It also allows for the synthesis of an HDL description into a netlist (a specification of physical electronic components and how they are connected together), which can then be placed and routed to produce the set of masks used to create an integrated circuit.
HDLs were created to implement register-transfer level abstraction, a model of the data flow and timing of a circuit.
Example of dataflow of VHDL: HDLs are standard text-based expressions of the structure of electronic systems and their behaviour over time.
However, in contrast to most software programming languages, HDLs also include an explicit notion of time, which is a primary attribute of hardware.
Simulators capable of supporting discrete-event (digital) and continuous-time (analog) modeling exist, and HDLs targeted for each are available.
Before the introduction of System Verilog in 2002, C++ integration with a logic simulator was one of the few ways to use object-oriented programming in hardware verification.
[3] This text introduced the concept of register transfer level, first used in the ISP language to describe the behavior of the Digital Equipment Corporation (DEC) PDP-8.
The RTM products never succeeded commercially and DEC stopped marketing them in the mid-1980s, as new methods grew more popular, more so very-large-scale integration (VLSI).
In the mid-1980s, a VLSI design framework was implemented around KARL and ABL by an international consortium funded by the Commission of the European Union.
[10] Initially, Verilog and VHDL were used to document and simulate circuit designs already captured and described in another form (such as schematic files).
Synthesis tools compiled HDL source files (written in a constrained format called RTL) into a manufacturable netlist description in terms of gates and transistors.
Specialized HDLs (such as Confluence) were introduced with the explicit goal of fixing specific limitations of Verilog and VHDL, though none were ever intended to replace them.
As a result of the efficiency gains realized using HDL, a majority of modern digital circuit design revolves around it.
The process of writing the HDL description is highly dependent on the nature of the circuit and the designer's preference for coding style.
Special text editors offer features for automatic indentation, syntax-dependent coloration, and macro-based expansion of the entity/architecture/signal declaration.
The testbench code is event driven: the engineer writes HDL statements to implement the (testbench-generated) reset-signal, to model interface transactions (such as a host–bus read/write), and to monitor the DUT's output.
Most programming languages are inherently procedural (single-threaded), with limited syntactical and semantic support to handle concurrency.
HDLs, on the other hand, resemble concurrent programming languages in their ability to model multiple parallel processes (such as flip-flops and adders) that automatically execute independently of one another.
Both programming languages and HDLs are processed by a compiler (often called a synthesizer in the HDL case), but with different goals.
For HDLs, "compiling" refers to logic synthesis; the process of transforming the HDL code listing into a physically realizable gate netlist.
On the other hand, a software compiler converts the source-code listing into a microprocessor-specific object code for execution on the target microprocessor.
[citation needed] There are attempts to raise the abstraction level of hardware design in order to reduce the complexity of programming in HDLs, creating a sub-field called high-level synthesis.
Annapolis Micro Systems, Inc.'s CoreFire Design Suite[11] and National Instruments LabVIEW FPGA provide a graphical dataflow approach to high-level design entry and languages such as SystemVerilog, SystemVHDL, and Handel-C seek to accomplish the same goal, but are aimed at making existing hardware engineers more productive, rather than making FPGAs more accessible to existing software engineers.
Several projects exist for defining printed circuit board connectivity using language based, textual-entry methods.
These modern methodologies employ open-source design languages specifically tailored for electronics, adopting declarative markup to specify what circuits should achieve.
This shift integrates software development principles into hardware design, streamlining the process and emphasizing automation, reuse, and validation.