Slide 34 of 53
Abstract data types (ADTs) are objects which can be used to represent an activity or component in behavioral modeling. An ADT supports data hiding, encapsulation, and parameterized reuse. As such they give VHDL some object-oriented capability.
An ADT is both a data structure (such as a stack, queue, tree, etc.) and a set of functions (e.g. operators) that provide useful services of the data. For example, a stack ADT would have functions for pushing an element onto the stack, retrieving an item from the stack, and perhaps several user-accessible attributes such as whether the stack is full or empty.