Windows can already run on ARM processors, so moving the operating system from x86 to a mobile-style architecture can look deceptively simple. A modern Windows PC can boot on ARM hardware, launch familiar desktop applications, use a large part of the traditional Windows ecosystem, and translate older x86 and x64 programs when a native ARM build is unavailable. Yet that does not mean the underlying transition is just a matter of recompiling Windows for another processor.
The difficult part is preserving decades of assumptions about instruction sets, drivers, applications, system libraries, installers, security software, hardware interfaces, and performance while the processor underneath follows a different architecture. Windows on ARM already uses a substantial compatibility layer, but that technology also shows where the real complexity lies.
x86 and ARM begin with different assumptions about how software runs
The most obvious difference is the instruction set architecture. Traditional Windows PCs were built around x86 and later x64 processors, while ARM computers use the ARM instruction set. The distinction is deeper than the names of individual instructions. Compilers, operating-system components, application binaries, optimization strategies, and low-level software all have to agree about the processor architecture they target.
A native ARM64 application contains machine code intended for an ARM64 processor. An x64 application contains instructions intended for an x64 processor. The two binaries are not interchangeable simply because both are 64-bit programs.
That matters because Windows is not just a graphical shell. It contains a huge amount of low-level code that communicates directly with the processor and with hardware. An operating system has to manage memory, processes, threads, interrupts, security boundaries, device access, power states, virtualization, and many other functions. Each layer contains assumptions about the environment in which it operates.
This is why the phrase "rewrite Windows for ARM" can be misleading. Much of the operating system can be rebuilt for another instruction set, and Microsoft has already done that. The harder engineering problem is maintaining the enormous ecosystem around Windows while changing the machine code and some low-level behavior underneath it.
The result is a layered approach. Native ARM64 components provide the foundation, while compatibility mechanisms allow older software to keep working. That is far more practical than demanding that every application developer rebuild an existing program at the same time.
Compatibility becomes difficult when old Windows software expects x86 behavior
A major advantage of Windows on ARM is that users do not necessarily have to abandon their existing software. Windows 11 on ARM supports x86 and x64 applications through emulation, while native ARM64 applications can run directly on the processor.
Emulation does not magically turn an x64 program into an ARM program. The operating system has to translate processor instructions from one architecture into instructions the ARM processor understands. Modern Windows uses the Prism emulator for this task. Its basic technique is dynamic translation: blocks of x86 instructions are converted into ARM64 instructions while the program runs, and translated blocks can be cached so the same work does not have to be repeated unnecessarily.
That can work surprisingly well, but it adds another layer between the application and the CPU. A native ARM64 application executes ARM64 instructions directly. An emulated x64 application has to pass through translation machinery before those instructions can execute on the ARM processor.
The difference becomes especially relevant in software that performs large amounts of CPU work, uses unusual processor instructions, depends on precise timing, or interacts with low-level components. A compatibility layer can reproduce a great deal of expected behavior, but it cannot erase every architectural difference.
There is another important boundary: emulation handles user-mode code, not kernel-mode drivers. An ordinary application can often survive the transition through emulation. A driver cannot simply pretend that an x86 or x64 processor exists underneath it. Kernel components have to be compiled for ARM64.
That distinction explains why one Windows program can work perfectly on an ARM computer while a seemingly unrelated utility, peripheral, security component, or specialized device fails.
Drivers are one of the biggest obstacles because they sit below ordinary applications
Applications usually communicate with hardware through operating-system interfaces. Drivers sit much closer to the hardware itself. They translate requests from Windows into operations that a particular device understands.
A printer, storage controller, specialized USB device, security product, virtualization system, or other peripheral can depend on a driver designed specifically for x86 or x64 Windows. If that driver is not available for ARM64, an application cannot simply emulate it in the same way that Windows can emulate ordinary user-mode instructions.
This creates a compatibility chain. The application may be compatible. Windows may be compatible. The processor may be fast enough. Yet the complete product can still fail because one low-level component has not been ported.
That is why ARM compatibility cannot be judged only by asking whether a particular executable starts. The more useful question is whether the entire software and hardware stack has an ARM64 path.
For ordinary office software, browsers, media tools, and many other applications, the situation is increasingly favorable because developers can ship native ARM versions or rely on improved emulation. Specialized hardware is different. A device manufacturer has to create, test, sign, distribute, and maintain an ARM64 driver.
