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

296 Process Part of POOSL
It is executed by first evaluating expressions E1 ¥¡ ¡ ¡ ¥ E6 in the order of appearance. Each
branch of which the guard evaluates to false is discarded. For a branch of which the guard
evaluates to bunk, a non-deterministic choice is taken whether it is discarded. For each
non-discarded branch, an attempt is made to execute the corresponding communication
statement. If the attempt succeeds, the communication actually takes place and the rest
statement (after the then) is executed 9 .
In fact, in imitation of POOL [ABKR86], earlier versions of POOSL indeed incorporated
select statements. The semantics of these statements was quite complex though. This
complexity became unmanageable when message-receive statements ch?m(p1 , ¡ ¡ ¥ pm)
were replaced by conditional message-receive statements ch?m(p1 ¥¡ ¡ ¡ ¥ pm ¡ E). The formalisation
of these statements in terms of the one-layered semantics appeared to be
extremely cumbersome. For this reason it was decided to split the semantics in two
layers. As a result, the select statements could be replaced by guarded commands and
choice statements. Through this decision the semantics was considerably simplified.
Furthermore, the introduction of guarded commands and choice statements increased
the expressive power of POOSL.
9.7.4 Tail Recursion
Reactive behaviour of complex (real-time) hardware/software systems is often most
naturally described in terms of finite state machine-like descriptions. Now each finite
state machine is more or less expressible in terms of if and do constructs. However, the
required transformations can be quite complicated and the results are often unreadable.
For this reason, POOSL has incorporated a construct that allows state machine behaviour
to be expressed directly and naturally. Consider the state diagram of a 1-place buffer Buf
given in Figure 9.5. In process calculi such as CCS [Mil89], this behaviour is naturally
specified as
ch?in(data)
Empty Full(data)
ch!out(data)
Figure 9.5: State Diagram of a 1-place Buffer
9Note that, although this execution resembles the execution of E1¡ statement
¢ E2 ch!m 1 or
¡ ch ?m¤¢ E4 £ p1 S p
2 or ch!m¥¢ E6 £ S E5¡ p
3 in the layered semantics, it is not the same. If one of the expressions
has side-effects, different observable behaviour can be obtained.
E3 £ S p