Difference between pages "Tasking Event-B Overview" and "Tasking Event-B Tutorial"

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=== Tasking Event-B ===
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For more information contact Andy Edmunds - University of Southampton - mailto:ae2@ecs.soton.ac.uk
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The following text can be read in conjunction with the slides<ref name = "Zurich2010Slides">http://deploy-eprints.ecs.soton.ac.uk/260/2/CGSlidesAndy%2520Edmunds%2520-%2520Code%2520Generation%2520Slides.pdf</ref> from the Deploy Plenary Meeting - Zurich 2010.
+
=== Tasking Event-B Tutorial Overview ===
 +
This tutorial follows on from the abstract development described [http://wiki.event-b.org/index.php/Development_of_a_Heating_Controller_System here].
  
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Tasking Event-B can be viewed as an extension of the existing Event-B language. We use the existing approaches of refinement and decomposition to structure a development that is suitable for construction of a Tasking Development. At some point during the modelling phase parameters may have to be introduced to facilitate decomposition. This constitutes a natural part of the refinement process as it moves towards decomposition and on to the implementation level. During decomposition parameters form part of the interface that enables event synchronization. We make use of this interface and add information (see [[#Implementing Events]]) to facilitate code generation.
+
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 [http://wiki.event-b.org/index.php/Tasking_Event-B_Overview here].
  
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=== Tasking Developments ===
+
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.
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A Tasking Development is generated programmatically, at the direction of the user; the Tasking Development consists of a number of machines (and perhaps associated contexts). In our approach we make use of the Event-B EMF extension mechanism which allows addition of new constructs to a model. The tasking extension consists of the constructs in the following table.
+
 
 +
The example/tutorial projects are are available in the [http://deploy-eprints.ecs.soton.ac.uk/304/ e-prints archive], or on [https://codegenerationd.svn.sourceforge.net/svnroot/codegenerationd/Examples/HeatingController_Tutorial_v0.1.4/ SVN].
  
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<center>
 
 
{| border="1"
 
{| border="1"
−
|Construct
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|Heating_ControllerTutorial_Completed
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|Options
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|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.
−
|-
 
−
|Machine Type
 
−
|DeclaredTask, AutoTask, SharedMachine
 
−
|-
 
−
|Control
 
−
|Sequence, Loop, Branch, EventSynch
 
−
|-
 
−
|Task Type
 
−
|Periodic(n), Triggered, Repeating, OneShot
 
−
|-
 
−
|Priority
 
−
| -
 
 
|-
 
|-
−
|Event Type
+
|Heating_ControllerTutorial_Step1
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|Branch, Loop, ProcedureDef, ProcedureSynch
+
|A bare project for step 1 of the [[Code_Generation_Tutorial#The_Tutorial |tutorial]].
 
|-
 
|-
−
|Parameter Type
+
|Heating_ControllerTutorial_Step2
−
|ActualIn, ActualOut, FormalIn, FormalOut
+
|A partially completed tasking development for steps 2, and 4 of the [[Code_Generation_Tutorial#The_Tutorial |tutorial]] (step 3 not required here).
 +
<!--|- >
 +
<!--|Heating_Controller5AddressedSim_Completed>
 +
<!--|A completed example that uses Addressed Variables in the tasks, and also in the environment simulation. Generates Memory Mapped IO for sensing and actuation.  >
 +
<!--|->
 +
<!--|Heating_Controller5AddressedNotSim_Completed>
 +
<!--|A completed example that uses Addressed Variables in the tasks only. Generates Deployable Memory Mapped IO for sensing and actuation. Similar to above but we discard the environment task from generated code.> 
 
|}
 
|}
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</center>
 
  
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The machines in the Tasking Development are extended with the constructs shown in the table, and may be viewed as keywords in a textual representation of the language. With extensions added, a Tasking Development can be translated to a common language model for mapping to implementation source code. There is also a translator that constructs new machines/contexts modelling the implementation, and these should refine/extend the existing elements of the Event-B project.
+
== 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 ''Heating_ControllerTutorial_Completed'' 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 ''Heating_ControllerTutorial_Completed'' 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.
 +
 
 +
== Using the Tasking Extension ==
 +
The steps needed to generate code from an Event-B model, in this tutorial, are as follows,
 +
* Step 1 - [[Tasking Event-B_Tutorial#Creating The Tasking Development|Create the tasking development]].
 +
* Step 2 - [[Tasking Event-B_Tutorial#Providing the Annotations for Implementations|Add Tasking annotations]].
 +
* Step 3 - [[Tasking Event-B_Tutorial#Optional Annotations for Addressed Variables|Add annotations for addressed variables (optional)]].
 +
* Step 4 - [[Tasking Event-B_Tutorial#Invoking the Translation|Invoke translators]].
 +
 
