Specifying the behaviour of a continuous-time Markov chain (CTMC) is done in similar fashion to a DTMC or an MDP, as discussed so far. The main difference is that updates in commands are labelled with (positive-valued) rates, rather than probabilities. The notation used in commands, however, to associate rates to transitions is identical to the one used to assign probabilities:
In a CTMC, when multiple possible transitions are available in a state, a race condition occurs (see e.g. [KNP07a] for more details). In terms of PRISM commands, this can arise in several ways. Firstly, within in a module, multiple transitions can be specified either as several different updates in a command, or as multiple commands with overlapping guards. The following, for example. are equivalent:
Furthermore, parallel composition between modules in a CTMC is modelled as a race condition, rather as a nondeterministic choice, like for MDPs.