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The operation and proper management of reservoirs requires the prediction of reservoir performance. This prediction is generally done by computer simulation. Since simulation software generates static models in the number of millions and even billions, dynamic simulation on these models is difficult and sometimes impossible; therefore, multi-scale block generation is necessary. In this study, we introduce a methodology that was inspired by Earth’s pattern to generate multi-scale grids on a multi-phase, heterogeneous reservoir, where the enhanced oil recovery process is steam injection. From a reservoir engineer’s point of view, Earth is equivalent to a multi-scale grid model that could be a pattern for multi-scale grid generation in a hydrocarbon reservoir to minimize central processing unit time for dynamic simulation. The principle of multi-scale grid generation in hydrocarbon reservoirs is as follows: regions with a high Darcy’s velocity should be fine-scale and other segments with low intensity of heterogeneity regions could resize into the up-scale. The intersection of latitude and longitude lines means that Earth’s model could be a practical pattern for dynamic simulation. This intersection creates segments with fine-size blocks, like the South and North Poles, that are equal to the injection and production wells in the reservoir model and other segments that have different intensities of coarse-size blocks. Earth’s magnetic field, which enters from South and exits from the North Pole, leads us to could consider Earth as a hydrocarbon model. The results of the multi-scale grid generation method, which was inspired by Earth’s pattern, were compared with the fine-mesh (geological) model; the results show that the proposed method predicts with high accuracy and less run-time compared with the fine-mesh model.

To meet the requirements of connected vehicle (CV) system simulation evaluations, different simulators (i.e., traffic flow simulation, network communication simulation and CV application simulation) must be combined and interaction must be enabled among them during the run-time of the simulation. This paper proposes a CV simulation prototype system based on integration of three components in one coupling. Considering the characteristics of the distributed interactive simulation of multi-emulators, a high-level architecture simulation and modeling concept is used to construct the simulation framework, and the three types of simulation elements are defined as the primary federates. The integration simulation system is constructed by defining the federation object model and developing the support environment of the run-time infrastructure during simulation. In the simulation implementation procedure, a commercial-off-the-shelf time-stepped VISSIM simulator and an open-source event-driven NS2 simulator are selected as the basic simulation tools to develop the system. In addition, selected related technologies are investigated, including the NS2 simulation model transformation, time synchronization management strategy, track consistency strategy, and integration of application members. Finally, two case studies of intersection speed guidance and collision warning are used to verify the validity of the simulation prototype system.

The primary focus of computer simulation in transportation engineering has been to model individual systems using modeling software packages designed for the specific system under investigation. However, this limits the ability to explore interactions between multiple disparate transportation systems in a dynamic modeling environment. To address this gap, this study develops and tests a technique to federate two transportation models, each constructed using different simulation software packages: (1) a discrete event-based simulation model of a freight trucking terminal and (2) a discrete time-step-based traffic microscopic simulation model of the network serving the terminal. The federation technique is tested to consider potential feedback loops that may arise between the models during a single simulation run. It is then applied in a notional case study of the Port of Savannah to further assess its capabilities. The findings suggest that this federated simulation technique captures the dynamic interaction of the two systems being modeled. A comparison of the observed versus expected time-based characteristics of the interactions are shown to yield statistically significant correlation.

In this paper, a general Stewart Platform Simulation tool (STEWSIM) is developed for robot designers and researchers. The STEWSIM uses an efficient connection matrix algorithm that has been developed for kinematic design of all possible types of general Stewart platforms (GSPs), such as 3×3, 3×4, 3×5 … 6×5 and 6×6 with an arbitrary number of legs varying between three and six. This new interactive simulation software based on the MATLAB Graphical User Interface (GUI) provides a suite of analyses, such as inverse and forward kinematics, Jacobian matrix, dexterity, workspace, singularity analysis and trajectory planning, using different kinds of roll-pitch-yaw or Euler angle sets. The STEWSIM also provides the visualization of the GSPs, graphical representation of the workspace, and graphic animation of trajectories specified by inverse kinematics in three-dimensional space. Furthermore, it provides an environment to compare the kinematic performances of the designed GSPs with arbitrary configuration. As a kinematic design, analysis and simulation software, the STEWSIM provides researchers with the ability to change the parameters interactively for designing their manipulators with optimum specifications, such as leg lengths and attachment points. The usage of this simulation tool may result in new types of feasible robotic manipulators that can be used for practical applications in industry.

