New Windows emulator on Arm x86 Prism, tested

With the next major update to Windows 11, Microsoft has promised some performance improvements for emulating Intel and AMD code on an Arm-based Windows laptop. The new x86-64 emulator is called Prism and is now available. With a variety of Copilot Plus PCs powered by 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 move on to our testing, here’s a quick overview of why this is important 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 historical software written for the operating system has been compiled, by default, to run on that architecture.

After several false starts, interference with Arm support, remember Microsoft Surface RT? – we are now entering the era of a dual-architecture Windows ecosystem: x86-64 and Arm-64, or Arch64 as it is technically called. This is due, in part, to Qualcomm’s Snapdragon PC processors, of which the Snapdragon X is the most recent. 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, say, Visual Studio (available for both architectures), it can produce binaries for x86-64 and Arm, meaning the application can run natively on both types of processors. However, this obviously only applies to new and updated apps; many older applications may never be compiled to run natively on Arm.

Although the Windows on Arm initiative has been around for over seven years, many applications are still x86-64 only. So we need an emulator to run them on an Arm processor. Microsoft has supported Arm emulation since Windows 10, but it has always resulted in performance hits. However, 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 ship with.

Even without an emulator, Microsoft claims that 90% of total app minutes—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 for non-gaming apps in the US and UK. Above is a slide from Arm itself, showing the growing support for native Arm apps.

On the consumer side, you can see that 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 more will run natively on Copilot Plus PCs from day one. So there’s plenty of software and growing support for Arm-native versions that you won’t need an emulator for, but obviously, there’s plenty of software (including games) that still require emulation.

During the launch of Copilot Plus PC, Microsoft also claimed that apps emulated on PCs running Snapdragon X Elite were more than twice as fast as previous generation Windows Arm devices running Windows 11 22H2. It says 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 the 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 CPU 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’s what matters most. Of course, this isn’t an apples-to-apples comparison; To know exactly how much better Prism is, you need to test the same device before and after the update.

To test the performance improvement on the software side, we tested x86-64 versions of Firefox, Cinebench Release 23, and Handbrake on a Surface Pro X 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, overall performance has been improved by 8%. In this case, encoding a file to H.264 using software encoding took just over a minute less on the new Windows 11 24H2 machine.

Keep in mind that these are improvements to existing hardware. Microsoft’s 100% claim is for newer hardware, and to be fair, we’ve seen more impressive results in our early testing on Copilot Plus PCs. There are still some hiccups, though, so it’s not a truly transformative experience.

Ultimately, it’s nice to see a little boost for anyone running older Windows machines on Arm. Microsoft still has a ways to go before the platform is fully realized, but hopefully this will at least be enough to tide us over until more developers jump into native Arm apps.