New Windows on Arm x86 emulator Prism, tested

With the next major update to Windows 11, Microsoft has promised some performance improvements on emulating Intel and AMD code on an Arm-based Windows laptop. The new x86-64 emulator is called Prism, and it’s now available. With a variety of Copilot Plus PCs sporting the new Snapdragon X processor now on the market, we thought we’d take a look at just how much of a difference Prism can make — even on older hardware.

Before we get to our tests, here’s a little background on why this is significant and why emulation continues to be an absolute necessity on Arm-based Windows machines.

For decades, Windows has been synonymous with x86 and x86-64 (AMD and Intel processors, in other words), and all the historic software written for the OS has been compiled, by default, to run on that architecture.

After several false starts meddling with Arm support — remember the Microsoft Surface RT? — we are now actually entering into an era of a dual-architecture Windows ecosystem: x86-64 and Arm-64, or Arch64 as it’s technically known. This is thanks, in part, to Qualcomm’s Snapdragon PC processors, of which Snapdragon X is the newest. The problem is that, without emulation, you can’t run x86/x64 code directly on an Arm processor because they don’t use the same instruction set.

Now, when you compile an application using, for example, Visual Studio (which is available for both architectures), it can produce binaries for both x86-64 and Arm, meaning that the application can run natively on both types of processors. However, this obviously only applies to new and updated applications; many older applications may never be compiled to run natively on Arm.

Although the Windows on Arm initiative has been around for more than seven years, countless apps are still x86-64-only. So, we need an emulator to run those on an Arm processor. Microsoft has supported Arm emulation since Windows 10, but this has always come with a performance penalty. However, now with Windows 11 24H2, it has been revamped, and this revamped version is called Prism.

Why Windows 11 24H2? Because that’s the version these new Copilot Plus PCs are shipping with.

Even without an emulator, Microsoft says that 90% of the total app minutes, meaning where you spend most of your time on a PC, are in apps that have a native Arm version. This claim is based on a survey of user behavior in non-gaming apps in the US and UK. Above is a slide from Arm itself, showing the growing support for Arm-native applications.

On the consumer side, you can see big names like Netflix, Kindle, Handbrake, Unity, VLC, and Microsoft Office are now available in Arm-native versions. Top apps like Google Chrome, WhatsApp, Zoom, Adobe Photoshop, Disney+, Spotify, and many more will run natively on Copilot Plus PCs from day one. So, there’s a lot of software and growing support for Arm-native versions where you won’t need an emulator, but obviously, plenty of software (including games) still require emulation.

During the Copilot Plus PC launch, Microsoft also claimed that emulated apps on PCs running the Snapdragon X Elite are more than two times faster than previous-generation Windows Arm devices running Windows 11 22H2. It states that you’ll see twice the emulated performance if you’re running Windows 11 24H2 on a Snapdragon X Elite platform.

This improvement is not only due to software but also to the Snapdragon X Elite itself. Microsoft compared a Qualcomm 8cx Gen 3 machine to a Snapdragon X Elite, showing big improvements in processor performance and some performance improvements in the Prism emulator. The idea is that today’s user experience will be better than before, and some argue that this matters most. Of course, that’s not an apples-to-apples comparison; to know exactly how much better Prism performs, you’d need to test the same device before and after the update.

To test the performance uplift on the software side, we tested the x86-64 versions of Firefox, Cinebench Release 23, and Handbrake on a Surface Pro X both before and after the Prism update. Here’s what we found:

• Firefox (Speedometer 3): 10%
• Cinebench r23 (Single-core): +8%
• Cinebench r23 (Multi-core): +4.5%
• HandBrake (encoding time in seconds): +8%

Note that for HandBrake, there was an 8% improvement in overall performance. In this case, encoding a file in H.264 using software encoding took just over a minute less on the new Windows 11 24H2 machine.

Remember, these are improvements for existing hardware. Microsoft’s 100% claim is for newer hardware, and to be fair, we have seen more impressive results in our initial testing of Copilot Plus PCs. There are still hiccups, though, so it’s not a truly transformative experience.

Ultimately, it’s nice to see a little boost for everyone using older Windows on Arm machines. Microsoft still has a way to go before the platform is fully realized, but hopefully, it’s at least enough to tide us over until more developers get on board with native Arm apps.