Wikipedia
DexOS
DexOS is a 32-bit operating system written in x86 assembly. It is designed for coders that want direct access to all hardware (including CPU and graphics), with well-commented code and documentation. It's licensed under a free to use license for commercial and non-commercial use. It is written and maintained by Craig Bamford and other voluntary developers from around the world. DexOS website has been shut down due to account reasons.