IAR PowerPac RTOS User Guide

Basic conceptsMAILBOXES AND QUEUESA mailbox is basically a data buffer managed by the RTOS and is used for sending a message to a task. It works withoutconflicts even if multiple tasks and interrupts try to access it simultaneously. IAR PowerPac RTOS also automaticallyactivates any task that is waiting for a message in a mailbox the moment it receives new data and, if necessary,automatically switches to this task.A queue works in a similar manner, but handle larger messages than mailboxes, and every message may have aindividual size.For more information, see the Chapter Mailboxes on page 61 and Chapter Queues on page 71.SEMAPHORESTwo types of semaphores are used for synchronizing tasks and to manage resources. The most common are resourcesemaphores, although counting semaphores are also used. For details and samples, refer to the Chapter Resourcesemaphores on page 49 and Chapter Counting Semaphores on page 55.EVENTSA task can wait for a particular event without using any calculation time. The idea is as simple as it is convincing; thereis no sense in polling if we can simply activate a task the moment the event that it is waiting for occurs. This saves agreat deal of calculation power and ensures that the task can respond to the event without delay. Typical applicationsfor events are those where a task waits for data, a pressed key, a received command or character, or the pulse of anexternal real-time clock.For further details, refer to the Chapter Task events on page 77 and Chapter Event objects on page 83.How task-switching worksA real-time multitasking system lets multiple tasks run like multiple single-task programs, quasi-simultaneously, on asingle CPU. A task consists of three parts in the multitasking world:● The program code, which usually resides in ROM (though it does not have to)● A stack, residing in a RAM area that can be accessed by the stack pointer● A task control block, residing in RAM.The stack has the same function as in a single-task system: storage of return addresses of function calls, parameters andlocal variables, and temporary storage of intermediate calculation results and register values. Each task can have adifferent stack size. More information can be found in chapter Stacks on page 97.The task control block (TCB) is a data structure assigned to a task when it is created. It contains status information ofthe task, including the stack pointer, task priority, current task status (ready, waiting, reason for suspension) and othermanagement data. This information allows an interrupted task to continue execution exactly where it left off. TCBs areonly accessed by the RTOS.Switching stacksThe following diagram demonstrates the process of switching from one stack to another.PPRTOS-217