An operating system is considered real-time if it invariably enables its programs to perform tasks within specific time constraints, usually those expected by the user. To meet this definition, some or all of the following methods are employed:

* The RTOS performs few tasks, thus ensuring that the tasks will always be executed before the deadline
* The RTOS drops or reduces certain functions when they cannot be executed within the time constraints ("load shedding")
* The RTOS monitors input consistently and in a timely manner
* The RTOS monitors resources and can interrupt background processes as needed to ensure real-time execution
* The RTOS anticipates potential requests and frees enough of the system to allow timely reaction to the user's request
* The RTOS keeps track of how much of each resource (CPU time per timeslice, RAM, communications bandwidth, etc.) might possibly be used in the worst-case by the currently-running tasks, and refuses to accept a new task unless it "fits" in the remaining un-allocated resources.

Chapters in this section will discuss how an RTOS works, some general methods for working with an RTOS, and a few popular RTOSes. Finally, in some later chapters, we will discuss how to write your own RTOS.