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

6.6 Dynamic Behaviour 205
even for a coarse grain partitioning of the behaviour. We want to be able to identify
and distinguish states for discussions about the system. For this reason we decided to
define various concepts of state in addition to the concept of behaviour state, such as
named behaviour state, variables state, global state and execution state. We start with
the description of some aspects of the concept state itself.
6.6.3.2 State and Finite State Machines
There are various notations for the concept of state if dynamic behaviour is denoted
in finite state machine models. An automaton describing a language can be denoted by
regular expressions. A sequential machine implemented in hardware can be described as
an ASM-chart, as a state diagram or as a state transition table. For hardware design, the
notion of state is usually interpreted as the value of an internal memory (state register)
that remembers the events that happened (inputs of the system), and that determines
the response of the (sub-)system. However, state can also be interpreted as a temporary
form of behaviour. Both interpretations have their own value.
During the design courses we gave for experienced industrial hardware designers, we
noticed that they interpret state rather statically as the value of a state register. State
is considered to be a stable situation between two clock ticks. It is not interpreted as
the behaviour that is performed by the system. In the static interpretation, state is
connected with static output. State machines are often used to switch alternative or
possibly parallel forms of behaviour on and off. This leads to a notion of state that
matches to the concept of behaviour state. The state of the controller can be named
according to the observable behaviour of the (sub)system, that is switched on. Examples
of nameable forms of possibly complex (parallel) behaviour are for example: normal
operation mode, test mode, maintenance mode, shutting down, configuring, initialising,
resetting, loading, etcetera. The behavioural notion of state is often called mode. The
value of a state register of an automaton can correspond to behaviour switched on by a
state automaton. In the following paragraphs we define various sorts of states. These
concepts enable to distinguish between various forms that can or cannot be named in
practice.
6.6.3.3 Named Behaviour States
Naming of behaviour states is very useful when specifying a system. Because of the
various notions of state we are forced to create a separate definition for a named state.
A named state is a kind of behaviour of a module that can be identified by partitioning
the complete behaviour of a module into sub-behaviours, that can be given a name
preferably according to a notion in the problem domain. We will also refer to named
behaviour states as modes of behaviour or briefly modes. The state belongs to an
operating entity (module) that already exists as part of the system. The named grain
of behaviour of an entity is preferably externally observable. However sometimes it is
useful to use named states for non-externally observable (internal) behaviour too. This
is for instance useful when an external event must be remembered because it will cause
a change of behaviour, but which has no immediate visible external effect.