SIMSCRIPT II.5 Programming Language

5. Discrete Simulation Concepts
5.1 Introduction
Chapters 1 through 4 of this book described the features of a general-purpose programming language
which provides for the high-level description of data structures and their manipulation. Level
5 of SIMSCRIPT II.5 provides concepts and language features for application in the simulation of
discrete systems. This chapter assumes familiarity with the aims and techniques of discrete-event
simulation. It describes the features by which SIMSCRIPT II.5 provides a uniquely powerful tool
for aiding such systems study. The topic is described in a number of current texts. The principal
text, which develops the principles of simulation using SIMSCRIPT II.5 as the modelling language,
is Building Simulation Models with SIMSCRIPT II.5, by E. C. Russell, published by CACI Products
Company.
Simulation, as described here, is the use of a numeric model of a system to study its behavior as it
operates over time. Discrete-event simulation deals specifically with modelling of those systems in
which the system state is deemed to change instantaneously at discrete points in time, rather than
continuously. This chapter presents language concepts and features designed to aid in conceptualizing
such systems and in modelling them in a computer program.
5.2 Describing a System Model
The basic components of a dynamic system are activities. The analysis of a supermarket operation,
for example, might yield such activities as the selection of merchandise by a customer, or the checking
out of goods for a customer, among others that deal with different aspects of supermarket operations.
Two important characteristics of activities are: (1) that they take time, and (2) that they
(potentially) change the state of a system.
When constructing a simulation model, the activities must be identified and represented in a way
that enables the model, when operating, to reproduce the time-dependent behavior of the system being
simulated. That is, the activities must be modelled in such a way that, when each occurs, the
system state changes in the proper way. This imposes requirements for (1) correctly modelling the
characteristics of activities, and for (2) sequencing the simulated execution of activities, so that the
order of performance of activities within a model corresponds to the order in which the same activities
occur in the real system.
The concepts embodied in Levels 1 through 4 are the essence of activity descriptions. Systems are
described (modelled) in SIMSCRIPT II.5 in the language of entities, attributes, and sets. Keeping
track of simulated time and organizing the execution of subprograms through which system activities
are represented are the essential functions provided by Level 5.
An activity within a system is bounded by two instantaneous events: when the activity starts, and
when it stops. Thus, the event is the simplest component of an activity description. The important
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