Chris O’Byrne created YAVRTOS for understanding how RTOS can be built and how they Tick.
What is a real-time operating system?
Processors are only actually only able to do one thing at a time, whereas we need them to do more than one thing at a time (e.g. update the display while checking for keypresses while reading the serial port). A real-time operating system (RTOS) provides facilities for achieving that.
What is a task?
A task does one of the things (e.g. update the display, check for keypresses, read the serial port) that the application needs done. The real-time operating system allows tasks to execute concurrently, while providing facilities for e.g. tasks to communicate with each other.
What is a tick?
The RTOS needs to be able to switch between tasks – they way it does this is by using a regular processor interrupt (e.g. one of the timer interrupts). Every such interrupt is called a “tick”. These interrupts must be set up before the RTOS is started.