 +
==== Creating The Tasking Development ====
 +
* Change to the Event-B Perspective.
 +
* Open the ''Heating_ControllerTutorial_Step1'' Project.
 +
* Select the following Machines: Display_Update_Task1, Envir1, Heater_Monitor_Task1, Shared_Object1, Temp_Ctrl_Task1 and HC_CONTEXT.
 +
* 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 the tasking development, if necessary, so that the machine that models the environment is at the top of the list of machines.
 +
 
 +
This is due to a temporary technical limitation in the translator.
 +
 
 +
Change to the Project ''Heating_ControllerTutorial_Step2'' 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 ''Temp_Ctrl_Task1Impl'', ''Envir1'' and ''Shared_Object1'' machines are incomplete. We will take the necessary steps to provide implementation details.
 +
 
 +
===== 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 added the ''Periodic Task'' extension to the ''Auto Task'', and set a period of 250 milliseconds. We have provided a PrettyPrint view of the completed ''Temp_Ctrl_Task1Impl'' [http://wiki.event-b.org/images/Temp_Ctrl_Task1Impl.pdf here], it can be read in conjunction with the tutorial.
 +
 
 +
The next step is to construct the task body using control constructs such as sequence, branch, loop and output. These constructs are discussed in the [http://wiki.event-b.org/index.php/Tasking_Event-B_Overview overview] of Tasking Event-B. We will now complete the sequence that has been partially defined in the task body.
 +
 
 +
*'''Synchronize Sensing between TCSense_Temperatures and ENSense_Temperatures'''.
 +
** Expand the Temp_Ctrl_Task1Impl ''Auto Task Machine'' node.
 +
** Expand the ''Seq'' sub-tree.
 +
** Right-click on the first ''Seq'' node and select ''New Child/Left Branch EventWrapper''.
 +
** Provide the event label ''tc1'' 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 in the properties view.
 +
** At this point the drop-down box displays a number of event names, select the '''second''' ''TCSense_Temperatures'' event.
 +
** Go to the drop-down menu of the ''Remote Event'' property.
 +
** From the list of events select the '''second''' ''ENSense_Temperatures'' event.
 +
 
 +
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.
 +
 
 +
Note that the Synch Events construct is used in several ways. We use it to implement [[Tasking Event-B Overview#Control Constructs|Event Synchronisation]]; sensing and actuation; and as a simple event wrapper. An example of its use in a simple event wrapper follows. The simple event wrapper is used to update local state; there is no synchronisation, as such, but we re-use the constructs that already exist rather than create new ones. We now add a wrapped event to the sequence:
 +
 
 +
*'''Add the Wrapped Event TCCalculate_Average_Temperature'''.
 +
** Expand the sub-tree of the second ''Seq'' node.
 +
** Right-click on this (same as above) ''Seq'' node and select ''New Child/Left Branch EventWrapper''.
 +
** Provide the event label ''tc2'' 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 '''second''' ''TCCalculate_Average_Temperature'' event.
 +
 
 +
The addition of the wrapped event, to the sequence, is simply specification of event ordering. It is implemented in code as a sequential subroutine call statement. We now specify event synchronisation between the task and shared object:
 +
 
 +
*'''Add Synchronisation between TCGet_Target_Temperature2 and SOGet_Target_Temperature2'''.
 +
** Further expand the ''Seq'' sub-tree until Eventwrapper tc3 appears.
 +
** Right-click on the sibling ''Seq'' node (lowest in the tree) and select ''New Child/Left Branch EventWrapper''.
 +
** Provide the event label ''tc4'' 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 '''second''' ''TCGet_Target_Temperature2'' event.
 +
** Go to the drop-down menu of the ''Remote Event'' property.
 +
** From the list of events select the '''second''' ''SOGet_Target_Temperature2'' event.
 +
 
 +
We have now completed the task body, and next provide additional details for events. In the first instance we focus on the the ''TCGet_Target_Temperature2 '' event in ''Temp_Ctrl_Task1Impl'' which is to be synchronized with the ''SOGet_Target_Temperature2 '' event in ''Shared_Object1Impl''.
 +
 
 +
*'''Add The Event Synchronisation Extension'''.
 +
** Navigate to the list of events in the machine.
 +
** Right-click on the ''TCGet_Target_Temperature2'' Event node.
 +
** Select ''New Child/Implementation'' from the menu.
 +
** Go to the Implementation properties view and set the ''Implementation Type'' property to ''ProcedureSynch''.
 +
 
