Semantics of a HIardware Design Language for
Hiroto Yasuura, Nagisa Ishiura · 1989
We propose a new approach to define a formal semantics of a hardware design language (HDL) in Japanese LSI design language standardization project. Our approach is to separate the definition of semailtics from simulators. Since the semantics includes nondeter- minism, we can describe vagueness of circuit behavior such as dispersion of delays without linguistic ambiguity. We introduce a new computation model of hardware behaviour called NES (Nondeterministic Event Sequence) model. NES model is a very simple model of the compu- tation in digital systems and provides an intuitive un'der- standing of concurrent behaviour of HDL description without loss of mathematical strictness. speed-up of simulators. For the standard language, how- ever, many simulators for the language may be developed in different organizations. In order to give a criterion of correctness of simulators, the language designer must present a formal and strict semantics of the language to simulator designers. The formal semantics is also impor- tant for designers of logic synthesizers and verifiers which are deeply concerned with the behavioral semantics of the language. In Japanese LX design language standardization project, we intend to define a:nd present a formal and strict semantics of the standard language. Our basic idea is that the semantics of the language should be defined independently of simulators. The language provides an abstract (or approximation) model of hardware. Simula- tor designers try to simulate the behavior of the abstract model, not the real hardware, under the trade-off of computation time and preciseness of simulation. The role of the language designer is to provide the abstract model of hardware which is natural and. general for hardware designers and CAD engineers. The language designer should be free from state-of-the-art simulation techniques and consider other CAD tools such as logic synthesizers as well as simulators. Giving a formal and strict. semantics, we present an operational semantics of the language which may be easy to understand behavior of described hardware for users. A new computation model called a nondeterministic event sequence (NES) model is defined as an interpretation mechanism of hardware behavior. A abst:ract model of hardware is described as a program on NES. In NES, all behavior of hardware is considered as a sequence of events. Nondeterminism is introduced explicitly into interpretation scheme of NES. Thus we can describe vagueness of hardware behavior such as dispersion of delay without loss of strictness of semantics.