Chapter 1. Introduction


Predicting the behavior of complex real world systems -- that's what GPSS World is all about.

Many expensive projects in the past have failed because the end result was not characterized accurately. From maximum capacity to cost of operation, it is essential for detailed knowledge of the behavior of the system under construction to be known as soon as possible. Although purely mathematical models are extremely valuable, and should be used where feasible, the complexity of most real world systems requires the use of computer simulation to get the necessary answers. That's where GPSS World comes in.

GPSS World is based on the seminal language of computer simulation, GPSS, which stands for General Purpose Simulation System. This language was developed primarily by Geoffrey Gordon at IBM around 1960, and has contributed important concepts to every commercial discrete event Computer Simulation Language developed ever since. GPSS World is a direct descendent of GPSS/PC, an early implementation of GPSS for personal computers. Since it's introduction in 1984, GPSS/PC and its successors have saved thousands of users millions of dollars. Now, the Windows implementation of GPSS World extends these capabilities into an Internet aware environment.

GPSS World is designed to deliver answers quickly and reliably, with the least effort, achieving the highest reliability of results. Consistent with these objectives, visualization of running simulations is highly stylized and a default statistical treatment is built in. This approach means that animations are "free" requiring no additional effort to produce, but are not photo-realistic. GPSS World's forte is transparency, not photo-realism. Third party animation systems are available which can provide pictorial animations based on GPSS World simulations.

Transparency is valuable for three reasons. First, it is dangerous to rely on an opaque "Black-Box" simulation whose internal mechanisms cannot be observed. Not only can you not be sure it fits your situation, but it is difficult to be assured that it even works as intended. Second, successful simulations are valuable and have a surprisingly long lifetime. It is possible that new staff members will be required to become familiar with the internal workings of the simulation -- a near-impossible task unless provision has already been made for a high level of transparency. Third, one of the most effective but least mentioned benefits of computer simulation is the insight on system behavior achieved when an experienced simulation professional can see the internal dynamics at crucial times in the simulation.

GPSS World was designed to address these issues. Its visualizable nature allows the internal mechanisms of models to be revealed and captured. Its interactivity allows one to explore and manipulate simulations. Its built-in data analysis facility can calculate confidence intervals and an Analysis of Variance easily. And now, it can even create and run sophisticated screening and optimization experiments automatically, with relatively little effort from you.

Most systems can be modeled in any of several ways using GPSS World. Usually only a small subset of the features available need to be used. However, the greatest proficiency requires familiarity with all that GPSS World has to offer. This manual is the primary source of that information.

This chapter consists of 5 sections. The first is a brief overview of the most important aspects of GPSS World. The second explores a few basic concepts that are necessary for moving on to later chapters. The third section discusses the advanced architecture of GPSS World and what you must know to take advantage of it. The fourth section explores the modeling language of GPSS World, and the fifth section is for GPSS/PC users who want to take advantage of the Compatibility Mode of GPSS World. This last feature is available only in the Commercial Version of GPSS World.

Installation and operating procedures are discussed in Chapter 2.


    1.1. Highlights

GPSS World is object oriented. Its inhabitants include Model Objects which are used to create Simulation Objects. Simulation Objects, in turn, are used to play out simulations and create Report Objects. Finally, Text Objects can be used as Include-files to support code sharing and a user source code library and they are often used as files which can be read from or written to by the simulation.

Simulation projects require several steps. They normally include model building and data collection, testing and verification, simulation, experimentation, and the analysis of results. GPSS World has a large number of capabilities addressing each of these steps. In GPSS World, you will create and modify a model using the full-screen text editor. If you prefer, you can insert GPSS Block Statements using special Block Input Dialogs where you just fill in the blanks. The resulting Block Statements are placed sequentially at the insertion point in your Model Object. You then Create a Simulation Object by selecting Command / Create Simulation in the Main Window Menu. Thereafter, you have a powerful set of Commands at your disposal to control the running of the simulation. You can enter Commands interactively, or you can include them in the original model. During the testing and verification phase, a large number of window types are available for online viewing and to take snapshots of the simulation. Hot Keys and Point and Shoot breakpoint control makes verification and debugging easy. After the simulation is tested, you can use the Automatic Experiment Generators or you can design your own experiments. GPSS World will help you every step of the way.

GPSS World was designed to exploit your computing environment. The use of virtual memory allows your models to grow to a billion bytes. Preemptive multitasking and multithreading mean higher responsiveness and permit GPSS World to be doing many things at the same time. It also means that the simulation environment can utilize the computing capacity of Symmetric Multi-Processing architectures.