Discrete-event systems have gained a lot of interest due to their wide range of applications, and discrete-event simulation is a useful method for the performance evaluation of such systems. In this domain, model-based evaluation methods play an important role and there are many formalisms and realistic experiments using these methods. In this paper, we introduce SimGine, a multi-formalism simulation engine for stochastic discrete-event systems based on SDES, which is a unified abstract description for stochastic discrete-event systems. The engine is also capable of rare-event simulation of models using the importance sampling technique, which makes it the first multi-formalism simulation tool with rare-event simulation capability. The XML-based input language of SimGine allows for definition of the required methods. The body of each method is expressed by codes in a high-level programming language and this provides a powerful and flexible approach for defining events with complex behavior. For the simulation of an existing model, a tool for translating models into the SimGine input language should be prepared. SimGine can be used as a stand-alone simulation tool or as a simulation engine in other tools.

The Discrete Event System Specification (DEVS) formal modeling and simulation (M&S) framework (which supports hierarchical and modular model composition) has been widely used to understand, analyze and develop a variety of systems. Numerous DEVS simulators have been developed; nevertheless, evaluating the performance of such simulators is a complex process and it has been usually done using ad hoc methods. DEVStone, instead, is a synthetic benchmark that can be used to automate the evaluation of the performance of DEVS-based simulators. DEVStone generates a suite of models with varied structure and behavior automatically. To do so, it uses a standardized mechanism that can be the basis for comparisons between simulation software tools. As a proof of the concept, we present various tests in which DEVStone was used to study the efficiency of five different simulation engines. In this case, we compared various versions of the CD++ simulator, and then compared its performance with the ‘A Discrete Event System Simulator’ (ADEVS) M&S tool. This is the first effort in which these simulation tools have been thoroughly compared with a very demanding set of experiments. The use of DEVStone allowed a standardized and exhaustive method to compare different features of the simulation software. We show how the basic ideas used for DEVStone facilitates performance analysis for upgrades and updates of a given simulation engine, while also providing a common metric to compare different M&S environments.

It is a challenge to depict the shape of flexible components in solid modeling software. This is because flexible components in an automobile are subject to large elastic deformations during the movement of the rigid components to which they are attached. The principles of computation of the orientation of entities in a simplified coordinate system, vector mathematics, the file I/O features and structured C++ programming were used to view the rubber hose in CAD after modeling the hose using simulation software. The deviations of the hose shape at 0°, 90°, and 180° twist position obtained from the non-linear mathematical model and the actual location of the real hose obtained by using ATOS II scanning equipment were reduced by modifying various parameters. At the end of the trials the mean deviation was below 1 mm at the 0° position and below 6 mm at the 90° and 180° twist positions. A regression analysis was performed for generalization of the force deformation values characterizing the non-linear behavior of off-axis elements. This analysis makes this modeling technique applicable for predicting the shape of hose lengths in the range of 227.8— 2474 mm between any two attachments in space. Subsequently, the regression model was used to predict the hose shape for a different length. The results show that non-linear model with off-axis elements predicts the hose shape much more accurately than the linear model with beams.

Nonstationary Poisson processes are appropriate in many applications, including disease studies, transportation, finance, and social policy. The authors review the risks of ignoring nonstationarity in Poisson processes and demonstrate three algorithms for generation of Poisson processes with piecewise-constant instantaneous rate functions, a capability that has been implemented in commercial simulation software. They test these algorithms in C programs and make comparisons of accuracy, speed, and variability across disparate rate functions and microprocessor architectures. Choice of optimal algorithm could not be predicted without knowledge of microprocessor architecture.

The simulation process involves the collaboration of different participants, such as simulation analysts, programmers, statisticians, and users of the simulation software. Many simulation tasks such as modeling, verification, validation, and design for experimentation require the participants to meet. It is understood that these meetings are time-consuming and expensive. This paper proposes a collaborative environment to help with the tasks of discrete event simulation software development using the World Wide Web platform. The environment, named GroupSim, is based on a collaborative computer system and uses the concepts of distributed modeling with automatic program generation and distributed control of experimentation. The authors show some examples to illustrate the use of the environment and discuss some issues related to collaborative environments such as concurrency control, access control, awareness, and performance.

