Then and Now, Part 1: RT (Premium)

A lot has changed since Microsoft first announced that it would port Windows to the Arm processor architecture. I've referenced some of this history throughout my recent writing about Windows 11 on Arm, Qualcomm's Snapdragon X chips, and Copilot+ PCs as needed for context and perspective. But the full story is, I think, fascinating, and could become as important to the history of Windows as the shift from MS-DOS, the development of NT, or Windows 95.

That story could be told different ways. But there have been three major milestones in the history of Windows on Arm, the initial reveal in February 2012, the Windows 10 on Arm announcement in late 2016, and the beginning of the Snapdragon X-based Copilot+ PC era in mid-2024. So that makes some sense to me: Three separate stories that together form a cohesive whole. This is part one.

That said, there are also threads that run throughout this story. For example, Intel—the dominant chipmaker in the PC market—has sought to undermine Arm at every turn. Unfortunately, it has done so to date solely by flexing its market power: Each time Microsoft announced a major Arm milestone, Intel was on hand, like some uninvited house guest, to remind the world that it wasn't going anywhere. This is a major part of the Windows on Arm story as well, which makes sense: Moving to Arm is all about embracing mobility and efficiency and rejecting the past.

Rejecting the past can be difficult.
An architecture for the future
Many readers are familiar with how NT was originally designed for non-Intel processors, and that the team was forced to embrace Intel's 32-bit x86 architecture because it was ubiquitous with PCs. Likewise, many will point to the various platforms to which NT-based Windows versions were ported over the years.

But less well known, or perhaps less well-remembered, is that Microsoft also adapted Windows—before NT and since—to a series of architectural shifts within the x86 architecture. After initially supporting the 8/16-bit Intel 8088 processor, subsequent Windows versions were tailored for the unique architectures of the 16-bit Intel 80286 and its segmented memory model, the 32-bit 808386 and its flat memory model, and then finally 64-bit x86 ("x64") designs that continued with a flat memory model but added 64-bit pointers and data types.

The transition to 64 bits began with Windows XP, initially for the Intel/HP Itanium, but later for x86-64, first via Windows XP Professional x64 Edition. That shift, which led to the modern 64-bit Windows 11 we use today, is particularly relevant to this story, and for two reasons.

Even on x86, 64-bit Windows versions run 32-bit code using an emulator called WOW64 (Windows on Windows 64-bit) that, among other things, abstracts the hardware for 32-bit apps and makes the system appear to them like a native 32-bit version of Windows. This design choice was tied to how Microsoft implemented backward compatibility with 32-bit x86 apps on the 64-b...