Tasking Event-B Tutorial

For more information contact Andy Edmunds - University of Southampton - mailto:ae2@ecs.soton.ac.uk

Tasking Event-B Tutorial Overview

This tutorial follows on from the abstract development described here.

This code generation tutorial extends the Heating Controller tutorial example, and makes use of example projects from the download site. The code generation stage produces implementable Ada code, and also an Event-B model. It is a model of the implementation, and contains flow control variables that model the flow of execution through the task body. The Ada code is produced from an intermediate model that is not visible to the user. The Common Language model (CLM), is generated from the Tasking Event-B by a translation tool. Ada (and other implementations) may be generated from the CLM. An overview of Tasking Event-B can be found here.

In the example so far, the Heating Controller has been refined to the point where we wish to add implementation constructs. The Event-B language is not expressive enough to fully describe the implementation. Tasking Event-B facilitates this final step to implementation, by extending Event-B with the necessary constructs. Event-B machines modelling tasks, shared objects and the environment are identified, and extended with the appropriate implementation details.

The example/tutorial projects are are available in the e-prints archive, or on SVN.

Heating_ControllerTutorial_Completed	An example project generating an environment simulation. Generates code, where environment variables are monitored and controlled using subroutine calls. Contains a completed Tasking Development with generated Event-B and Ada code.
Heating_ControllerTutorial_Step1	A bare project for step 1 of the tutorial.
Heating_ControllerTutorial_Step2	A partially completed tasking development for steps 2, and 4 of the tutorial (step 3 not required here).

Using the Tasking Extension

The steps needed to generate code from an Event-B model, in this tutorial, are as follows,

• Step 1 - Add Tasking annotations.
• Step 2 - Invoke translators.

'Flattening' the Implementation Machines

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Providing the correct Composed Machine

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The Temp_Ctrl_Task1Impl Machine

In the partially complete tutorial project we have already identified the Temp_Ctrl_Task1Impl as an Auto Task Tasking Machine, by adding the Auto Task extension. Auto Tasks are tasks that will be declared and defined in the Main procedure of the implementation. The effect of this is that the Auto Tasks are created when the program first loads, and then activated (made ready to run) before the Main procedure body runs. We have set the task type to Periodic, and set a period of 250 milliseconds. We have provided a screenshot of the completed Temp_Ctrl_Task1Impl here, it can be read in conjunction with the tutorial.

The next step is to construct the task body using the TaskBody Editor, with control constructs such as sequence, branch, loop and output. These constructs are discussed in the overview of Tasking Event-B. We will now complete the sequence that has been partially defined in the task body.

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By relating the sensing events in this way we describe a simulation of the interaction between the task and environment. The details of the interaction are embodied in the events themselves; and this is implemented in the simulation code by reading the values of the environment variables.

Next we look at the sensing event TCSense_Temperatures event in Temp_Ctrl_Task1Impl. Sensing (and actuating) can be viewed as a kind of synchronisation. Synchronisation between tasks and shared objects are represented as subroutine calls. The sensing/actuating synchronisations only occur between tasks and the environment.

• Add The Sensed Event Extension.
  • Right-click on the TCSense_Temperatures Event node.
  • Select New Child/Implementation from the menu.
  • Go to the Implementation properties view and set the Implementation Type property to Sensing.

The Shared Machine

The next step is to identify the Shared_Object1Impl machine as a Shared Machine. A screenshot of the Shared_Object1Impl shared machine can be read in conjunction with the text.

• Optionally collapse open branches of the EMF editor to remove clutter.
• Right-click on the Shared_Object Machine node in the Rose Editor.
• Select New Child/Shared Machine from the menu.

The Environ Machine

In the prepared machine we have identified the Envir1Impl as an Environ Machine, by adding the Environ Machine extension. Envir1Impl models a task that simulates the environment, and can be used to generate simulation code. For deployment in a non-simulated environment the environ machine's generated code can be ignored; we provide details of non-simulated code using addressed variables later. As before, a screenshot is available here. In the prepared Environment Machine we have already set task type to Periodic extension, and set a period of 100 milliseconds.

We will now complete the sequence that has been partially defined in the task body. The following specification models simulation of a temperature change; the temperature value is represented by a monitored variable in the environment. The generated code simulates the temperature change in the environment by changing the monitored value.

• Model Temperature Change in the environment.

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• Output to the screen during the simulation can be specified as follows:

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The generated code will print the text, and the value of the variable, to the screen.

The final step is to complete the ENSense_Temperatures event. The event is a sensing event, sensing is a kind of synchronisation, it synchronises with the TCSense_Temperatures event in the Temp_Ctrl_Task1 tasking machine. We add formal parameters annotations corresponding to the actual parameters that we have already defined in the task.

• Add The Sensed Event Extension.
  • Right-click on the ENSense_Temperatures Event node.
  • Select New Child/Implementation from the menu.
  • Go to the Implementation properties view and set the Implementation Type property to Sensing.

We have identified the event as a sensing event. Now we add the parameter direction:

A Summary of Steps

For a Tasking Machine definition:

1. Add the Tasking Machine type (Auto etc).
2. Set the task type (Periodic etc.).
3. Set the task priority.
4. Specify the task body.
5. For sensing/actuating events, add the Event Type.

For a Shared Machine definition:

1. Add the SharedMachine Machine type.

For an Environ Machine definition:

1. Make the type an Environ Machine type.
2. Set the task type Periodic; a shorter period than the shortest task period is best for simulation.
3. Set the task priority.
4. Specify the task body, it will contain a simulation of changes in the environment.
5. For each sensing/actuating event, add the Event Type.

Invoking the Translators

• To Generate Ada,
  • Right-Click on the composed machine, or any machine in the development, select Code Generation/Translate Event-B to Ada.
  • Open the generated code directory in the project to view the source files. A refresh will be necessary to make the code visible. The .gpr file has been provided for AdaCore GPS users.

• To create the Event-B model of the implementation,
  • Right-Click on the composed machine, or any machine in the development, select Code Generation/Translate Tasking Event-B to Event-B.

Optional Annotations for Addressed Variables

To use memory mapped IO (Addressed Variables) in our generated code we can specify which addresses to use in our sensing and actuating events. The addresses are added to the event parameters of a Tasking Machine's sensing and actuating events. The addresses may also be added to the Environ machine's machine variables, for use in simulation. It should be noted that the use of addressed variables, in simulation, has to be done cautiously to prevent memory errors. In the current release the translator generates code for all of these situations, and the environment task should be discarded if simulation is not required.

We now add addressed variable to the TCSense_Temperatures event in Temp_Ctrl_Task1Impl, a PrettyPrint view is available here.

• Add Address Information to Event Parameters.
  • Right-click on the parameter node.
  • Select New Child/Addressed Variable.
  • Go to the Addressed Variable properties view and set the Address and Base properties to appropriate values.

Reads of the monitored variables of the sensing event can therefore be made directly from the address specified. Their is also a base property which can be set to indicate the base of the property value. The default value is 16. The environment simulation may also make use of addressed variables, but in this case the extension is made to the Environ Machine machine variables and used as shown here.

Invocation of the translators proceeds as detailed above.

Generated Code

The Ada Code generated by the translator is available at the following links:

for simulation of environment without addressed variables, Heating_ControllerTutorial_Completed

for simulation of environment with addressed variables, Heating_Controller5AddressedSim_Completed

Removal of the environment task from the Heating_Controller5AddressedSim_Completed should be deployable.