An Atomic DEVS is a functional atom in a model, which cannot be further divided into sub-components. Its behavior is described by implementation-independent mathematical functions and symbols.
Atomic DEVS is a tuple as shown in Table 1.1. All the states of the DEVS are in the admissible state set . An execution of the model is to sequentially change its states, until ended explicitly. The change in its states is defined by two functions: internal transition function and external transition function .
defines the autonomous internal behavior. The time when these changes take place is defined by function . It takes a state as a parameter and returns a non-negative real value denoting the time interval between state changes. The time for a DEVS is not discrete, because the simulation is not based on time-slicing. An internal transition can be scheduled at any point in the future on the real time-line (Figure 1.8).
An external event may occur at any time. is the external transition function. It defines which new state the DEVS should be changed to, when a certain external event is received. The new state depends on the old state and how long the DEVS has been in the old state (elapsed time). The old state and the elapsed time are usually represented as a tuple , where and .
Only internal transitions are allowed to produce output. is the set of all possible output values. The output produced by a transition from the old state to any other state can be calculated with the function. Value means no output is produced.
All the input values are defined in and all the output values are defined in . They can be viewed as an interface exposed to the outside world. The outside world communicates with the Atomic DEVS only through input events and output events.