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µC/OS-III is a preemptive kernel, which means that µC/OS-III always runs the most important task that is ready-to-run as shown in Figure 1-2.
(1)A low-priority task is executing.
(2)An interrupt occurs, and the CPU vectors to the ISR responsible for servicing the interrupting device.
(3)The ISR services the interrupt device, but actually does very little work. The ISR will typically signal or send a message to a higher-priority task that will be responsible for most of the processing of the interrupting device. For example, if the interrupt comes from an Ethernet controller, the ISR simply signals a task, which will process the received packet.
(4)When the ISR finishes, µC/OS-III notices that a more important task has been made ready-to-run by the ISR and will not return to the interrupted task, but instead context switch to the more important task.
(5)The higher-priority task executes and performs the necessary processing in response to the interrupt device.
(6)When the higher-priority task completes its work, it loops back to the beginning of the task code and makes a µC/OS-III function call to wait for the next interrupt from the device.
(7)The low-priority task resumes exactly at the point where it was interrupted, not knowing what happened.
Kernels such as µC/OS-III are also responsible for managing communication between tasks, and managing system resources (memory and I/O devices).
A kernel adds overhead to a system because the services provided by the kernel require time to execute. The amount of overhead depends on how often these services are invoked. In a well-designed application, a kernel uses between 2% and 4% of a CPU’s time. And, since µC/OS-III is software that is added to an application, it requires extra ROM (code space) and RAM (data space).
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