ARM : RTX

I would never do this but you did ask. I don't think this is a good use of tasks.

Create a controll task with say a priority of 200

set you os_tick_tmr to be 1ms

// this is the controll task running at priority 200

create tasks a..d with priority 100 (save the tid's)

while (1)
{
    os_tsk_prio(task_id_of_next_task_to_run,150)
    os_dly_wait(1);
}

if any task calls os_block - save the controll task which definitely does (you did not state that this was the case) then a task that is priority 100 will run and you may not easily be able to tell which one.

One of the main advantages of using tasks is that they may call os_block and allow another task to run. It might be best to have the communications task be the highest priority and have it os_block, waiting for a communication event to happen and wake up to service that event. Then schedule your other tasks to be round robin and switch every 1ms, if that is really what you need.

Every time I look at this I just see more and more problems going down this path. But once again you did ask.