In many cases, it is not appropriate to describe a module structurally. One such case is a module which is at the bottom of the hierarchy of some other structural description. For example, if you are designing a system using IC packages bought from an IC shop, you do not need to describe the internal structure of an IC. In such cases, a description of the function performed by the module is required, without reference to its actual internal structure. Such a description is called a functional or behavioural description.
To illustrate this, suppose that the function of the entity F in Figur 1-1(a) is the exclusive-or function. Then a behavioural description of F could be the Boolean function
Y = /A . B + A . /B
More complex behaviours cannot be described purely as a function of inputs. In systems with feedback, the outputs are also a function of time. VHDL solves this problem by allowing description of behaviour in the form of an executable program. Chapter 2 and 4 describe the programming language facilities.