Switching from high priority task to low priority task in uCOS II

Question

I'm new to RTOS (uCOS II) and learning it by reading the book written by uCOS author. I have a doubt and I'm unable to find the answer to it.

In uCOS the task with highest priority is given CPU as per the scheduling algorithm. So, if I create write a uCOS application by creating two tasks One with High priority ( Prio = 1 for ex) and the other with low priority ( for ex Prio = 9). If for example the highest priority task is waiting for an event, then the scheduler should start executing the next higher priority task ? If thats correct then what part of the code switches High priority with low priority ?

The three arch dependent codes are : 1. Interrupt level context switch 2. Start highest priority task ready to run 3. Task level context switch

In case 1 after serving the interrupt the scheduler returns to the highest priority task. In case 2, its called when we start the OS by OSStart() In case 3, When ever a higher priority task is made ready and its called by timer interrupt

Now, where exactly or how exactly will the scheduler assigns CPU to a lower priority task given the high priority task is in wait ??

Thanks

Answer 1

Another way to consider your question is to ask yourself how did the high priority task get into the waiting state. The answer to both questions is that the high priority task calls an RTOS routine such as GetEvent(). (I don't know whether that is a real uCOS-II routine -- I'm just generalizing.). The RTOS routine puts the high priority task into the waiting state (i.e. blocked) and then the RTOS scheduler finds the next highest priority task that is ready to run and switches to that task's context. The RTOS will have several blocking functions that allow for a task context switch. For example when you read from a queue or mailbox or when you wait for a semaphore or mutex.

Answer 2

The scheduler runs whenever a scheduling event occurs. In your example, that occurs when the high priority task calls the event wait. In general OS calls that may block or yield cause the scheduler to run. The scheduler also runs on exit from ISRs including the IS timer ISR.

In general, when the scheduler performs a context switch, it copies the current processor core registers to the task's control block, and copies the stored register values for the task being switched to into the processor registers, with the stack pointer and program-counter copies last. The change to the program-counter causes execution to continue in the new task with the task's own stack, in the state it was when it last blocked or was preemted. Preemption can occur when a scheduling event occurs in an ISR that causes a higher priority task to become ready.

The thing about uC/OS-II is that it is described in intricate detail in Jean Labrosse's book. The general principles of RTOS with examples using uC/OS-II are described in this online course by Jack Ganssle.

Answer 3

Interrupt level context switch is used for preemptive, for example, you have an low priority task running, and high priority need to run (OSTimeDly timeout, for example), in this situation, Interrupt level context switch will pause low priority task, then switch to high priority one.

For high to low priority switch, it need high one give up CPU resource by calling OS_Sched