Code Generation Tutorial

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Tutorial Overview

The aim of the tutorial is to allow users to explore the approach with a relatively simple example. The example uses a shared buffer with reader and writer processes. The tutorial is presented in three stages, making use of the example projects from the download site. There are two translations performed, one is to a common language model (IL1). The second is to an Event-B project which includes a model of the implementation. There is a PrettyPrinter for Ada source code, which uses the common language model. An overview of Tasking Event-B can be found at http://wiki.event-b.org/index.php/Tasking_Event-B_Overview.

A typical Event-B development may be refined to the point where it is ready for implementation, but 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 that are to be implemented (and their seen Contexts) are selected and added to a Tasking Development; the Tasking Development files have the file extension .tasking. The machines in the Tasking Development are then extended with implementation details.

The example/tutorial projects are,

SharedBuffer20100819Demo	An example project with a completed Tasking Development and IL1 model (post IL1 translation, but before Event-B translation).
Sharedbuffer20100819Tasking	Same as the example project above, but with Event-B model translations. The difference being that this development includes a model of the implementation. These are refinements that include a program counter to describe flow of execution in each task.
SharedBuffer20100819Tutorial	A bare project for step 1 of the tutorial.
Sharedbuffer20100819Tutorial2	A partially completed tasking development for steps 2 and 3 of the tutorial.

Preliminaries

Before further discussion of the modelling aspects, we take a look at the PrettyPrint viewers. The PrettyPrinters make the viewing of IL1 and tasking models easier; it also provides a route to generate source code. The source code can easily be pasted from the IL1 Pretty Printer window into an the Ada source file .

The PrettyPrint View of a Tasking Development

To open the Tasking PrettyPrint viewer,

• from the top-menu select Window/Show View/Other/Tasking Pretty Printer.

Note that the Tasking PrettyPrinter may have to be closed when editing the Tasking Development, since it can give rise to exceptions. The PrettyPrinter would need further work to make it robust, however it is intended only as a short-term solution.

• Open the SharedBuffer20100819Demo Project and switch to the Resource Perspective.
• Open the .tasking model and inspect it. Clicking on the Main, Machine or Event nodes updates the pretty print window.

Viewing Source Code

aka. The PrettyPrint View of an IL1 Model.

To view Ada source code,

• from the top-menu select Window/Show View/Other/IL1 Pretty Printer.
• Open the SharedBuffer20100819Demo Project and switch to the Resource Perspective.
• Open the .il1 model and inspect it. Clicking on the Protected, Main Entry, or Task nodes updates the pretty print window.

Cleaning the Tasking Development

If the .tasking file has errors, then it may need cleaning. To do this right-click on the Main node, select Epsilon Translation/CleanUp. If a model has errors it can still be viewed by clicking on the Selection tab at the bottom of the tasking editor window.

The Tutorial

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

• Step 1 - Create the tasking development.
• Step 2 - Add annotations
• Step 3 - Invoke translators.

Creating The Tasking Development

• Change to the Event-B Perspective.
• Open the SharedBuffer20100819Tutorial Project.
• Select the following Machines: Reader, Writer and Shared.
• Right-click and select Make Tasking Development/Generate Tasking Development.

The new Tasking Development will not be visible in the Event-B perspective, change to the resource perspective, open and inspect the new .tasking file. The Tasking Development contains (the EMF representation of) the machines that we wish to provide implementations for. In order to introduce the new concepts we have prepared a partially complete development.

Change to the Project SharedBuffer20100819Tutorial2 to begin the next step.

Providing the Annotations for Implementations

• Close any Tasking Pretty Print Viewers that remain open. The incomplete model will give rise to exceptions.
• Go to the to the Resource Perspective.
• Open and inspect the .tasking machine.

The WriterTsk and SharedObj machines are incomplete. We will take the steps to necessary to provide implementation details.

The WriterTsk Machine

In the partially complete tutorial project we already identified the WriterTsk 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 added the Periodic Task extension to the Auto Task, and set a period of 250 milliseconds. We will now complete the sequence that has been partially defined in the task body.