 +
We have identified the event as one that partakes in a synchronisation. The corresponding event in the Shared machine is dealt with [http://wiki.event-b.org/index.php/Tasking_Event-B_Tutorial#The_Shared_Machine here]
 +
 
 +
*'''Identify a parameter direction'''.
 +
** Right-click on the ''tm'' node.
 +
** Select''New Child/Parameter Type''.
 +
** Go to the ''Parameter Type'' properties view and set the ''Parameter Type'' property to ''actualIn''.
 +
 
 +
We have now identified the parameter as an actualIn (this models a call's return value).
 +
 
 +
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''.
 +
 
 +
We have identified the event as a sensing event. Now we add the parameter direction:
 +
 
 +
*'''Identify parameter directions'''.
 +
** Right-click on the ''t1'' node.
 +
** Select''New Child/Parameter Type''.
 +
** Go to the ''Parameter Type'' properties view and set the ''Parameter Type'' property to ''actualIn''.
 +
** Right-click on the ''t2'' node.
 +
** Select''New Child/Parameter Type''.
 +
** Go to the ''Parameter Type'' properties view and set the ''Parameter Type'' property to ''actualIn''.
 +
 
 +
We have now identified the parameters as an actualIn (modelling a received value from the environment).
 +
 
 +
===== The Shared Machine =====
 +
 
 +
The next step is to identify the ''Shared_Object1Impl'' machine as a ''Shared Machine''. A PrettyPrint view of the [http://wiki.event-b.org/images/Shared_Object1Impl.pdf 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 ''.tasking'' file.
 +
* Select ''New Child/Shared Machine'' from the menu.
 +
 
 +
We now show how to extend the ''SOGet_Target_Temperature2'' event of the Shared Machine with details about its implementation. The ''SOGet_Target_Temperature2'' event in ''Shared_Object1Impl'' synchronizes with the ''TCGet_Target_Temperature2'' event in the '' Temp_Ctrl_Task1Impl''.
 +
 
 +
* '''Identify SOGet_Target_Temperature2 as a Synchronized event'''.
 +
** Right-click on the ''SOGet_Target_Temperature2 '' Event node.
 +
** Select ''New Child/Implementation'' from the menu.
 +
** Go to the Implementation properties view and set the ''Implementation Type'' property to ''ProcedureSynch''.
 +
 
 +
* '''Identify the outgoing (return) parameter'''.
 +
** Right-click on the ''tm'' node.
 +
** Select ''New Child/Parameter Type''.
 +
** Go to the ''Parameter Type'' properties view and set the ''Parameter Type'' property to ''formalOut''.
 +
 
 +
===== 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 PrettyPrint view is available [http://wiki.event-b.org/images/Envir1Impl_2.pdf here]. In the prepared Environment Machine we have already added a ''Periodic Task'' 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'''.
 +
** Optionally collapse open branches of the EMF editor to remove clutter. 
 +
** Expand the Environ1Impl ''Environ Machine'' node.
 +
** Expand the ''Seq'' sub-tree fully.
 +
** Right-click on the last ''Seq'' node in the tree and and select ''New Child/Left Branch EventWrapper''.
 +
** Provide the event label ''e4'' 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 '''second''' ''ENAlter_Temperature_Sensor1'' event.
 +
 
 +
Output to the screen during the simulation can be specified as follows:
 +
 
 +
*'''Text Output during Simulation.'''.
 +
** Right-click on the last ''Seq'' node in the tree and and select ''New Child/Right Branch Output''.
 +
** Select the ''Output'' node, and in the properties menu select the ''Element'' property drop down box.
 +
** Select the ''last'' variable ''ctd'' that appears in the list.
 +
** In the ''Text'' property field, add a textual description to accompany the text output.
 +
 
 +
The generated code will print the text, and the value of the variable, to the screen. The next step is to identify the ''ENAlter_Temperature_Sensor1'' as a ''ProcedureDef''. This event enables the environment to manipulate the monitored variable.
 +
 
 +
* '''Identify ENAlter_Temperature_Sensor1 as a ProcedureDef event'''.
 +
** Right-click on the ''ENAlter_Temperature_Sensor1'' Event node.
 +
** Select ''New Child/Implementation'' from the menu.
 +
** Go to the Implementation properties view and set the ''Implementation Type'' property to ''ProcedureDef''.
 +
 