The authors take a ground vehicle simulation software architecture, which was designed for stationary simulation in crewstations in a laboratory environment, and reuse it to add embedded simulation capabilities to fielded ground vehicles. The purpose of embedded simulation is to enhance soldier effectiveness both before and during actual battlefield conditions. As a result, in a fielded ground vehicle, the computer specialist responsible for setting up, starting, controlling, monitoring, and stopping the simulation is not present. Unlike in a laboratory environment, in a fielded vehicle, the simulation software itself is responsible for performing these functions. The authors provide a case study illustrating characteristics of a system architecture that allow it to be long-lived by evolving not only to changing requirements due to technological and methodological changes but also to changes due to new and previously unthought-of patterns of system utilization.

This article reviews the characteristics of the model selection problem for simulation software or analyses and presents a multiobjective decision analysis approach for model selection that addresses these characteristics. The approach is illustrated with an application to selecting a modeling approach for simulating transport agents. The authors discovered that using multiobjective decision analysis for model selection can assist decision makers in more fully understanding all aspects of the simulation design problem and can uncover insights into the particular simulation project being designed.

A simulation model of the flow of intermodal terminal units (ITUs) among inland intermodal terminals is presented. The intermodal terminals are interconnected by rail corridors. Each terminal serves a user catchment area via a road network. The terminal is modelled as a set of platforms that are served by a number of gantry cranes and front lifters. Given the schedule of train connections among the terminals, an agent-based system, the intermodal transport planner (ITP), books ITUs on trains and assigns trucks to deliver them to the source terminal and to pick them up in the destination terminal. The terminal and rail corridor simulation software has been implemented as a discrete event simulation model, using MODSIM III as development tool. The ITP has been implemented on the basis of the TELETRUCK system. This research has been developed within the PLATFORM project, funded by the Directorate General VII of the European Community.

In recent years, the complexity of computer simulations has increased steadily, and it is now common to see very complex software development for simulation studies. These tasks are particularly challenging in the field of computer systems and networking simulations. Although there have been numerous efforts in the field of modeling and simulation of computer systems and computer networks, there have been few opportunities for researchers to discuss and highlight the main issues they face when building new software tools, techniques and architectures for these applications. Following the success of the first SIMUTools International Conference on Simulation Tools and Techniques (Marseille, France, March 2008), and its follow-up editions in Rome (2009) and Malaga (2010), this special issue aims at giving such an opportunity. The idea of this issue is to provide a timely reference of the current best practices and thus contribute to help the simulation community to tackle new problems and improve the current state-of-the-art techniques and tools. The first edition of SIMUTools received 114 submissions in this field out of which 33 were accepted for publication as full papers in the conference proceedings. The authors of the best of these papers and numerous other contributions in this area were submitted to this issue. After a long review process, this double-volume issue includes a number of varied topics briefly summarized hereafter, including a few extended versions of the original papers from SIMUTools, and many other interesting articles related to simulation tools and techniques. ‘A Component-Based Simulator for MIPS32 Processors’, by Yu Chen and Hessam Sarjoughian, introduces the use of DEVS-Suite for creating a library for simulation of computer architecture, based on MIPS32 processors. The authors compare single-cycle, multi-cycle and pipeline MIPS 32 architectures, by modeling them using their physical and functional architectures. Each component is built as a DEVS atomic/ coupled component, and a simulation is automatically executed based on the DEVS-Suite environment. The authors also discuss educational activities based on their model and tools, including an interesting survey. The paper ‘A Simulation Environment for Hierarchical Process Chains Based on OMNETþþ’, by Falko Bause, Peter Buchholz, Jan Kriege and Sebastian Vastag, introduces the use of the networking tool OMNETþþ in a different application domain: process chains. The authors introduce ProC/B, a library for hierarchical modeling that can be mapped into discrete-event simulation models. The idea is to map a graphical specification model and convert it into an OMNETþþ library for simulation purposes. The authors present the design of such a library, and discuss validation results of the transformation process. The article ‘Application-Level Simulation for Network Security’, by Stephan Schmidt, Rainer Bye, Joel Chinnow, Karsten Bsufka, Ahmet Camtepe and Sahin Albayrak, presents the design of NeSSi, a network simulator that incorporates varied network-security models (automated attack generators, trace analysis, intrusion detection, etc.). The main goal is to allow network service providers to improve security through simulation analysis, using a library tailored specifically for security experts and network administrators. The tool allows the user to model hierarchical networks, and it provides an advanced GUI to create network topologies and varied scenarios. The models can be executed with high performance, based on a parallel

