ReactOS 0.4.3 was released recently. It is a free and open source alternative to Microsoft Windows. It is not based on Windows code and uses API calls to run Windows software. It is based on WINE for Linux that runs Windows apps under Linux.
ReactOS 0.4.3 has an NTVDM layer that runs 16 bit DOS and 16 bit Windows apps. The 64-bit version of Windows can’t run 16-bit apps anymore. They had to break 16-bit compatibility in 64-bit mode to get more performance.
ReactOS 0.4.3 added better compatibility with DOS and Windows games. It is trying to replace Windows XP, while modern Windows are Windows 7, 8.1, and 10 that run more modern programs. So ReactOS is better off with older Windows programs.