Pirbhai and Hatley Extensions
The Pirbhai and Hatley extensions to essential structured analysis notation focus less on the establishment of additional graphical symbols and more on the representation and specification of the control oriented suspects of the software. Such that mellor and ward pirbhai and hately suggest which dashed and solid notation be represented separately. Thus a control flow diagram CFD is described. The CFD contains the similar procedure as the DFD but shows control flow rather than data flow. Alternative of representing control process straight within the flow model a notational reference to a control specification is used. In fundamental nature the solid bar can be viewed as a window into an executive which controls the procedure represented in the DFD based on the event which is passed by the window. CSPEC is used to indicate (1) how the software behaves when an control or event signal is sensed and (2) which process are invoked as an consequence of the occurrence of the event. A process specification is used to define the inner workings of a procedure represented in a flow chart.
By using the notation with additional information contained in CSPECs and PSPECs pirbhai and hatley create a model of a real time system. The Data flow diagrams are used to represent data and the processes which manipulate it. The Control flow diagrams show how events flow among processes and describe those external events which cause several process to be activated. The interrelationship between 2 process several control models and processes. The process model is connected to the control model by data conditions. Control model is connected to the process model by process activation information contained in the CSPEC.
A data condition occurs whenever data input to a procedure results in a control output. The process checks & convert pressure implements the algorithm defines in the PSPEC pseudocode describe. When the absolute tank pressure is greater than an allowable maximum an above pressure event is generated. Note that when pirbhai and hatley 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. To determine what occur when this event occurs we must check the CSPEC.
The control specification contains a number of important modeling tools. A procedure activation table is used to indicate that processes are activated through a given event which flows by the vertical bar. Example for, a process activation table PAT might indicate in which the above pressure event would cause a process reduce tank pressure to be invoked. While to the PAT the CDPEC may contain a state transition diagram STD. The STD is a behavioral model which relies on the definition of a group of system states and is define in the following section.