 +
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:
 +
 
 +
*'''Identify parameter directions'''.
 +
** Right-click on the ''t1'' node.
 +
** Select''New Child/Parameter Type''.
 +
** Go to the ''Parameter Type'' properties view and set the ''Parameter Type'' property to ''formalOut''.
 +
** Right-click on the ''t2'' node.
 +
** Select''New Child/Parameter Type''.
 +
** Go to the ''Parameter Type'' properties view and set the ''Parameter Type'' property to ''formalOut''.
 +
 
 +
We have now identified the parameters as an formalOut (modelling a simulation's subroutine call return value). This completes the necessary annotations for the simulation, and we can proceed to the translation step. In the event that memory mapped IO is required (non-simulation) then addresses can be added to the model at this stage, before translation takes place. See section on [http://wiki.event-b.org/index.php/Tasking_Event-B_Tutorial#Optional_Annotations_for_Addressed_Variables Addressed Variables] for details.
 +
 
 +
==== A Quick Check ====
 +
 
 +
It should now be possible to open the tasking PrettyPrinter view (Window menu) without errors. If the PrettyPrint fails, then this is a sign that the model has been incorrectly constructed. The point of failure (the extent of the printout before failure) may indicate the location of the error in the model.
 +
 
 +
===== A Summary of Steps =====
 +
If generating environment simulation code:
 +
# Ensure the Environ Machine is first machine in the development.
 +
 
 +
For a Tasking Machine definition:
 +
# Add the Tasking Machine type (Auto etc).
 +
# Add the task type (Periodic etc.).
 +
# Define the task priority.
 +
# Define the task body.
 +
# For each event, add the Event Type.
 +
# For each event parameter, add the Parameter Type.
 +
# Optionally define addressed variables.
 +
 
 +
For a Shared Machine definition:
 +
# Add the ''SharedMachine'' Machine type.
 +
# For each event, define the Event Type.
 +
# For each event parameter, define the Parameter Type.
 +
 
 +
For an Environ Machine definition:
 +
# Make the type an Environ Machine type.
 +
# Make the task type Periodic; a shorter period than the shortest task period is best for simulation.
 +
# Define the task priority.
 +
# Define the task body, it will contain a simulation of changes in the environment.
 +
# For each event, add the Event Type.
 +
# For each event parameter, add the Parameter Type.
 +
# Optionally define addressed variables.
 +
 
 +
== Invoking the Translators ==
 +
 
 +
* 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 ''Heating_ControllerTutorial_Step2'' 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.
 +
 
 +
In the event that the translator fails check that the correct events have been selected in the ''synchEvents'' construct. This can be done by looking at each task's ''taskBody'' construct, in the Tasking Pretty Printer view. In the PrettyPrinter view, each synchronization has a ''target'' and ''event'' seprated by dot-notation. The target is either the name of a shared machine or an environ machine. In this example check that the ''target'' refers to the correct machine, i.e. ''Envir1Impl'' rather than ''Envir1''; ''Temp_Ctrl_Task1Impl'' rather than ''Temp_Ctrl_Task1''; and ''SharedObject1Impl'', rather than ''SharedObject1''. After correcting any errors, invoke the translator again.
  
−
=== Tasking Machines ===
+
* To create the Event-B model of the implementation,
−
The following constructs relate only to Tasking Machines, and provide implementation details. Timing of periodic tasks is not modelled formally. Tasking Machines are related to the concept of an Ada task. These can be implemented in Ada using tasks, in C using the pthread library C, or in Java using threads.
+
** Return to the Rodin Modelling Perspective.
 +
** Right-Click on the Main node, select ''Epsilon Translation/Translate Task Mch 2 Event-B EMF''.
 +
** The ''Heating_Controller5AGen'' project is generated, it can be opened and inspected.
  
−
* Tasking Machines may be characterised by the following types:
+
There may be errors in the generated machines (the issue will disappear in a future release); these can be fixed in the following way.
−
** AutoTasks - Singleton Tasks.
+
* Open a Machine in the Event-B Machine Editor.
−
** Declared tasks - (Not currently used) A task template relating to an Ada ''tasktype'' declaration.  
+
* Select the Edit tab.
−
** TaskType - Defines the scheduling, cycle and lifetime of a task. i.e. one-shot periodic or triggered.
+
* Open the REFINES section, the error lies here.
−
** Priority - An integer value is supplied, the task with the highest value priority takes precedence when being scheduled.
+
* 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.
  
