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|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).