TABLE OF CONTENTS Recall Figure 4-4 shown previously, the software family located above device drivers covers OS (Operating System) and Firmware.
OS is a higher-level software family responsible for integrating all lower-level software pieces to set up a coherent view of the hardware management. Such responsibility is generally entitled to Operating System or Executive Runtime, which presents a programming interface to higher-level applications.
A key difference exists: Operating System shares CPU time between a large variable set of tasks that are scheduled only when they have something to do, whereas Executive Runtime shares CPU time between a small fixed set of tasks that are called at regular intervals for testing event occurrences and performing potential job. A common point between them is the ability to manage the conflicts of resource accesses for an optimal use of the hardware platform.
Operating System is essentially a piece of complex software for managing task preemption and switching, hardware interrupt dispatches, collaboration between low-level device drivers, etc. The task management role of Operating System is even more distinct when it provides real time functions. Executive Runtime, on the contrary, is considered as a simple task scheduler that neither has potential preemptions between tasks nor interrupt handling; and it has virtually no overhead for task switching. Both of them are of course relatively far from each other in terms of functionality, but they will be considered and described collectively as a single entity called OS to cover the two...
