About 10 months ago I posted OSle here [1], an OS in x86 assembly that fits in the 510 bytes of a bootloader. It got great discussion and I kept working on it. All userland programs were 16-bit assembly until last week. I started playing around with giving it a C runtime and here we are: you can now write programs for OSle in C. The SDK ships a small C runtime that provides the same API the assembly SDK has — file I/O, process management, screen output. Guest programs compile with a standard toolchain and run on the same 510-byte kernel.

Try it in the browser: https://shikaan.github.io/osle/ [1]: https://news.ycombinator.com/item?id=43866585