Software Design Methods
for Real-Time Systems
Software Design. A software design strategy is an
Capsule Description
overall plan and direction for performing design.
For example, functional decomposition is a software
This module describes the concepts and methods
design strategy.
used in the software design of real-time systems. It
outlines the characteristics of real-time systems, de-
A software design concept is a fundamental idea that
scribes the role of software design in real-time sys-
can be applied to designing a system. Information
tem development, surveys and compares some soft-
hiding is a software design concept.
ware design methods for real-time systems, and out-
A software design notation or representation is a
lines techniques for the verification and validation of
means of describing a software design. It may be
real-time designs. For each design method treated,
diagrammatic, symbolic, or textual. Structure charts
its emphasis, concepts on which it is based, steps
and pseudocode are software design notations.
used in its application, and an assessment of the
method are provided.
A software design method is a systematic approach
for carrying out design. It typically describes a se-
quence of steps for producing a design. A design
method is based on a set of design concepts,
employs a design strategy or strategies, and docu-
Philosophy
ments the resulting design using one or more design
notations.
Real-Time Systems. Real-time systems have wide-
spread use in industrial, commercial, and military
A software design method does not provide a cook-
applications. These systems are often complex be-
book approach to performing design. A designer
cause they have to deal with multiple independent
must use his skill and judgement in applying the
streams of input events. These events have arrival
method. It should be pointed out that when a meth-
rates that are often unpredictable, although they must
od is deficient in a certain aspect, it is often the case
be responded to within predefined timing con-
that experienced designers will compensate for this
straints.
by developing an ad hoc solution.
Real-time systems are frequently classified as “hard
Module Organization. This module builds on the
real-time systems” or “soft real-time systems.” A
module Introduction to Software Design
[Budgen89]
hard real-time system has time-critical deadlines that
by focusing on the real-time system domain. It
must be met; otherwise a catastrophic system failure
points out the differences between this domain and
can occur. In a soft real-time system, it is consid-
other application domains. It describes design con-
ered undesirable, but not catastrophic, if deadlines
cepts that are of particular importance to real-time
are occasionally missed.
system design, such as concurrent tasks and finite
state machines. Life-cycle considerations specific to
In spite of the importance of timing constraints in
real-time systems are examined. Design represen-
real-time systems, it is a characteristic (and a
tations for expressing real-time designs are outlined.
limitation) of the current state of the art in software
This module references material in Introduction to
design methods for real-time systems that the meth-
Software Design, in particular, for those design
ods tend to emphasize structural and behavioral as-
topics that are of importance to the design of all soft-
pects of real-time systems and generally pay signifi-
ware systems.
cantly less attention to timing constraints.
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