This paper introduces the computer model established for a new family of modular type, multi-station walking beam high-speed chip mounter (also called chip shooter) machines, and the implemented high-accuracy simula tion software. The objective of the project was the de velopment of a comprehensive simulation tool that can replace the real machines in experiments regarding sys tem analysis and setup, yet provide accurate cycle time estimation. The thorough analysis of the machines led to the understanding of the characteristics of each part of the system, as well as the hierarchy of the parts and the interaction between them. This well-defined hierar chy and separability of different parts suggest the ob ject-oriented paradigm for the implementation of the simulation software. This paper highlights the major goals successfully addressed by the developed software: the identification of the system hierarchy with its rep resentation in a C++ class structure, the implementa tion providing code efficiency, and the design of I/O features that prepare the simulator to be embedded in an optimization procedure.

Due to the increasing popularity of simula tion, there are numerous simulation software tools available on the market. This paper pre sents the results of a survey on users' re quirements of discrete-event simulation software. The survey involved members of the Simulation Study Group of the Operational Research Society of Great Britain. Findings of the survey indicate which types of simula tion software are primarily being used, the most common application areas of simulation, and users' opinions about software and pos sible ways of improving simulation software.

Thermal contact resistances (TCR) play an im portant role in heat transfer. TCR is therefore of significance in the simulation of injection mold ing processes where one of the factors determin ing productivity is the cooling time. Simulation software packages either consider TCR as negli gible or use a constant value based on steady- state conduction, whereas in injection molding, TCR is time-dependent. We use simulation of the injection molding process to show that using TCR values within the range obtained from steady state experiments may lead to underpre dicting cooling times by up to 15% and, hence, there is a need to use time-dependent TCR in the heat transfer simulations. Our simulation result indicates that the value of TCR also affects the frozen layer fraction in the early postfilling period.

In this paper we present a discrete event simulation of the Archive and Analysis Subsystem (AAS) to be installed by the Japanese space agency, NASDA, as part of the Japanese contribution to the Interna tional Arctic Research Center (IARC) at the Uni versity of Alaska, Fairbanks (UAF). Scientists will utilize the AAS to support their Arctic research. The objective of this effort is to develop a simula tion model that will allow engineers to anticipate the loading of certain system components, especially the storage and network subsystems. At the start of this effort we determined that the simulation mod eling software Extend™, a product of Imagine That, Inc., of San Francisco, would be most appropriate in meeting our objectives within the given cost and time constraints. We present our architecture and model assumptions within the context of the engi neering design of the AAS. We also present results in the form of graphical displays from the model output, based upon our assumptions. We describe some of the difficulties encountered in making use of the Extend™ simulation software in modeling our subsystem and our approach to overcoming these difficulties.

Computer spreadsheets are widely used for a variety of modelling tasks and provide an interface that is well understood by most users. This paper presents a discrete-event simulation system using a spreadsheet as an input/output device which provides a platform on which to widen the base of simulation users. In an educational setting the system can provide an introduction to the student of the concepts behind the discrete-event method. Once the students grasp these concepts, they are better equipped to construct models using specialised simulation software. In a business setting the system could provide an introduction to the technique to a new set of end-users. A case study of a supermarket system is presented to show how the software can be used in an educational context.

Simulation is emerging as a general purpose planning, modeling, and problem-solving tool for a wide range of manufacturing processes. Availability of different simulation software has made it easier for engineers and managers to approach complex problems with a tool that allows them to evaluate "what if" scenarios. Extend™ is one of the newer simulation software packages on the market that has some unique features that may prove beneficial in modeling continuous as well as discrete-event problems. It is a powerful tool for decision-support systems, and provides insight into complex manufacturing and business problems. In this paper, a brief description of a fairly complex nuclear fuel transfer project and the features of Extend in modeling complex problems are discussed.

The Commonwealth Scientific and Industrial Research Organisation (CSIRO) Forestry and Forest Products has developed a PC-based fire spread prediction application that can be used by fire control officers as an aid in strategic planning and resource allocation. This computer application, called Siro Fire, is a DOS protected mode application that incorporates several fire spread models that can be applied to the two major fuel types found in Australia-grass and forest. It uses GIS-derived geographic maps and digital elevation models to calculate the probable spread of a fire across the landscape and displays the results on a graphical representation of the map of the area of concern. The application is being used in the South Australian Country Fire Service as both an operational and training tool. This paper outlines the evolution of previous fire spread simulation software, the development of SiroFire and how SiroFire carries out fire spread simulations using existing fire spread prediction algorithms.

