Actions are the units of behavior. In many kinds of systems, including computer
systems, behaviors are made up of discrete, discontinuous steps. Each step is an
action. An action can be composed of several smaller activities, but an action is
atomic: once it begins, it runs to completion, without being interrupted or preempted
by another action. For each kind of action in the implementation, there is a method
in the model program. When the model program runs, each method call represents
an action in the implementation.1
State is the information stored in a program or system at one point in time.
The concept of state captures our intuition of a ???situation??? or ???state of affairs???: the
important properties of a system at one moment. The state of the implementation is
represented by variables of the model program. Each particular assignment of values
to variables in the model program represents a particular state (a situation) of the
implementation. The variables in the model program that represent implementation
state are called state variables (to distinguish them from the model program??™s
local variables, parameters, etc.). State variables in the model program need not
correspond exactly to program variables in the implementation; they might represent
information that is not stored in implementation variables (messages in transit, for
example).
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