Ward and Mellor Extensions
12.4.3 Ward and Mellor Extensions
Ward and Mellor [WAR85] extend basic structured analysis notation to accommodate the following demands imposed by a real-time system:
• Information flow is gathered or produced on a time-continuous basis. • Control information is passed throughout the system and associated control
processing.
F I G U R E 12.12
Monitored
Input
Time- temperature
data flow
“continuous”
Monitor and adjust
Temperature set point
• Multiple instances of the same transformation are sometimes encountered in multitasking situations.
• Systems have states and a mechanism causes transition between states. In a significant percentage of real-time applications, the system must monitor time-
continuous information generated by some real-world process. For example, a real- To adequately model a time test monitoring system for gas turbine engines might be required to monitor
real-time system, turbine speed, combustor temperature, and a variety of pressure probes on a con- structured analysis
tinuous basis. Conventional data flow notation does not make a distinction between notation must be
available for time- discrete data and time-continuous data. One extension to basic structured analysis continuous data and
notation, shown in Figure 12.12, provides a mechanism for representing time-con- event processing.
tinuous data flow. The double headed arrow is used to represent time-continuous flow while a single headed arrow is used to indicate discrete data flow. In the figure, mon-
itored temperature is measured continuously while a single value for tempera-
ture set point is also provided. The process shown in the figure produces a time-continuous output, corrected value.
The distinction between discrete and time-continuous data flow has important implications for both the system engineer and the software designer. During the cre- ation of the system model, a system engineer will be better able to isolate those processes that may be performance critical (it is often likely that the input and out- put of time-continuous data will be performance sensitive). As the physical or imple- mentation model is created, the designer must establish a mechanism for collection of time-continuous data. Obviously, the digital system collects data in a quasi-con- tinuous fashion using techniques such as high-speed polling. The notation indicates where analog-to-digital hardware will be required and which transforms are likely to demand high-performance software.
In conventional data flow diagrams, control or event flows are not represented explicitly. In fact, the software engineer is cautioned to specifically exclude the
PA R T T H R E E C O N V E N T I O N A L M E T H O D S F O R S O F T WA R E E N G I N E E R I N G
F I G U R E 12.13
Movement
Data and
alarm
control flows Status of each
using Ward fixture
and Mellor Parts status buffer
Start/stop
control
Bit string
operator interface
Operator Position
robot commands
Robot movement record Robot command file
representation of control flow from the data flow diagram. This exclusion is overly restrictive when real-time applications are considered, and for this reason, a spe- cialized notation for representing event flows and control processing has been devel- oped. Continuing the convention established for data flow diagrams, data flow is represented using a solid arrow. Control flow, however, is represented using a dashed or shaded arrow. A process that handles only control flows, called a control process, is similarly represented using a dashed bubble.
Control flow can be input directly to a conventional process or into a control process. Figure 12.13 illustrates control flow and processing as it would be represented using
Parts
» The Concurrent Development Model
» SUMMARY Software engineering is a discipline that integrates process, methods, and tools for
» PEOPLE In a study published by the IEEE [CUR88], the engineering vice presidents of three
» THE PROCESS The generic phases that characterize the software process—definition, development,
» THE PROJECT In order to manage a successful software project, we must understand what can go
» METRICS IN THE PROCESS AND PROJECT DOMAINS
» Extended Function Point Metrics
» METRICS FOR SOFTWARE QUALITY
» INTEGRATING METRICS WITHIN THE SOFTWARE PROCESS
» METRICS FOR SMALL ORGANIZATIONS
» ESTABLISHING A SOFTWARE METRICS PROGRAM
» Obtaining Information Necessary for Scope
» An Example of LOC-Based Estimation
» QUALITY CONCEPTS 1 It has been said that no two snowflakes are alike. Certainly when we watch snow
» SUMMARY Software quality assurance is an umbrella activity that is applied at each step in the
» R diagram 1.4 <part-of> data model; data model <part-of> design specification;
» SYSTEM MODELING Every computer-based system can be modeled as an information transform using an
» Facilitated Application Specification Techniques
» Data Objects, Attributes, and Relationships
» Entity/Relationship Diagrams
» Hatley and Pirbhai Extensions
» Creating an Entity/Relationship Diagram
» SUMMARY Design is the technical kernel of software engineering. During design, progressive
» Data Modeling, Data Structures, Databases, and the Data Warehouse
» Data Design at the Component Level
» A Brief Taxonomy of Styles and Patterns
» Quantitative Guidance for Architectural Design
» Isolate the transform center by specifying incoming and outgoing
» SUMMARY Software architecture provides a holistic view of the system to be built. It depicts the
» The User Interface Design Process
» Defining Interface Objects and Actions
» D E S I G N E VA L U AT I O N
» Testing for Real-Time Systems
» Organizing for Software Testing
» Criteria for Completion of Testing
» The Transition to a Quantitative View
» The Attributes of Effective Software Metrics
» Architectural Design Metrics
» Component-Level Design Metrics
» SUMMARY Software metrics provide a quantitative way to assess the quality of internal product
» Encapsulation, Inheritance, and Polymorphism
» Identifying Classes and Objects
» The Common Process Framework for OO
» OO Project Metrics and Estimation
» Event Identification with Use-Cases
» SUMMARY Object-oriented analysis methods enable a software engineer to model a problem by
» Partitioning the Analysis Model
» Designing Algorithms and Data Structures
» Program Components and Interfaces
» SUMMARY Object-oriented design translates the OOA model of the real world into an
» Testing Surface Structure and Deep Structure
» Deficiencies of Less Formal Approaches 1
» What Makes Cleanroom Different?
» Design Refinement and Verification
» SUMMARY Cleanroom software engineering is a formal approach to software development that
» Structural Modeling and Structure Points
» Describing Reusable Components
» SUMMARY Component-based software engineering offers inherent benefits in software quality,
» Guidelines for Distributing Application Subsystems
» Middleware and Object Request Broker Architectures
» An Overview of a Design Approach
» Consider expert Web developer will create a complete design, but time and cost can be appropriate
» A Software Reengineering Process Model
» Reverse Engineering to Understand Data
» Forward Engineering for Client/Server Architectures
» SUMMARY Reengineering occurs at two different levels of abstraction. At the business level,
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