The structure of the modeling language has been simplified in GPSS World. A Model is now defined as a sequence of Model Statements. Each can be a GPSS Statement or a PLUS Procedure definition (possibly a PLUS Experiment definition). A Model Object is used to create the corresponding Simulation Object, which can then be activated. Any Model Statement can be sent to an existing Simulation Object. Such statements are called Interactive Statements. These concepts are discussed in more detail in the next section.

New in GPSS World are the GPSS Block creation dialogs. These are great in a teaching environment. Using the Edit / Insert Block menu command, you can open the Block Menu Window which has a button for each GPSS Block. Passing your mouse over each button is all you need to do to see a description of that Block. When you click on a button, a Dialog opens with all the details you need to create a GPSS Block. Just fill in the blanks. The Help button takes you to the appropriate place in the Reference Manual describing the block. When you click OK the resulting block is inserted in the Model Object. Don't worry, any syntax errors are reported immediately so you can correct them on the spot. For formatting purposes, the inserted GPSS Block Statements contain tabs. You can alter the tabstops to your liking in the Settings of the Model Object. In any event, since inserted Block Statements and PLUS Experiments are in the form of source code, you can always edit them before Creating the Simulation Object.

The simulation language within GPSS World has been extended by PLUS, the Programming Language Under Simulation. This simple but powerful programming language removes the restrictions that existed in the older GPSS implementations. Data within this environment are untyped, with conversions occurring automatically as needed. In addition, powerful function and probability distribution libraries are directly available to be used in PLUS Expressions. The Procedure Library supports string manipulations, numeric calculations and probability distributions. User written PLUS Procedures can be used in the same way. An INCLUDE Command can bring in existing User Procedure Libraries containing tested PLUS Procedures for use throughout the simulation.

User-defined PLUS Procedures and can be accessed anywhere in the Model. Expressions, defined in PLUS, can incorporate data elements and System Numeric Attributes. When parenthesized, PLUS Expressions can appear outside PLUS Procedures, in GPSS Statements. In fact, most arguments in the GPSS Statements can now be in the form of a parenthesized expression.

In Version 4 of GPSS World, the PLUS Language has been extended for definition of Experiments. This powerful feature permits programmable control and can even be based on simulation results. Thus, completely automatic operation is possible including the exploration of response surfaces. PLUS Experiments, which are invoked by the CONDUCT Command, can be used to control the running of simulations over a parameter space. Experiments can be created with minimal involvement on your part. GPSS World now includes two new powerful experiment generators that can be accessed through the Edit Menu of the Main Window. The Screening Experiment Generator will create a fractional factorial experiment under your direction, and insert its PLUS source code into your model. Similarly, the Optimizing experiment generator will insert a sophisticated response surface exploration experiment that searches for minimum or maximum yields. The CONDUCT Command necessary to initiate the experiments is by default loaded into a Function Key. To execute a Screening or Optimizing Experiment and analyze the results, all you have to do is press a button.

GPSS World has comprehensive discrete and continuous modeling capabilities. The tightly knit continuous modeling feature allows for easy transition between the continuous and the discrete phases. From the continuous phase thresholds can be established that trigger the creation of Transactions for the discrete phase. Conversely, the INTEGRATION Block and INTEGRATE Command, control the processing in the continuous phase.

Several new GPSS Blocks have been added to cover the control of integrations, Transaction rescheduling, changes to Assembly Sets, user-defined PLUS Blocks, and Data Streams. Data types now include integer and real numeric values, as well as strings. Each type is automatically coerced to the required form, as needed. The new data type "UNSPECIFIED" now is available to indicate the absence of valid data, such as that from a missing run of an experiment.

Even matrix structures have been improved. They can now incorporate up to 6 dimensions. The new multiway ANOVA Library Procedure analyzes data from a "Result Matrix", which is nothing more than a GPSS Matrix Entity where the yields of an experiment have been stored in a conventional way.

GPSS World is easy to operate. A full screen text editor can be used in any of the Text Windows. Even the Journal Window and Reports can be customized and annotated. You can use bookmarks as placeholders in any text window. Then since a single keystroke takes you to successive bookmarks, jumping from place to place in a large Model is a trivial matter. When you Create a Simulation, if any errors are detected, they are remembered so that you can correct them one at a time. When you go to each successive error, the cursor moves automatically to the location in the Model where the error was detected to help you locate each problem. After the Simulation is Created, several Hot Keys can be used in place of Interactive Commands. Not only that, using Model and Simulation Settings, you can load your own Commands into one or more function keys in order to get single keystroke response. Windows are easy to open. Generally, only a menu selection is needed. During debugging, the Debug Toolbar allows even breakpoint control to be accomplished in a Point and Shoot fashion.

