Functional- and behavioural-level description languages and simulators  represent the highest levels of simulation available to circuit designers. These levels enable designers to model circuits in terms of interacting abstract units that may not even be capable of fabrication. As such, designers are not limited by the restricted behaviour of fundamental circuit devices. In addition, these levels also provide designers a viable means of quickly exploring alternatives without becoming overwhelmed with the impact that design decisions would have on the circuit at lower levels.
Functional-level simulators are generally closer to the actual hardware representation than behaviour-level simulators. An abstract unit in a functional-level simulation would accept input and produce output just like its corresponding hardware component. However, more flexibility is permitted with respect to how the input is presented to the unit and how it is processed to produce output. For example, an adder at the gate-level may consist of several half-adders which adds to numbers by operating directly on their bits. The equivalent functional-level unit would simply take two integers and add them using arithmetic constructs available in the hardware description language.
Behavioural-level simulators go one level higher and permit designers to model abstract control processing which may not be realizable in hardware. The purpose of these simulators is to give the designer a general overview of the design and to experiment with high-level alternatives. The usefulness of design tools and simulators that operate at this level has been the subject of debate due to the difficulty in translating such high level designs into compact, high-performance circuits. However, advances in silicon compilation have made the translation process easier and more efficient. In addition, high level simulators are still useful for rapid prototyping, even if the design is not actually physically fabricated. Rapid prototyping allow designers to study the feasibility of a high level design before actually delving into the tedious, low-level details such as placement and routing.
One of the more popular languages for describing, simulating and eventually synthesizing circuits at the functional and behavioural level is VHDL .