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data
flow dominated systems. Computations are executed only where the operands
are available. Communication between processes is done via unbounded FIFO
buffering Scheme (Stoy and Zebo
1994
). Data Flow models support hierarchy since
the nodes can represent complex functions or other Data Flow.
Several variations of Data Flow Graphs have been proposed in the literature such
as Synchronous Data Flow (SDF) and Asynchronous Data Flow (ADF). In SDF, a
fl
fixed number of tokens are consumed, where in ADF the number of tokens con-
sumed is variable.
Synchronous/Reactive Models
Synchronous modeling is based on the synchrony hypothesis. Outputs are produced
instantly in reaction to inputs and there is no observable delay in the outputs.
Synchronous models are used for modeling reactive real-time Systems. Stoy and
Zebo (
1994
) mentioned two styles for modeling reactive real time systems. First
multiple clocked recurrent systems (MCRS) which are suitable for data dominated
by real time systems. Second, state base formalisms which are suitable for control
dominated real time systems. Synchronous languages, such as Esterel, are used for
capturing Synchronous/Reactive model computation.
Heterogeneous Models
Heterogeneous Models combine features of different models of computations. Two
examples of heterogeneous models are presented: Programming languages and
Program State Machine (PSM). Programming languages provide a heterogeneous
model that can support data, activity and control modeling. Two types of pro-
gramming languages are presented, imperative language such as C, and declarative
languages such as LISP and PROLOG. In imperative languages, statements are
executed in the same order speci
ed in the speci
cation. On the other hand, exe-
cution order in not speci
ed in declarative languages since the sequence of exe-
cution is based on a set of logic rules or functions.
Program State Machine (PSM) is a merger between HCFSM and programming
languages. The Spec Charts language, which was designed as an extension to
VHDL, is capable of capturing the PSM model. The SpecC is another language
capable of capturing the PSM model. The following Table
3
attempts to set a
comparison between different models of computation.
2.2.2 Speci
cation Using Language
The goal of a speci
cation using language is to describe the intended functionality
of non-ambiguous systems. A large number of speci
cations using languages are
currently being used in embedded system design since there is no language that is
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