Specification of Reactive Hardware/Software Systems - Electronic ...

6.2 Concurrency and Synchronisation 153
modelled by processes and clusters. Sequential circuits can be modelled by:
collaborating data objects;
a single process object;
collaborating processes in a concurrency boundary.
In general, a synchronous hardware system requires a synchronous clock object that
sends clock-tick messages to all objects that must be synchronised.
Software can be implemented in various ways. The sort of concurrency can be described
using the notion of thread. The number of threads that can be active simultaneously
determines the sort of system. Software can be implemented as a:
sequential program (the implementation has a single thread of control);
quasi-concurrent program by multiprogramming (the implementation has multiple
threads but only one active thread);
concurrent program (the implementation has multiple active threads).
The concepts we offer enable the modelling of all these sorts of systems. The modelling
of sequential software is according to what has been described for hardware in
this paragraph. Concurrency can be described by process objects and clusters. A quasiconcurrent
system is in principle a sequential system. Such systems can be implemented
rather effectively using an available (real-time) operating system as basis for communication
and task scheduling. A first exploration of the path from a POOSL model to
a quasi-concurrent implementation in C++ is described by J.D. van Felius [vF96]. He
describes the development of a PEC (POOSL Extension of C++) library that enables
the mapping of the communication primitives of POOSL to C++. Quasi concurrency
is achieved by using threads based on the IEEE Portable Operating System Interface
(POSIX) standard 1003.1c (also called Pthreads).
6.2.2.8 Interleaving Semantics
So far we treated concurrency mainly from the point of view of analysis and implementation.
A more formal approach towards concurrency was necessary for the development
of the formal semantics of POOSL. The formal semantics are a computational interleaving
semantics. The execution of a system of collaborating instances is modelled as the
interleaving of all atomic actions (see Section 9.5). This means that the behaviour of a
system is interpreted as a sequential execution of actions. In the semantic model nothing
happens really simultaneously, except for a rendezvous. This can be accepted if actions
are assumed to take zero time. Concurrency of two processes then boils down to a mutual
independence of their actions. It does not matter whether a process performs some
action after or before an action of another process. This reasoning enables concurrency
with an interleaving semantics.