provide a seamless thunking layer, and let the hardware market mature before forcing the OS—are baked deeply into the engineering culture of Windows NT.

This era ended when DEC faltered, and Intel, pushing its own ill-fated 64-bit architecture (IA-64 / Itanium), forced Microsoft to choose sides. By 1999, support for Alpha was dropped. Intel’s Itanium (IA-64) was a pure 64-bit architecture that abandoned x86 backward compatibility entirely. It relied on a complex technology called EPIC (Explicitly Parallel Instruction Computing). Microsoft, needing Intel’s volume, committed fully. Windows 2000 (NT 5.0) had a limited, unreleased 64-bit version for Itanium. But the first commercially available 64-bit Windows was Windows XP 64-Bit Edition for Itanium-based Systems (2001), based on the same codebase as Windows XP (NT 5.1).

This was a true 64-bit operating system with a native 64-bit kernel, 64-bit system processes (like the Session Manager and Plug and Play), and support for a massive 16 terabytes of virtual memory. However, it was a commercial disaster. Because Itanium could not run legacy x86 code efficiently (using a slow software emulation layer), users found that their existing 32-bit applications ran like molasses. Moreover, device drivers had to be rewritten for IA-64, a market that never materialized outside of high-end servers.

The DEC Alpha was, in many ways, the first true 64-bit platform for NT. The Alpha 21064, released in 1992, was a native 64-bit processor. Microsoft and DEC had a tight partnership: Windows NT was the premier OS for Alpha workstations. For a brief period in the mid-1990s, if you wanted raw 64-bit computing power for scientific or engineering tasks, you ran Windows NT 4.0 on an Alpha. However, these systems were not what we call "64-bit Windows" today in the consumer sense. They ran 32-bit NT code compiled for Alpha, but the kernel and drivers could take advantage of 64-bit registers and memory addressing. The user experience was identical to 32-bit x86 NT, but under the hood, the HAL (Hardware Abstraction Layer) was managing a 64-bit address space.

In conclusion, 64-bit Windows NT is not a single product but a living architecture that began with a portable kernel on RISC workstations, stumbled through Itanium’s noble but failed purity, found its savior in AMD’s pragmatic x86-64, and finally reached ubiquity in the last decade. Every time you open Task Manager on a modern PC and see "64-bit operating system, x64-based processor," you are looking at the result of a thirty-year war for memory addressing—a war that Windows NT ultimately won by refusing to abandon its users, even as it rewired its deepest foundations.