5.3 Moving into Design

Like the analysis phase, there are also models in the design phase most important of them are system implementation model (includes processor model & task model) and program implementation model.

Processor model aims to allocate essential models to different CPUs and how CPUs should communicate among themselves e.g. entire essential model can be allocated to a single processor or each bubble in DFD can be allocated to a different CPU. Data stores should be allocated to different processors as well.

In any of the cases which more than 1 CPU is used, communication mechanism among CPUs should be considered. This communication can happen by direct link between processors with a cable or channel or LAN or indirectly by writing to a magnetic type or anything which can be read by another processor as input.

A very important issue to be considered is that communication between processes (bubbles) is much much faster than CPU-to-CPU communication therefore system designer should group processes & stores to have high volume of communication within each process. Of course other factors such as cost, efficiency, security and reliability should be considered too.

Task model aims to assign tasks & data stores to individual tasks on each processor. All the terms partition, job step, overlay or control point implies the notion of task. The types of activities (synchronized or asynchronized) have impact on processors’ activity as well.

The program implementation model

At level of each individual task where usually only one activity can take place at the same time, program implementation model is used to organize activities of a single unit in structured chart.

Two of the most design goals and objectives are high cohesion and low coupling as well as other objectives such as module size, span of control and scope of effect/scope of control:

Cohesion: the degree to which components of a module are sufficient and necessary to carry out one single well-defined function. Essential processes shouldn’t be split to different modules and no unrelated tasks are gathered in a module. High cohesion is desired which means each module carries out one single task.

Coupling: the degree to which modules are interconnected or related or linked to one another. Strong coupling between modules i.e. high level on communication between modules in system makes the system complex & difficult to implement and maintain. Also a change in one part of a system may have unpredictable effect in another part of the system. Low or loose coupling is desired which means keeping modules small, simple with minimum number of data element shared between modules.

Span of control refers to number of immediate subordinates that can be called by manager module. To increase span of control an intermediate level of manage module can be put in place

Scope of effect/scope of control: refers to how much a module is dependent or affected by outcome of decision of other modules. If this is the case, the module should be subordinate to the module that makes decision.