Itís easy to visualize running simulations. GPSS World can create stylized animations of any GPSS entity type with no more than a mouse click or two, from you. These windows are dynamic -- showing the changing state of running simulations. In several different ways, you can view the changing condition of any GPSS entity. In addition to the entity specific views, one or more Expression Windows can be opened to display the changing values of any number of your own Expressions. For visualization of any of the variables in your simulation, any number of scrollable PLOT Windows can be opened to show dynamics graphically. And finally, the Table Window opens a changing view of any frequency distribution you care to see.

GPSS World provides for a set of snapshots, as well. These are advanced features intended for professionals who need detailed knowledge of microstates of the simulation. Static snapshots can be taken of any Transaction, the Future Events Chain, the Current Events Chain, or the membership of the Numeric and Transaction Groups.

GPSS World is highly interactive. All Model Statements can be used interactively. When you send a Command, Block Statement, Procedure Definition, or Experiment Definition to an existing Simulation Object, the interaction is used to redefine or manipulate the state of the simulation. This level of control is useful while the behavior of the simulation is being verified. In essence, you can do almost anything, except Block insertion, to the simulation after it is created.

Simulations can communicate with the outside world. You can now use five new GPSS Blocks and/or PLUS Procedures to manipulate Data Streams. They are OPEN, CLOSE, READ, WRITE, and SEEK. The last of these, SEEK, provides for direct access into a database. Data Streams have many purposes. You can use them to access data files, to create Result Files and custom reports, to access internal data directly. There are now PLUS Library Routines with the same names that perform nearly the same functions. That means that complex I/O operations can now be handled inside User-created PLUS Procedures. GPSS World simulations can even communicate with other products by direct invocation. The PLUS Procedure Library now contains the Call(), Call_Integer(), Call_String(), and the Call_Real() procedures which can invoke external functions existing on your system in executable files (i.e. EXE and DLL files). These features are described in Chapter 8.

The analysis of results is easy in GPSS World. It has facilities that support the capture and printing of graphical windows. The Journal Window records the activities associated with the Simulation Object. An automatic Report numbering system ensures the safe keeping of each Standard Report. The new ANOVA Library Procedure can perform a complete multiway Analysis of Variance when passed a Result Matrix. That's all there is to the generation of an ANOVA table and a set of Confidence Intervals. And now, the Automatic Experiment Generators can create sophisticated screening and optimizing experiments based on dialog-window input from you. The basic analyses of these experiments are done automatically and reported in the Journal Window.

A new Batch Mode of operation can be used to run simulations in the background. If you specify a Model File or a Simulation File on the DOS command line followed by the word "BATCH", GPSS World will run in a minimized window. It first opens the file and then passes it a Create Command (if a Model Object) or a CONTINUE Command (if a Simulation Object). In the former case, you would normally append the desired run control Commands in the Model itself. You can use the new EXIT 1 Command (or Exit(1) Library Procedure) to shut down the resulting Session by itself, automatically saving all the newly created or modified Objects. EXIT is discussed in Chapter 6.

GPSS World does inherit some characteristics from GPSS/PC. For example, normally it retains the use of the # character as the multiplication operator, reserving the * character for GPSS Indirect Addressing. You can switch the function of these characters so that * denotes multiplication, if you like, by changing a Model Setting. Although GPSS Statements, except Function Followers, must still fit on a single line of text, the maximum line length has been increased to 250 characters. PLUS Statements are not so restricted. They can span any number of lines.


    1.2. GPSS World Concepts

GPSS World is populated by 4 kinds of objects. Model, Simulation, and Report Objects form the 3 basic types that are used in all GPSS World simulations. Typically, a Model is developed by editing the statements in a Model Object. Then, a Create Simulation Command is issued, thereby creating a Simulation Object structured according to the statements in the Model. Simulations are run by sending them GPSS Commands, or by including commands in the Model Object, itself. Normally, when a simulation completes, a Report Object is created automatically.

Overall control of multiple simulation runs can be handled within this framework by including a PLUS Experiment in the Model Object. (PLUS is the "Programming Language Under Simulation" that can be used to define procedures and experiments.) Then, a CONDUCT Command can be sent to the Simulation Object to control a series of simulation runs, and deal with the results.

The Text Object completes the 4 types of GPSS World Objects. These are named in INCLUDE Statements to aid the development of large simulations, and for a few other handy uses such as the creation of a source code library. In addition, Text Objects are often used by Simulation Objects, when OPEN, CLOSE, READ, WRITE, and SEEK operations are performed.

