A processor built around the ARM instruction set physically cannot execute a program compiled for a traditional x86 chip, since the two families understand completely different sets of machine level instructions, the same way two people speaking unrelated languages cannot simply understand each other by talking louder. Yet a Windows laptop running on an ARM chip can open an ordinary x86 program and run it successfully anyway, without the program itself being rewritten or even aware anything unusual is happening. The layer of software making that possible translates one processor's language into another's in real time, a genuinely difficult engineering problem that Windows has spent years refining.
Why x86 Software Cannot Simply Run on an ARM Chip Directly
Every processor understands only its own specific instruction set, the fixed vocabulary of low level operations, such as adding two numbers or moving data between memory locations, that its physical circuitry is built to recognize and execute. When software is compiled, the resulting program is translated from human readable code into a sequence of these exact instructions, tailored specifically to one particular processor family. A program compiled for the x86 instruction set that traditional desktop and laptop processors use contains instructions an ARM processor's circuitry simply does not recognize at all, making direct execution physically impossible regardless of how the software itself is packaged or installed.
This incompatibility exists at the most fundamental hardware level, well below anything an operating system alone could normally paper over. Solving it requires either recompiling the original software from source code specifically for the ARM instruction set, which depends entirely on the software's developer choosing to do so, or building a separate layer capable of converting the existing x86 instructions into equivalent ARM instructions on the fly, without needing the original developer's involvement at all. Windows on ARM relies heavily on this second approach specifically because the vast majority of existing Windows software was never compiled with ARM in mind, and waiting for every developer to voluntarily provide a native ARM version was never a realistic path to a usable operating system on day one.
How Dynamic Binary Translation Converts Code on the Fly
The core technique Windows uses to run x86 software on ARM hardware is called dynamic binary translation, a process that examines blocks of the original x86 instructions as the program actually runs and converts each block into an equivalent sequence of ARM64 instructions immediately before executing it, rather than translating the entire program up front before it even starts. This just in time approach mirrors the same general strategy used by just in time compilers in other contexts, translating only the code paths a program actually exercises during a given run rather than exhaustively translating every possible instruction the program could theoretically ever execute.
Because this translation work costs real processing time, Windows caches the translated ARM64 code once it has been produced, storing it so that the next time the same block of original instructions needs to run, whether later in the same session or during an entirely separate future launch of the same program, the system can reuse the already translated version instead of repeating the translation work from scratch. This caching is what allows a program's performance under emulation to noticeably improve after its first run, since the expensive part of the process, the actual instruction translation itself, only has to happen once per unique block of code rather than every single time that code executes.
These caches are organized on a per module basis rather than tied to one single running instance of a program, which means the benefit extends beyond just the specific application that happened to trigger the original translation. If two entirely separate programs both depend on the same shared system library, the translated cache produced the first time either program loads that library can be reused by the other as well, sparing it from repeating translation work someone else's program had already paid for. Windows further applies optimization techniques informed by how code actually behaves once it is running, adjusting the translated output based on real execution patterns rather than treating every instruction block as equally important to optimize aggressively, concentrating the heaviest optimization effort specifically on the code paths a program spends the most time actually running.
What the Emulation Layer Deliberately Does Not Cover
Emulation on Windows for ARM is scoped specifically to ordinary user mode applications, the category of software that runs with standard, restricted permissions and interacts with the rest of the system entirely through defined operating system interfaces rather than reaching directly into hardware. Device drivers and other kernel mode components sit outside this coverage entirely and must be compiled natively for ARM64 to function at all, since kernel mode code operates with a level of trust and direct hardware access that translating on the fly would make both impractically slow and genuinely risky from a stability and security standpoint.
This scoping decision has a very concrete practical consequence: a piece of hardware whose manufacturer has only ever provided an x86 or x64 driver, without a native ARM64 version, simply will not function correctly on a Windows on ARM machine, regardless of how well the emulation layer otherwise performs for ordinary applications. Peripheral support has historically been one of the more persistent friction points for Windows on ARM as a result, improving steadily as more hardware manufacturers produce native ARM64 drivers, but remaining a real, structural limitation the emulation layer was never designed to solve on its own.
