A control sub system takes responsibility for

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A control sub-system takes responsibility for managing the execution of other sub-systems Call-return model Top-down subroutine model where control starts at the top of a subroutine hierarchy and moves downwards. Applicable to sequential systems Manager model Applicable to concurrent systems. One system component controls the stopping, starting and coordination of other system processes. Can be implemented in sequential systems as a case statement ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 10 Slide 20 Call-return model Routine 1.2 Routine 1.1 Routine 3.2 Routine 3.1 Routine 2 Routine 3 Routine 1 Main program ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 10 Slide 21 Real-time system control System controller User interface Fault handler Computation processes Actuator processes Sensor processes ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 10 Slide 22 Event-driven systems Driven by externally generated events where the timing of the event is outwith the control of the sub-systems which process the event Two principal event-driven models Broadcast models. An event is broadcast to all sub- systems. Any sub-system which can handle the event may do so Interrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 10 Slide 23 Broadcast model Effective in integrating sub-systems on different computers in a network Sub-systems register an interest in specific events. When these occur, control is transferred to the sub- system which can handle the event Control policy is not embedded in the event and message handler. Sub-systems decide on events of interest to them However, sub-systems don t know if or when an event will be handled ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 10 Slide 24 Selective broadcasting Sub-system 1 Event and message handler Sub-system 2 Sub-system 3 Sub-system 4
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©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 10 Slide 25 Interrupt-driven systems Used in real-time systems where fast response to an event is essential There are known interrupt types with a handler defined for each type Each type is associated with a memory location and a hardware switch causes transfer to its handler Allows fast response but complex to program and difficult to validate ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 10 Slide 26 Interrupt-driven control Handler 1 Handler 2 Handler 3 Handler 4 Process 1 Process 2 Process 3 Process 4 Interrupts Interrupt vector ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 10 Slide 27 Object models Structure the system into a set of loosely coupled objects with well-defined interfaces Object-oriented decomposition is concerned with identifying object classes, their attributes and operations
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