As simulation has become a major tool in studying discrete manufacturing systems, experimental design issues have started to receive much attention. Although the importance of proper experimentation is often emphasized in the literature on discrete event simulation, it is neglected in most practical simulation studies. The main reason for this neglect is that the design and analysis of a simulation experiment require expertise in experimental design methodology as well as familiarity with traditional statistical output analysis methods. This article presents an attempt to structure the process of planning and designing simulation experiments, and proposes a knowledge- based system, namely Design Of Experi ments for Simulation (DOES), that assists the inexperienced analyst in this process. The proposed system takes the analyst through a planning process based on the simulation objectives.

This paper presents a design of a knowledge-based simulation environment based on techniques from conventional simulation methods, object-oriented programming methods, and knowledge- based programming methods. The environment has cooperating knowledge- based and simulation parts integrated into one modeling system. An idealized knowledge-based simulation model is presented along with a language for implementing such a model. This language is based on Symbolics Common Lisp with Flavors package. A model of the software development process is presented to illustrate the use of the knowledge-based simulation environment. This software development model is not intended to be a rigorous model of the software development process, but only to illustrate the use of the knowledge-based development environment which is presented in this paper.

Although most simulation practitioners are aware that output analysis is of extreme importance in a discrete-event system simulation, many are not versed in the process for conducting that analysis. Hence, it is not unusual that a simulation practitioner will conduct just one replication of the simulation, and provide the resulting measure-of-performance as the estimated response. Point estimates like this can be very dangerous, since they represent but one of many possible values of an experiment when random variation is present. It is much more appropriate to provide a statistical confidence interval, as a minimal form of output analysis. This article uses an example that describes a context in which variation in output performance measures can occur. The actual output from a simulation of that example system is used to develop confidence intervals. The output features of simulation software are briefly described. This is followed by a listing of these same output features and their availability on five simulation software products. The intention of the article is to draw attention to simulation output analysis and to show that support for it is readily available in simulation software. It is not the intention of the article to compare the five products, or to indicate that other simulation software products do or do not have the same or similar features.

In today's process plant design environment, steady state flowsheet simulations are fairly routine practices. With today's microprocessor power, with availability of capable and cost effective software, desktop dynamic models of entire plants are a practical reality. With such information age power tools, possibilities are infinite. Not the least is that the many what-if scenarios surely will enlarge the design parameter space, ensured by a systematic procedure in quality design, all at reasonable cost. With the multitude of software platforms, reusable (universal) models are cost effective. More significantly, these can be a boon to the credibility issue. The ability to take a complex model, developed elsewhere on an unfamiliar platform, plug into one's favorite simulation software and be operative without the pain/effort in re-engineering the entire model database can surely promote increase in usage of dynamic simulation among plant designers. A universal database is proposed to facilitate portability of models among language platforms. In this paper, portability of large complex models is proven by two examples (from plant design) on two platforms (DAP and ACSL). This paper illustrates the database requirements to allow models to commute between vastly different platforms, using specific examples to arrive at a more universal conclusion.

The development of a simulation software for port operation is described, with special emphasis on considerations for choosing both software and hardware. These consider ations, along with input and output requirements, led to a decision to write the software in C and to use simulation library SSS. The resulting software is portable, flexible and comprehensive, it addresses multiple port functions, various ship arrival patterns, fleet composition, labor and productivity issues. It is the only simulation model known to deal with coordination between terminals in more than one port.

Modern Simulation Program Generators (SPGs) must address four key issues: portability, user interface, consistency and extensibility. Portable SPGs allow the user to edit the model outside of the SPG giving the user the flexibility to add features not supported by the SPG. An SPG's user interface should mimic the user's perception of the system to be modelled. Often the user visualizes the model as a network diagram. The user interface and the SPG's consistency deter mine ease-of-use. SPGs should be developed under a standard environment to provide consistency between programs. They should be modular, to allow the code generator to be modified as new simulation tools are developed. Xsimcode addresses each of these issues. It is a portable SPG, providing flexibitity for experienced modelers. It uses a graphical, direct manipulation interface to represent the simulation model. A noun-verb dialogue limits the user's choices once an object is selected. As an X-Windows program, Xsimcode's interface is consistent with many other X-Windows programs. Xsimcode's user interface module and code generation module are separated by a data structure, allowing Xsimcode to be extended to support other simulation languages or ported to other windowing environments.