The economics matter as well. A company may decide that supporting a relatively small part of the PC market does not justify years of driver maintenance. That decision can leave an otherwise capable computer with a compatibility gap that Microsoft cannot solve inside Windows alone.
Native ARM applications remove translation and change the performance equation
Emulation is valuable because it protects the existing Windows software ecosystem. Native ARM64 software is valuable because it eliminates the need for instruction translation.
Microsoft distinguishes between the two paths for exactly this reason. x86 and x64 applications can run on ARM, but applications rebuilt specifically for ARM can provide better performance, responsiveness, and battery life. Native code does not need to be translated from another instruction set before execution.
This does not mean that every emulated application is slow. Modern ARM processors can be extremely capable, and the translation technology has improved considerably. Windows 11 24H2 introduced Prism with optimizations intended to improve performance and reduce processor usage for emulated applications.
Still, there is a fundamental tradeoff. Every compatibility layer consumes engineering effort and can introduce overhead. The more software becomes native, the less Windows has to compensate for an architectural mismatch.
The transition therefore has to happen layer by layer. The operating system needs native components. Major applications need ARM64 builds. Development tools need ARM support. Drivers need ARM64 versions. Hardware vendors need compatible firmware and utilities. Software installers and update systems need to understand multiple architectures.
The transition is consequently an ecosystem migration rather than a single rewrite.
Windows has to preserve decades of interfaces while changing the processor underneath
Windows has accumulated an enormous amount of compatibility baggage. That can sound like a disadvantage, but it is also one of the reasons Windows remains useful to people with old software.
A new architecture creates a difficult balancing act. Microsoft can redesign internal components where necessary, but it cannot casually change every external assumption that existing programs depend on. Application programming interfaces, executable formats, system libraries, registry behavior, filesystem behavior, security mechanisms, and compatibility conventions all have to continue working.
Windows on ARM therefore uses carefully designed mechanisms to make older software believe that it is running in a familiar environment. For x86 applications, Windows uses the WOW64 compatibility layer. For x64 applications, Windows uses ARM64X system binaries that can serve both x64 and ARM64 processes from the same locations.
This matters because compatibility is not only about translating CPU instructions. An application also expects a certain operating-system environment. It expects system libraries to expose familiar interfaces, files to appear in expected locations, registry operations to behave consistently, and system calls to have recognizable results.
A successful compatibility layer therefore has to reproduce more than instruction syntax. It has to preserve the contract between applications and Windows.
That contract is enormous.
The hardest cases are programs that reach below the normal application layer
The closer software gets to hardware, the harder an architecture transition becomes.
A conventional desktop application can often be isolated from processor-specific details. It calls Windows APIs, receives data, processes it, and displays a result. If its executable is x86 or x64, Windows can translate its user-mode instructions.
Low-level software has fewer opportunities to hide the architecture. Drivers, hypervisors, some security products, specialized performance tools, and applications with tightly integrated hardware components can depend on processor behavior that cannot simply be reproduced through ordinary application emulation.
Games demonstrate another version of the same problem. A game may use an ordinary executable together with a graphics stack, anti-cheat components, launchers, middleware, and drivers. If even one essential component requires unsupported low-level code, the entire experience can break.
The problem is not necessarily that ARM is incapable of running the workload. The problem is that the software stack was built around assumptions that were never designed for ARM64.
This is why compatibility can improve over time. A program that fails may work after its developer releases an ARM64 build or updates a dependent component. The same principle applies to peripherals: once a manufacturer supplies an ARM64 driver, a previously unusable device can become fully functional.
ARM's mobile heritage is an advantage for efficiency but not a shortcut for Windows compatibility
Calling ARM a "mobile architecture" can create the wrong impression. ARM processors are used across phones, tablets, laptops, servers, embedded systems, and high-performance computers. For modern PC discussions, the relevant characteristic is that ARM-based systems can be designed around strong power efficiency and highly integrated system-on-chip architectures.
An ARM PC platform can combine CPU cores with graphics, wireless connectivity, memory controllers, media engines, and neural processing hardware. That integration can help create thin and quiet computers with long battery life.
But hardware efficiency does not automatically solve software compatibility.
