AROS is an open-source, API-compatible rewrite of the Amiga OS,  the AROS project started in 1995 and settled on Amiga OS API version 3.1 and is designed to be sourcecode-compatible to Amiga programs. It is available for many platforms, native ones (x86, PPC, ARM) and hosted versions which uses the drivers of the host operation system (Windows, Mac OS X, Linux, Android). You have a full window-based operating system, with unlimited virtual screens. It also has an awesome DOS and a special programming language, ARexx, which allows interaction between programs. Because it is a full operating system, it is not limited to any restrictions. It’s also very leightweight, so it even works very fast on older computers. The 64-bit version comes with SMP support and is running on multicore AMD and Intel processors, handling up to 8 cores correctly.