>>In FreeRTOS, a lower priority task only runs if all
>>the higher priority tasks are sleeping.
>
> One scheme I have seen used in a co-operative OS is each task is assigned
> a
> priority number, the larger the number the lower priority of the task.
> Then
> as the task scheduler checks each task, it decrements the priority number
> ,
> and when it reaches 0, that task is run. This means that even the lowest
> priority task will eventually reach a point where it will get an
> opportunity
> to run, even with a large number of high priority tasks. When the task has
> had an opportunity to run, and goes back to sleep, the OS reloads the
> priority number with the original value, ready for the next round of
> decrementing.
>

That's clever! I've just done cooperative multitasking before with equal
priority. In my schemes (like
http://www.piclist.org/techref/microchip/language/c/MultiTask.c.htm), a
task runs until it gets stuck waiting for input. It then sits in a loop
with NextTask() waiting for input. Once it has input, it runs until it
again is waiting for input. On the output side, the tasks generally drive
a fifo (often an input fifo for another task or for an I/O device), so
they do not get stuck there unless the fifo fills.

Harold


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