Project XOS or eXperimental Operating System is a platform to develop a simple operating system. The OS is programmed using a custom language known as SPL (System Programmer's Language) and application programs, which run on the OS, are programmed using APL (Application Programmer's Language).
You can check this link http://xosnitc.github.io/ to know more about XOS and developing in XOS.
The set of programs provided here can be used as a guideline for programming.