The code in Resched,
when deciding on a new process to execute,
does not bother to verify if the ready list is empty.
It
assumes that one process is always available.
To ensure that a ready process always exists,
Xinu creates an extra process, called the null process,
when it initializes the system.
This process
has
process id 0,
its code consists of an infinite loop,
and it has
priority zero
(why?).