Windows has been running on ARM for ages. Windows RT was an early go, but even before that internal builds of Windows for ARM were kept around. Windows Phone 8

Windows has been running on ARM for ages. Windows RT was an early go, but even before that internal builds of Windows for ARM were kept around. Windows Phone 8 ran on the Windows 8 kernel, and Windows 10 Mobile ran on the Win10 kernel. It seems like the hard part here was doing the seamless emulation of x86 apps. Writing a production quality general purpose emulator is hard.