On Tue, May 26, 2009 at 3:15 PM, Isaac Marino Bavaresco <
isaacbavaresco@yahoo.com.br> wrote:

> Preemption may happen just at specific points (when the macro is
> called), but only if the tick has elapsed, unlike a "Yield" that forces
> a switch every time it is called.
>

Some posts ago I was suggesting something like that and there could be a
small advantage in rare cases where you need to do a loop for example and it
is safe to 'redo' the loop at any point. For example you mark the beginning
of the loop as a safe reentry point and then you do not need to yield the
scheduler at every cycle to make the decision if a task switch is needed --
less overhead for sure.

However, the point is that using an RTOS you are not controlling the
resources directly therefore not polling or wasting the CPU with delay loops
etc. For those you can use the OS provided functions -- if nothing else then
queues or semaphores/mutexes. In that way a cooperative scheduler is just as
fine especially when you have interrupt/event driven routines as well (so
your normal task can still be interrupted by any time by an ISR executing an
event handler and then returning to your task).

As Olin suggested this can be well done by normal programming style using
interrupts and well organised program structure. It may can be another
discussion though whenever in this case you are writing your own scheduler
for yourself without even realizing that it was a scheduler.

Tamas
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