VHDL stands for VHSIC (Very High Speed Integrated Circuits) Hardware Description Language. In the mid-1980’s the U.S. Department of Defense and the IEEE sponsored the development of this hardware description language with the goal to develop very high-speed integrated circuit. It has become now one of industry’s standard languages used to describe digital systems. The other widely used hardware description language is Verilog. Both are powerful languages that allow you to describe and simulate complex digital systems. A third HDL language is ABEL (Advanced Boolean Equation Language) which was specifically designed for Programmable Logic Devices (PLD). ABEL is less powerful than the other two languages and is less popular in industry. This tutorial deals with VHDL, as described by the IEEE standard 1076-1993.

Although these languages look similar as conventional programming languages, there are some important differences. A hardware description language is inherently parallel, i.e. commands, which correspond to logic gates, are executed (computed) in parallel, as soon as a new input arrives. A HDL program mimics the behavior of a physical, usually digital, system. It also allows incorporation of timing specifications (gate delays) as well as to describe a system as an interconnection of different components.