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Computer Hardware Description Languages (CHDL)

Computers can be used to simulate any machine – an ability, of course, that lies at the heart of Computer Aided Design (CAD). Since computers, too, are machines, they also need to be simulated when they are being designed. This is the subject area of Computer Hardware Simulation (CHS).

Simulation of the given system can only be performed if a description of the system and its environment is first given. Computer Hardware Description Languages (CHDL) are used for this purpose, and are simply the computer languages that are used for programming the given simulation.

Choosing the Level of Simulation

In theory, the computer could be programmed to simulate the target hardware down to an arbitrary level of detail – right down to simulating the movements of individual sub-atomic particles within the components of the system. In practice, of course, this would be both unnecessary, and prohibitively costly of computer resources (memory space and execution time).

The question of where the designer ought to draw the line can only be answered by considering the sort of information that is being sought from the simulation.

Assuming that computer hardware is implemented as digital circuitry, the traditional levels can be mapped out as follows:

System Level System described as a structure of sub-systems Simula, GPSS, Simscript
Sub-system Level Sub-system described as a structure of sub-sub-systems
Register Transfer Level (RTL) Sub-sub-system described as a structure of registers and buses ISPS, isp', Hilo
Gate Level Registers and buses described as a structure of logic gates HDL, DDL, Digsim
Circuit Level Logic gates described as a structure of transistors and other electronic components Spice2, NAP2/ANP3

As mentioned earlier, the levels do not stop there, but continue on down, allowing transistors to be described in terms of their component layers of semiconductor material, for example.

Inevitably, of course, general-purpose programming languages get used, too, as hardware description languages, at any or all of the above levels. Indeed, the use of functional programming languages in this role was of particular interest at the time of my doctorate (and the CHDLs listed in the last column of the table are the ones that also date from this time).

Since then, other languages have been developed. As with computer programming languages, though, the overall concepts remain the same (it is a simple matter to learn any new computer language once earlier ones have been mastered).

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© Malcolm Shute, Valley d'Aigues Research, 2005-2007