Computer software for modelling dynamic systems has existed for over three decades. During this period, computer modelling and simulation have been enhanced by powerful simulation languages, graphical modelling techniques, and statistical techniques. A recent trend in simulation software develop ment has been the addition of many analysis tools to existing simulation languages. An examination of these tools indicates that they are predominantly independent of the application area for which the simulation language is designed, suggesting that a general library of simulation analysis tools should be created for use with several simulation languages. A user-centered approach to simulation software design provides an application- independent description of the model analyst's needs. This description is the basis for a general environment within which simulation languages and models may be implemented, and prevents the redundant coding of application-oriented simulation software. It improves on the traditional language-centered approach, which first describes a simulation system in terms of an application language, and considers the experimental interface as secondary. This paper motivates and describes the design of user-centered software for visual interactive simulation.

Existing general purpose simulation software is not adequate for studying large, flexible spacecraft with digital controllers because it will not accept sampled-data equations or link to state-of-the-art structural modeling software. The MODEL S sim ulation language was developed to handle the first of these problems. It will accept mathematical models involving both differential equations and sampled-data equations. Computa tional delays can also be included. This makes MODEL S useful for the simulation of many digital controllers. Simple examples and an actual spacecraft controller show the language's power, flexibility, and ease of use. Planned enhancements include multirate sampling capability, software mode control and event sensing, and linkage to complex plant models.

A cognizant interface exhibits artificially intelligent knowledge and behavior toward the parties it is to serve in its interfacing role. These parties are the human user, on one hand, and the computer-based simulation software, on the other. The interface is therefore viewed as two-sided. To enable users who are neither computer specialists nor simulation experts to take advantage of sophisticated tools for system modeling and simulation, new approaches to interface design are necessary. They must encompass state-of-the-art technology (e.g. computer graphics and networks) as well as utilize techniques available in the areas of artificial intelligence and human-computer interaction. This paper has a three-fold purpose: to present results of a structured review of the simulation + ar tificial intelligence literature; to focus on criteria for engineering a modifiable interface; and to add to that interface the ability to adapt to the user. An illustrative prototype is outlined and a simulation-oriented framework of knowledge bases is presented.

SIMTOOLS is a collection of procedures and functions that allow discrete event simulation programs to be easily developed in Pascal. The package, which implements the event view, has pro cedures for creating and deleting entities, managing lists or queues, event scheduling and sequencing, system tracing and data collection. It is useful for any model-building effort, but especially those which do not fall neatly into the class of queu ing networks where specialized simulation software is available. The advantages of such an approach include the ability to apply top-down design, self-documentation, portability and the fact that the only development software required is the Pascal compiler.

An interactive threaded command language (TCL) was developed for design and simulation applications. The FORTRAN imple mentation of this language emphasizes a basic approach toward developing simulation software rather then the specific command language. TCL incorporates the extensibility and interactive features of threaded interpretive languages such as FORTH and STOIC, but it allows use of all the flexible data structures and numerical capabilities of FORTRAN 77. The approach presented here can easily be adapted to develop other higher level languages tailored to specific applications. Another interesting feature, within a FORTRAN 77 program, is the handling of various display terminals through an internal data structure and an ASCII text file that contains the terminal's "characteristics".

This paper describes a subprogram module which sim plifies the implementation of parameter optimization studies in continuous dynamic systems. The module is an enhancement to CSSL-based simulation software and relies on its host software for the specification and solution of the dynamic equations of the system under study. The module includes several different optimization algorithms -a feature which can be of great value in difficult problems. It has output documentation routines and is organized so as to be transparent to the user.

In any future standard for continuous system simulation lan guages, it is of primary importance that the data and program structures are made sufficiently general and flexible. In this paper, the use of matrix structures in modeling and simulation, and the impact of matrix environments on the design of simulation languages and their experimental frames will be discussed.

DISCO is a SIMULA-based language for the simulation of systems which contain both continuous and discrete- event processes. It is a true extension of SIMULA's concepts for discrete-event simulation and is capable of representing and analyzing models including both types of processes. With the addition of few new con cepts, a general yet simple simulation language has been achieved. This paper describes the basic concepts of DISCO and demonstrates through examples the descriptive power of the language. DISCO is described mainly from the user's point of view; its implementation is only sketched.