Decompose system into modules

A effective modularization is dependent on the criteria that divides system into modules

Modularization

Specify the interface between modules before work

Preparation

decompose a system into modules on the basis of a flowchart.

decomposes a system into modules on the basis of a hierarchical structure. 

modularization 1&2

Main differences: the way of devision work & interfaces

e.g. input format; 按需加载 to save memeory; different formats to pack input; 

2: line storage module hiddens the implementation of line from other modules. and its change will not affect other modules.              (它变，其他modules可以保持不变)

Changeability

1 has complex interfaces. modules are not really independent.

Independent development

2 easy to understand

Comprehensibility

Difference between 1&2

General criteria: each module hides its design decision from the rest of the system.

1: makes each major step in the processing a module. works like flowchart.

2: uses information hiding as criteria. The modules no longer correspond to steps in the processing.

Criteria

The sequence of instructions necessary to call a given routine and the routine itself are part of the same module.

The formats of control blocks used in queues in OS and similar programs must be hidden within a \"control block module\

The sequence in which certain items will be processed should be hidden within a single module. Various change rangign from equipm-ent additions to unavailability or resources in an OS make sequencing extremely variable.

Decomposition tips

总结: The several representations of the program must be maintained in the machine together with a program performing map-ping between them.

Efficiency implementation

It assures us to \"prune off\" the upper level of the tree and start a new tree on the old trunk.

Hierarchcal structure and \"clean\" decomposition are two desirable but independent properties of a system structure.

Hierarchical structure

Summary

Decompose Systems into Modules