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One important difference between most programming languages and HDLs is that HDLs explicitly include the notion of time. There are different types of description in them “dataflow, behavioral and structural”. HDLs are standard text-based expressions of the structure of electronic systems and their behaviour over time. HDLs also include an explicit notion of time, which is a primary attribute of hardware. HDLs are used to write executable specifications for hardware. A program designed to implement the underlying semantics of the language statements and simulate the progress of time provides the hardware designer with the ability to model a piece of hardware before it is created physically. HDLs targeted for each are available.
Generally, however, software programming languages do not include any capability for explicitly expressing time, and thus cannot function as hardware description languages. System Verilog is the first major HDL to offer object orientation and garbage collection. Synthesizers generally ignore the expression of any timing constructs in the text. The ability to have a synthesizable subset of the language does not itself make a hardware description language. The first hardware description languages appeared in the late 1960s, looking like more traditional languages. ISPS was well suited to describe relations between the inputs and the outputs of the design and was quickly adopted by commercial teams at DEC, as well as by a number of research teams both in the USA and among its NATO allies. Torino, Italy, producing the ABLED graphic VLSI design editor.
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. ABEL to fill that need. In 1987, a request from the U. HDL simulation enabled engineers to work at a higher level of abstraction than simulation at the schematic level, and thus increased design capacity from hundreds of transistors to thousands.
HDLs pushed HDLs from the background into the foreground of digital design. RTL synthesis: extremely high-speed, low-power, or asynchronous circuitry. Within a few years, VHDL and Verilog emerged as the dominant HDLs in the electronics industry, while older and less capable HDLs gradually disappeared from use. Verilog and VHDL, though none were ever intended to replace them. Over the years, much effort has been invested in improving HDLs. As a result of the efficiency gains realized using HDL, a majority of modern digital circuit design revolves around it.
Most designs begin as a set of requirements or a high-level architectural diagram. The process of writing the HDL description is highly dependent on the nature of the circuit and the designer’s preference for coding style. The HDL code then undergoes a code review, or auditing. In preparation for synthesis, the HDL description is subject to an array of automated checkers.
This process aids in resolving errors before the code is synthesized. In industry parlance, HDL design generally ends at the synthesis stage. Once the synthesis tool has mapped the HDL description into a gate netlist, the netlist is passed off to the back-end stage. HDLs may or may not play a significant role in the back-end flow.