Lecture 4 — History Junctions & Parallel Decomposition

What is a superstate?

A state that contains other states (substates)

Why use superstates in large designs?

To reduce complexity/clutter by grouping related modes and simplifying transitions (e.g., exception mechanism)

What is the “exception mechanism” idea in statecharts?

Group normal behaviour inside a superstate so an “exception” can transition out cleanly, without duplicating many transitions

What does a history junction (H) do?

It records prior substate activity so re-entering the superstate resumes the previously active substate(s)

On the first entry to a superstate with history, what happens?

The superstate uses its default transition to decide the active substate.

On later re-entry to a superstate with history, what happens?

The superstate skips the default transition and resumes the prior active configuration

What does “exclusive (OR) decomposition” mean?

Only one substate can be active at a time (mutually exclusive modes)

What does “parallel (AND) decomposition” mean?

All substates are active concurrently inside the parent state

In exams, what is an AND-superstate?

A superstate whose children run in parallel (AND)

In exams, what is an OR-superstate?

A superstate whose children are exclusive (OR) (one active at a time)

What’s the key exam effect of “H in Lift but not in Listener”?

Lift resumes its last substate on re-entry; Listener resets to its default substate

In the lecture’s Temperature Control example, what does a configuration like “PowerOn, Off1, Off2” represent?

Power is on, and (in parallel regions) both FAN1 and FAN2 are currently in their “Off” substates

Why is parallel decomposition useful in embedded controllers?

It models concurrent components (e.g., multiple subsystems operating simultaneously).

If a parent is parallel (AND), do you expect one or multiple active substates inside it?

Multiple—typically one active substate per parallel region

What’s a common past-paper question style connected to this lecture?

1. “List basic states / OR-superstates / AND-superstates” and explain the effect of history H.