Object-Oriented Software Development with UML - UNU-IIST ...

Mastering Complex Systems 17The problems of characterizing the behaviour of system In a software system for a large application,there may be hundreds or even thousands of variables as well as more than one thread of control. Thebehaviour of the system is how it changes from one state to another. Although a system can in practice haveonly finite number of states,in a large system, there is a combinatorial explosion that makes this number very large;¢each event external to the system may change the state of the system;¢the system may react to an external event nondeterministically, i.e. it may not be predictable which¢of a number of states the system will enter after a certain event.This observation indicates the importance and difficulty of the decomposition in the design phase.3.2 Mastering Complex SystemsHaving said what make software inherently complex, an essential problem of software engineering is howwe can master this complexity during the software development process.3.2.1 Examples of complex systemsA personal computer A personal computer is a device of moderate complexity. Most of them can bedecomposed into the same major elements:a central processing unit (CPU),¢a monitor,¢a keyboard, and¢some sort secondary storage device, either a floppy disk or a hard disk drive.¢Each of these parts can be further decomposed into subparts. For example, a CPU is composed ofa primary memory unit,¢an arithmetic/logic unit (ALU), and¢¢a bus to which peripheral devices are attached.Each of these parts may be in turn decomposed: an ALU may be divided into¢registers, and random control logicReport No. 259, July 2002UNU/IIST, P.O. Box 3058, Macau