Software Engineering-Hatley and Pirbhai Extensions

The Hatley and Pirbhai  extensions to basic structured analysis notation focus less on the creation of additional graphical symbols and more on the representation and specification of the control-oriented aspects of the software. The dashed arrow is once again used to represent control or event flow. Unlike Ward and Mellor, Hatley and Pirbhai suggest that dashed and solid notation be represented separately. Therefore, a control flow diagram is defined. The CFD contains the same processes as the DFD, but shows control flow, rather than data flow. Instead of representing control processes directly within the flow model, a notational reference (a solid bar) to a control specification (CSPEC) is used. In essence, the solid bar can be viewed as a "window" into an "executive" (the CSPEC) that controls the processes (functions) represented in the DFD based on the event that is passed through the window. The CSPEC, is used to indicate (1) how the software behaves when an event or control signal is sensed and (2) which processes are invoked as a consequence of the occurrence of the event. A process specification is used to describe the inner workings of a process represented in a flow diagram.

Using the notation described in earlier figures, along with additional information contained in PSPECs and CSPECs, Hatley and Pirbhai create a model of a real-time system. Data flow diagrams are used to represent data and the processes that manipulate it. Control flow diagrams show how events flow among processes and illustrate those external events that cause various processes to be activated. The interrelationship between the process and control models is shown schematically inthe figure below. The process model is "connected" to the control model through data conditions. The control model is "connected" to the process model through process activation information contained in the CSPEC.

A data condition occurs whenever data input to a process result in control output. This situation is illustrated in figure below, part of a flow model for an automated monitoring and control system for pressure vessels in an oil refinery. The process check and convert pressure implements the algorithm described in the PSPEC pseudocode shown. When the absolute tank pressure is greater than an allowable maximum, an above pressure event is generated. Note that when Hatley and Pirbhai notation is used, the data flow is shown as part of a DFD, while the control flow is noted separately as part of a control flow diagram. As we noted earlier, the vertical solid bar into which the above pressure event flows is a pointer to the CSPEC. Therefore, to determine what happens when this event occurs, we must check the CSPEC.

The control specification (CSPEC) contains a number of important modeling tools. A process activation table  is used to indicate which processes are activated by a given event. For example, a process activation table (PAT) for the above figure might indicate that the above pressure event would cause a process reduce tank pressure (not shown) to be invoked. In addition to the PAT, the CSPEC may contain a state transition diagram. The STD is a behavioral model that relies on the definition of a set of system states and is described in the following section.