Manux is an open-source, single-tasking and minimalist hobby kernel written in Z80 assembly and C. Currently, the kernel supports basic IO and monotasking, but new features will be added in the future. The kernel is designed to be loadable from BASIC.
The kernel is written in a combination of Z80 assembly and C, and can be built with the z88dk toolchain. A minimal kernel can be built with any Z80 assembler, without including the C sources.
Manux can run very simple POSIX programs. However, it's not fully POSIX-compliant, so it may not work with some programs. Currently the supported features are input/output and fork.