Currently, the first Windows 10 on ARM PCs can run 32-bit ARM applications, but not apps designed to run on 64-bit ARM processors. The new SDK will make good on Microsoft's promise from late last year to support 64-bit ARM apps.
What this SDK won't do, however, is enable x64-based applications to run on Windows 10 on ARM devices. As Microsoft officials have said, the company does support x86 apps running in emulation on ARM, thanks to its Windows-on-Windows (WoW) compatibility layer.
The Win32 emulation for these ARM-based devices is meant to win over those who need Win32 apps that may or may not be available in the Store. Microsoft is touting the availability of key, commonly used Win32 apps, especially full Microsoft Office, as working well on these devices. Over time, Microsoft and partners plan to broaden the set of tested Win32 apps that will work well via emulation, officials have said.
Microsoft has no plans to try to add x64 app emulation, as performance would be pretty terrible, officials have said. Instead, the company is trying to convince developers to recompile their apps to run natively on 64-bit ARM chips and have them available in the Microsoft Store. The new SDK is meant to try to make that more appealing to developers.
I've asked Microsoft officials if they've got further details to share about the SDK for 64-bit ARM apps. No word back so far.
Update: So there may be a bit more to this SDK than the original report indicated. Here's what a Microsoft spokesperson said is coming at Build:
"To clarify, Microsoft is planning to release a preview of the Windows 10 ARM64 SDK for Store and desktop apps, allowing developers to recompile their win32 desktop apps to ARM64 so they can run natively without emulation. With the SDK, x64 apps and x86 apps will also be able to recompile to ARM64 and run natively. We will be sharing more details on the ARM64 SDK Preview at Build."
It's the second part of this statement that might (or might not) be new. I'm not sure if we knew Microsoft was working on a way to allow x64-based apps to be recompiled for ARM64.