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Running PRISM /

Loading And Building A Model

Typically, when using PRISM, the first step is to load a model that has been specified in the PRISM modelling language. If using the GUI, select menu option "Model | Open Model" and choose a file. There are a selection of sample PRISM model files in the examples directory of the distribution.

The model will then be displayed in the editor in the "Model" tab of the GUI window. The file is parsed upon loading. If there are no errors, information about the modules, variables, and other components of the model is displayed in the panel to the left and a green tick will be visible. If there are errors in the file, a red cross will appear instead and the errors will be highlighted in the model editor. To view details of the error, position the mouse pointer over the source of the error (or over the red cross). Alternatively, select menu option "Model | Parse Model" and the error mIessage will be displayed in a message box. Model descriptions can, of course, also be typed from scratch into the GUI's editor.

Building the model

In order to perform model checking, PRISM will (in most cases) need to construct the corresponding probabilistic model, i.e. convert the PRISM model description to, for example, an MDP, DTMC or CTMC. During this process, PRISM computes the set of states in the model which are reachable from the initial states and the transition matrix which represents the model.

Model construction is done automatically when you perform model checking. However, you may always want to explicitly ask PRISM to build the model in order to test for errors or to see how large the model is. From the GUI, you can do this by by selecting "Model | Build Model". If there are no errors during model construction, the number of states and transitions in the model will be displayed in the bottom left corner of the window.

From the command-line, simply type:

prism model.nm

where model.nm is the name of the file containing the model description.

For some types of models, notably PTAs, models are not constructed in this way (because the models are infinite-state). In these cases, analysis of the model is not performed until model checking is performed.

Deadlocks

You should be aware of the possibility of deadlock states (or deadlocks) in the model, i.e. states which are reachable but from which there are no outgoing transitions. PRISM will automatically search your model for deadlocks and, by default, "fix" them by adding self-loops in these states. Since deadlocks are sometimes caused by modelling errors, PRISM will display a warning message in the log when deadlocks are fixed in this way.

You can control whether deadlocks are automatically fixed in this way using the "Automatically fix deadlocks" option (or with command-line switches -nofixdl and -fixdl). When fixing is disabled, PRISM will report and error when the model contains deadlocks (this used to be the default behaviour in older versions of PRISM).

If you have unwanted or unexpected deadlocks in your model, there are several ways you can detect then. Firstly, by disabling deadlock fixing (as described above), PRISM will display a list of deadlock states in the log. Alternatively, you can model check the filter property filter(print, "deadlock"), which has the safe effect.

To find out how deadlocks occur, i.e. which paths through the model lead to a deadlock state, there are several possibilities. Firstly, you can model check the CTL property E[F "deadlock"]. When checked from the GUI, this will provide you with the option of display a path to a deadlock in the simulator. From the command-line, for example with:

prism dice.pm -pf 'E[F "deadlock"]'

a path to a deadlock will be displayed in the log.

Finally, in the eventuality that the model is too large to be model checked, you can still use the simulator to search for deadlocks. This can be done either by manually generating random paths using the simulator in the GUI or, from the command-line, e.g. by running:

prism dice.pm -simpath deadlock stdout

PRISM Manual

Running PRISM

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