• Add Synchronisation between TWrite and SWrite.
  • Expand the Auto Task Machine node.
  • Expand the Seq sub-tree.
  • Right-click on the Seq node and select New Child/Left Branch EventWrapper.
  • Provide the event label w1 using the properties view.
  • Right-click on Event Wrapper and select New Child/ Synch Events.
  • Select Synch Events and go to the drop-down menu of the Local Event property.
  • At this point the drop-down box displays a number of event names, select the TWrite event.
  • Go to the drop-down menu of the Remote Event property.
  • From the list of events select the SWrite event.

The Synch Events construct is used to implement Event Synchronisation. The next step wraps an event in an Event Wrapper in order to update the local state; there is no synchronisation as such but we will re-use the constructs that already exist.

• Add the Wrapped Event TcalcWVal.
  • Expand the sub-tree of the second Seq node.
  • Right-click on the Seq node and select New Child/Left Branch EventWrapper.
  • Provide the event label w2 using the properties view.
  • Right-click on Event Wrapper and select New Child/ Synch Events.
  • Select Synch Events and go to the drop-down menu of the Local Event property.
  • From the list of events select the TcalcWVal event.

We have now completed the task body, and it just remains to complete provide details for the TWrite event. The TWrite event in WriterTsk is to be synchronized with the SWrite event in the SharedObj.

• Add Event Extensions.
  • Right-click on the TWrite Event node.
  • Select New Child/Extension.
  • Right-click on the Extension node and select New Child/Implementation from the menu.
  • Go to the Implementation properties view and set the Implementation Type property to ProcedureSynch.

• Identify Incoming and Outgoing parameters.
  • Right-click on the outAP node and add an Extension.
  • Right-click on the Extension and selectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to actualOut.
  • Right-click on the inAP node and add an Extension.
  • Right-click on the Extension and selectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to actualIn.

The Shared Machine

The next step is to identify the SharedObj machine as a Shared Machine. The SharedObj Machine will be extended using the Event-B EMF extension mechanism.

• Right-click on the SharedObj Machine node in the .tasking file.
• Select New Child/Extension.
• Right-click on the Extension node and select New Child/Shared Machine from the menu.

We now show how to extend the SWrite event of the Shared Machine with details about its implementation. The SWrite event in SharedObj is to be synchronized with the TWrite event in the WriterTsk.

• Identify SWrite as a Syncronisation.
  • Right-click on the SWrite Event node.
  • Select New Child/Extension.
  • Right-click on the Extension node and select New Child/Implementation from the menu.
  • Go to the Implementation properties view and set the Implementation Type property to ProcedureSynch.

• Identify incoming and outgoing parameters.
  • Right-click on the inFP node and add an Extension.
  • Right-click on the Extension and selectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to formalIn.
  • Right-click on the outFP node and add an Extension.
  • Right-click on the Extension and selectNew Child/Parameter Type.
  • Go to the Parameter Type properties view and set the Parameter Type property to formalOut.

A Summary of Steps

For a Tasking Machine definition:

1. Add the Tasking Machine type (Auto etc).
2. Add the task type (Periodic etc.).
3. Define the task priority.
4. Define the task body.
5. For each event, add the Event Type.
6. For each event parameter, add the Parameter Type.

For a Shared Machine definition:

1. Add the SharedMachine Machine type.
2. For each event, define the Event Type.
3. For each event parameter, define the Parameter Type.

Invoking the Translation

• To create the IL1 model,
  • Right-Click on the Main node, select Epsilon Translation/Translate Task Mch 2 IL1 EMF.
  • Open the Resource Perspective.
  • Right-click on the sharedbuffer20100819Tutorial2 project folder.
  • Select refresh, the .il1 file should appear in the project.
  • Open and inspect the file, and view the source code by opening the IL1 Pretty Print view if desired.

• To create the Event-B model of the implementation,
  • Return to the Rodin Modelling Perspective.
  • Right-Click on the Main node, select Epsilon Translation/Translate Task Mch 2 Event-B EMF.
  • The sharedbuffer20100819bTasking project is generated, it can be opened and inspected.

There are errors in the generated machines (not investigated the cause yet); these can be fixed in the following way.

• Open a Machine in the Event-B Machine Editor.
• Select the Edit tab.
• Open the REFINES section, the error lies here.
• The correct machine is refined, but choose a different machine to refine (any one, it doesn't matter).
• Select the original refined machine again.
• Save and clean the project, and the error should disappear.
• Repeat for the same errors in the other machines; save and clean again.
• The machines can viewed as normal using the Rodin editors.