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Tasks and Co-routines
[Getting Started]

See the How FreeRTOS Works section for an introduction to basic multitasking concepts.

The Tasks and Co-Routine documentation pages provide information to allow judgment as to when co-routine use may and may not be appropriate. Below is a brief summary. Note that an application can be designed using just tasks, just co-routines, or a mixture of both - however tasks and co-routines use different API functions and therefore a queue (or semaphore) cannot be used to pass data from a task to a co-routine or vice versa.

Co-routines are really only intended for use on very small processors that have severe RAM constraints.

Characteristics of a 'Task'

In brief: A real time application that uses an RTOS can be structured as a set of independent tasks. Each task executes within its own context with no coincidental dependency on other tasks within the system or the RTOS scheduler itself. Only one task within the application can be executing at any point in time and the real time RTOS scheduler is responsible for deciding which task this should be. The RTOS scheduler may therefore repeatedly start and stop each task (swap each task in and out) as the application executes. As a task has no knowledge of the RTOS scheduler activity it is the responsibility of the real time RTOS scheduler to ensure that the processor context (register values, stack contents, etc) when a task is swapped in is exactly that as when the same task was swapped out. To achieve this each task is provided with its own stack. When the task is swapped out the execution context is saved to the stack of that task so it can also be exactly restored when the same task is later swapped back in. See the How FreeRTOS Works section for more information.

Task Summary

No restrictions on use.
Supports full preemption.
Fully prioritised.
Each task maintains its own stack resulting in higher RAM usage.
Re-entrancy must be carefully considered if using preemption.

Characteristics of a 'Co-routine'

Note: Co-routines were implemented for use on very small devices, but are very rarely used in the field these days. For that reason, while there are no plans to remove co-routines from the code, there are also no plans to develop them further.

Co-routines are conceptually similar to tasks but have the following fundamental differences (elaborated further on the co-routine documentation page):

  1. Stack usage

    All the co-routines within an application share a single stack. This greatly reduces the amount of RAM required compared to a similar application written using tasks.

  2. Scheduling and priorities

    Co-routines use prioritised cooperative scheduling with respect to other co-routines, but can be included in an application that uses preemptive tasks.

  3. Macro implementation

    The co-routine implementation is provided through a set of macros.

  4. Restrictions on use

    The reduction in RAM usage comes at the cost of some stringent restrictions in how co-routines can be structured.

Co-Routine Summary

Sharing a stack between co-routines results in much lower RAM usage.
Cooperative operation makes re-entrancy less of an issue.
Very portable across architectures.
Fully prioritised relative to other co-routines, but can always be preempted by tasks if the two are mixed.
Lack of stack requires special consideration.
Restrictions on where API calls can be made.
Co-operative operation only amongst co-routines themselves.

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Copyright (C) 2004-2010 Richard Barry. Copyright (C) 2010-2016 Real Time Engineers Ltd.
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