−
=== Shared Machines ===
+
== Optional Annotations for Addressed Variables ==
−
A Shared Machine corresponds to the concept of a protected resource, such as a monitor. They may be implemented in Ada as a Protected Object, in C using mutex locking, or in Java as a monitor.
+
To use memory mapped IO (Addressed Variables) in our generated code we can specify which addresses to use in our [http://wiki.event-b.org/index.php/Tasking_Event-B_Overview#Implementing_Events 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.
  
−
* Applied to the Shared Machine we have:
+
We now add addressed variable to the ''TCSense_Temperatures'' event in ''Temp_Ctrl_Task1Impl'', a PrettyPrint view is available [http://wiki.event-b.org/images/AddressedVarsTask.pdf here].
−
** A SharedMachine ''keyword'' that identifies a machine as a Shared Machine.
 
  
−
=== Towards implementation ===
+
* '''Add Address Information to Event Parameters'''.
−
At the stage where we are considering how to implement the Event-B development we need to think about controlling the flow of execution, and how events should be implemented. The following section describes the constructs that we have introduced to facilitate this.
+
** Right-click on the parameter node.
−
==== Control Constructs ====
+
** Select ''New Child/Addressed Variable''.
−
Each Tasking Machine has a ''task body'' which contains the flow control, or algorithmic, constructs.  
+
** Go to the ''Addressed Variable'' properties view and set the ''Address'' and ''Base'' properties to appropriate values.
  
−
* We have the following constructs available in the Tasking Machine body:
+
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 [http://wiki.event-b.org/images/AddressedVarsEnvir.pdf here].
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** Sequence - for imposing an order on events.
 
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** Branch - choice between a number of mutually exclusive events.
 
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** Loop - event repetition while it's guard remains true.
 
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** Event Synchronisation - synchronization between an event in a Tasking Machine and an event in a Shared Machine. Synchronization corresponds to an subroutine call with atomic (with respect to an external viewer) updates. The updates in the protected resource are implemented by a procedure call to a protected object, and tasks do no share state.  The synchronization construct also provides the means to specify parameter passing, both in and out of the task.
 
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** Event wrappers - The event synchronization construct is contained in an event wrapper. The wrapper may also contain a single event (we re-use the synchronization construct, but do not use it for synchronizing). The event may belong to the Tasking Machine, or to a Shared Machine that is visible to the task. Single events in a wrapper correspond to a subroutine call in an implementation.
 
  
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==== Implementing Events ====
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Invocation of the translators proceeds as detailed above.
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An event's role in the implementation is identified using the following extensions which are added to the event. Events used in task bodies are 'references' that make use of existing event definitions from the abstract development. The events are extended. to assist with translation, with a keyword indicating their role in the implementation.
 
  
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* Event implementation.
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== Generated Code ==
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** Branch - In essence a task's event is split in the implementation; guards are mapped to branch conditions and actions are mapped to the branch body. If the branch refers to a Shared Machine event (procedureDef) then this is mapped to a simple procedure call.
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The Ada Code generated by the translator is available at the following links:
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** Loop - The task's event guard maps to the loop condition and actions to to loop body. If the loop refers to a Shared Machine event then it is mapped to a simple procedure call.
 
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** ProcedureSych - This usually indicates to the translator that the event maps to a subroutine, but an event in a task may not require a subroutine implementation if its role is simply to provide parameters for a procedure call.
 
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** ProcedureDef - Identifies an event that maps to a (potentially blocking) subroutine definition. Event guards are implemented as a conditional wait; in Ada this is an entry barrier, and in C may use a pthread condition variable .
 
  
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In an implementation, when an subroutine is defined, its formal parameters are replaced by actual parameter values at run-time. To assist the code generator we extend the Event-B parameters. We identify formal and actual parameters in the implementation, and add the following keywords to the event parameters, as follows:
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for simulation of environment without addressed variables, [http://wiki.event-b.org/images/Code_Heating_ControllerTutorial_Completed.pdf Heating_ControllerTutorial_Completed]
  
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* Event parameter types
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for simulation of environment with addressed variables, [http://wiki.event-b.org/images/Code_Heating_Controller5AddressedSim_Completed.pdf Heating_Controller5AddressedSim_Completed]
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** FormalIn or FormalOut - event parameters are extended with the ParameterType construct. Extension with formal parameters indicates a mapping to formal parameters in the implementation.
 
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** ActualIn or ActualOut - Extension with an actual parameter indicates a mapping to an actual parameter in the implementation.
 
  
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== References ==
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Removal of the environment task from the ''Heating_Controller5AddressedSim_Completed'' should be deployable.
  
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<references/>
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[[Category:User documentation]]

Revision as of 09:14, 29 November 2011

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).
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