Windows has to understand the specific platform, and applications have to understand the architecture. The processor may offer excellent performance per watt while an application still depends on an x64-only driver. A laptop may have impressive battery life while a specialized utility refuses to install. A program may run through emulation but consume more processor resources than its native ARM64 equivalent.
The hardware and software sides of the transition therefore move at different speeds.
That is why the modern ARM PC strategy is not simply "replace x64 with ARM and everything gets better." Instead, it combines native software, compatibility technology, and gradual ecosystem migration.
Prism makes the transition easier but cannot remove the need for native software
Prism is important because it makes the compatibility layer more efficient. Windows 11 24H2 uses Prism to emulate both x86 and x64 applications on supported ARM systems. The technology includes optimizations around instruction translation and caching, helping existing applications run more efficiently than they would under a simplistic emulator.
The principle remains the same, though. Prism translates code that was written for one processor architecture so that it can execute on another.
Imagine a program as a long set of instructions written in a language understood by an x64 processor. Prism acts as a translator that converts useful blocks into ARM64 instructions. Once those translated blocks have been generated, Windows can cache them and reuse them, reducing repeated translation work.
That is considerably more sophisticated than interpreting every instruction one by one. It allows applications to achieve much better performance than a simplistic emulator would provide.
Yet translation still has boundaries. If software expects a driver to execute x64 kernel code, emulation does not solve the problem. If an application depends on a processor feature that the emulated environment cannot reproduce correctly, compatibility can suffer. If software assumes a particular low-level behavior, the compatibility layer has to reproduce that behavior accurately.
The emulator is therefore best understood as a bridge. It helps existing software cross the architectural gap, but it does not make the gap disappear.
The real transition is from compatibility mode to an ARM-native Windows ecosystem
The long-term goal is not to run every x64 application through emulation forever. A more sustainable model is an ecosystem where the operating system, major applications, development tools, drivers, and hardware utilities are all available natively for ARM64.
That changes the performance equation. Native applications can avoid translation overhead and make better use of the characteristics of the platform. Developers can test ARM-specific behavior directly. Hardware vendors can provide native drivers. Users encounter fewer compatibility surprises.
The transition can already be seen in the growing number of applications with ARM64 versions. Microsoft provides native ARM development tools and maintains compatibility mechanisms for software that has not yet made the move.
This two-track model is what makes the transition practical. Users do not have to wait until every application has been rewritten. Developers can move when it makes sense. Microsoft can improve compatibility while the native ecosystem grows.
The alternative would be a hard break: a new ARM version of Windows that refuses to run the existing x86 and x64 software base. That would produce a cleaner architecture, but it would also destroy much of the reason people choose Windows in the first place.
Compatibility is therefore not an unfortunate side effect of the transition. It is one of its central engineering requirements.
Why a complete switch cannot be reduced to recompiling Windows
Recompiling a large part of Windows for ARM64 is a major engineering task, but it is only one part of the problem.
The operating system has to boot and manage ARM hardware. System libraries have to exist in native form. Applications need compatible interfaces. Drivers need ARM64 versions. Security software has to work correctly. Development environments need native toolchains. Installers and update mechanisms need to handle multiple architectures. Hardware manufacturers have to test firmware and utilities. Software companies have to decide whether and when to release ARM64 builds.
Every dependency adds another point where the transition can stall.
That is why the phrase "rewrite Windows" understates the challenge. Microsoft is not moving a single application from one CPU to another. It is maintaining a platform whose value comes from the interaction of millions of programs, devices, developers, and users.
The processor architecture is only the foundation. The real challenge is everything built on top of it.
ARM is an attractive foundation for efficient PCs, and modern Windows has already demonstrated that an ARM-based version can deliver a familiar desktop experience. But the path from x86 to ARM is difficult precisely because Windows is not an isolated piece of software. It is an ecosystem with decades of accumulated assumptions.
The most successful strategy is therefore not a sudden replacement of x86 with ARM. It is a gradual shift in which native ARM64 software becomes the preferred path while emulation protects compatibility with the existing Windows world.
That approach may look less dramatic than a complete rewrite, but technically it is far more practical. The difficult part was never making Windows start on an ARM processor. The difficult part is making the enormous world around Windows continue to work while the processor underneath it changes.