Re: SDL-News: Spontaneous Transition

Subject: Re: SDL-News: Spontaneous Transition
From: Rick Reed TSE (
Date: Mon Apr 22 2002 - 21:14:29 GMT

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mokhtar Nibouche at wrote on 21/04/2002 23:14:

> I am a bit confused with spontaneous transitions (NONE). Suppose a state with
> both normal transition and spontaneous one. Suppose a signal is ready for
> consumption for normal transition, what will happen?? Would the normal
> transition take place, or the spontaneous one??

The behaviour is not predictable. Because the state has a spontaneous
transition, this transition is able to be entered at any time while the
process is in the state regardless of the the input queue.

> What if the normal transition
> signal is not at the top of the input queue??

In this situation the either transition could be entered. The choice is non

> or what if it is not in the
> queue at all???

If there is no signal in the input queue that corresponds to one of the
normal transitions from the state. The process may or may not enter the
transition for the spontaneous transition. The spontaneous transition may
also be entered after an indeterminate delay, so some delay in between these
two extremes. The behaviour of the process corresponds to the SDL if the
spontaneous transition is enters immediately the process enters the state,
or if the spontaneous transition is never entered. Entering spontaneous
transition corresponds to some invisible internal event.

> I assume that spontaneous transition has priority over continuous signal,
> right?

No, this is not correct. There is no priority between the transitions -
either could occur.

Rick Reed -
Tel:+44 1455 55 96 55 Fax:+44 1455 55 96 58 Mob.:+44 7970 50 96 50

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