How Windows Runs Both 32 Bit and 64 Bit x86 Software Differently
Windows handles emulation of older 32 bit x86 software and modern 64 bit x64 software through two distinct underlying mechanisms rather than one unified approach. The 32 bit case builds on the same compatibility layer, generally called WOW64, that Windows has long used to let 32 bit programs run correctly on 64 bit Windows even without any ARM hardware involved at all, extending that same established redirection mechanism to also handle instruction translation when running on ARM64 hardware, including redirecting file system and registry access exactly the way it always has for 32 bit compatibility on any x64 machine.
The 64 bit case works differently, without relying on that same WOW64 mechanism at all, since 64 bit x64 emulation on ARM was added considerably later. Core Windows system files needed by 64 bit emulated applications are instead built as a special dual format binary that can be loaded directly by both native ARM64 processes and emulated x64 processes from the exact same physical file, letting an emulated 64 bit application access the operating system's genuine file system and registry directly, without the same kind of redirection layer the older 32 bit mechanism depends on.
This distinction matters in practice because it changes how an emulated program actually experiences the rest of the operating system around it. A 32 bit x86 application running through the older mechanism sees a filtered, redirected view of certain system folders and registry locations, the same behavior it would experience on an ordinary 64 bit x64 machine with no ARM hardware involved at all, simply inherited from decades of established 32 bit compatibility handling. A 64 bit x64 application running under the newer mechanism instead interacts with the genuine, unredirected system directly, which reduces the number of edge cases where a program's assumptions about file paths or registry locations end up subtly wrong purely as a side effect of running under emulation rather than natively.
Arm64EC and the Ability to Mix Native and Emulated Code Together
Beyond full emulation, Windows offers developers a distinct migration path called Arm64EC, short for Emulation Compatible, an application binary interface specifically designed to let a single running program mix natively compiled ARM64 code alongside emulated x64 code within the very same process. This allows a developer to recompile only the most performance sensitive parts of an existing x64 application as native ARM64 code, while leaving less critical portions of that same application running under ordinary emulation, without needing to rewrite and fully port the entire application before seeing any performance benefit at all.
This incremental approach reflects a deliberate, pragmatic compromise on Microsoft's part, recognizing that the realistic path for most existing Windows software toward genuine native ARM64 performance runs through gradual, prioritized migration rather than an all at once rewrite most developers would have little incentive to undertake for an already working x64 application. A developer can measure which specific parts of their program are actually consuming the most processing time under emulation and focus native recompilation effort specifically there, capturing much of the available performance benefit while leaving lower priority code running under emulation indefinitely.
The mechanism enabling this mixed execution relies on the compiled program itself carrying markers that let Windows recognize, at a very fine grained level, which specific sections of code have been recompiled as native ARM64 and which sections remain in their original x64 form. When execution reaches a boundary between the two, Windows transitions smoothly between running that code natively and running it under emulation, entirely transparent to the rest of the application, which continues calling into both kinds of code as though no distinction existed at all. This transparency is precisely what makes Arm64EC practical as an incremental migration strategy rather than requiring a hard, all or nothing cutover the moment a developer decides to begin porting an existing application toward native ARM64 performance.
Why Performance Under Emulation Has Improved Significantly Over Time
Early attempts at running x86 software on ARM based Windows devices suffered from performance that lagged noticeably behind genuinely native software, a combination of immature translation technology, ARM hardware that had not yet been specifically tuned with emulation workloads in mind, and Windows components that had not yet been optimized for the emulation scenario at all. More recent generations of the emulation layer have closed much of this gap through several combined improvements working together rather than any single fix: refined translation logic producing more efficient ARM64 output from the same original x86 instructions, expanded support for modern x86 instruction extensions that earlier emulation implementations lacked entirely, and ARM processors themselves increasingly including hardware features specifically useful for accelerating this kind of on the fly instruction translation.
Microsoft has also invested directly in recompiling many of the Windows operating system's own core components as native ARM64 or Arm64EC code, reducing how much of a typical user's overall computing session actually depends on emulation at all, since system level code that would otherwise need translation on every single call now runs natively instead. The practical result of these combined efforts is that a meaningful share of everyday usage on a modern Windows on ARM device increasingly runs on either genuinely native ARM64 software or system components, with emulation reserved specifically for the remaining applications that have not yet been recompiled, rather than emulation carrying the entire weight of the user experience the way it effectively had to during the platform's earliest years.