All GPSS World Objects can be saved at any time, in any state, and reopened in a later session. This includes Simulation Objects, which can be run any number of times from a saved state, thereby facilitating debugging and demonstrations. With some careful planning, Simulation Objects can be saved in the middle of complex Experiments, allowing you to complete the simulation runs at a later time.

All GPSS World Objects except Text Objects contain data in addition to the basic ASCII text. This is why you can use an external text editor to edit a Text Object, but not any of the others. Model, Simulation, and Report Objects contain Bookmarks for easy navigation, and a list of attribute values called Settings, which can be modified and are inherited by child objects. When a Simulation Object is Created it begins with a copy of the Settings of the Model that created it. The same goes for Report Objects which inherit their settings from the parent Simulation Object. In addition, Model Objects remember all the Translation errors encountered the last time you tried to Create a Simulation Object from it. That way you can make some corrections and save the rest for later.

Model Objects

A Model Object contains primarily a sequence of Model Statements and a set of built-in values called Settings. In addition, a set of Bookmarks and a circular Result List of syntax errors are part of the Model Object. When a Simulation Object is Created by Translating the Statements in a Model, it inherits all the Settings from the parent Model Object. The Command / Create Simulation menu command in the Main Window is used to create a new Simulation Object.

Model Statements

A Model is the set of all statements contained in a Model Object and all its Include-files. A Model Statement can be either a GPSS Statement, or a PLUS Procedure definition. GPSS Statements, in turn, can be either Block Statements or Commands. Model Statements define a Simulation Object when it is first Translated, and they can be sent as interactions to an existing simulation.

Commands are either Queued or Immediate. Actually, all Commands except HALT and SHOW are Queued. Each Simulation Object has a special chain called a Command Queue that holds all Queued Commands received by the Simulation Object. Commands are taken off the Command Queue, one after the other, for execution.

The HALT Command is a special case. It is an Immediate Command, and therefore is not placed on the simulationís Command Queue. However, when a HALT is executed, all remaining Commands are removed from the Command Queue and discarded. This ensures that all action on behalf of the simulation ceases when a HALT is received. Normally, a HALTed simulation is resumed by sending it a CONTINUE Command.

Multiple Model Files

Usually, a Model is the set of Statements in a single Model Object, and optionally, any number of plain text files, called Text Objects. It is sometimes useful to place a sequence of one or more Statements, PLUS Procedures, and/or PLUS Experiments into one or more separate Text Objects in order to modularize the model.

Text Objects, which are plain text files, can be named in INCLUDE Statements in the Model Object. Doing so causes all Statements in the Text Object to be inserted in place of the INCLUDE Command when the Simulation Object is created. Text Objects may themselves may contain INCLUDE Statements, to a maximum nesting level of 5. INCLUDE Commands can be used interactively, as well as within Models. They can even be loaded into Function Keys, thereby invoking a complex sequence with a single keystroke. This is discussed in detail in Chapter 2.

A Model is built from the Statements in exactly one Model Object and optionally any (nonnegative) number of Text Objects, which are mentioned in INCLUDE Statements encountered during the Translation of the Model. Only the Model Object can have Settings associated with it, which can be accessed through the Main Window's Edit / Settings menu command. These Settings are inherited by all Simulation Objects Created from that Model Object. To prevent the occurrence of multiple Settings, Model Objects may not be used in INCLUDE Statements, only Text Objects.

There are two kinds of numbering associated with files. Model and Include-files are numbered internally so they can be referenced by Status Messages coming from the simulation. The number is the 0-relative ordinal of the file in the order encountered by the Translator when the Simulation Object was created. For example, the primary Model Object is assigned Model File Number 0 and the Include-files are numbered as they are encountered starting with 1. This numbering is for identification in error messages only. A different numbering system is applied to newly created simulation and report files in order to keep the file names distinct. You can set the numbering of the filenames of child objects within the Settings of the parent object. For example, to change the numbering of simulation filenames, you should edit the Settings of the parent Model Object. Similarly, edit the Settings of the Simulation Object to set the numbering of its child Report Objects.

Model Settings

Each Model Object carries with it a set of choices called Model Settings, or simply "Settings". These are accessible using the Edit / Settings menu command in the Main Window's menu. Settings are inherited by the Simulation Object, and also by the resulting Report Objects. Some settings can be changed even after the Simulation Object is created. The details are in Chapter 2.

Simulation Objects

A Simulation Object is created by translating the statements in a Model Object. The Command / Create Simulation menu item is used for this purpose. After a Simulation Object is created successfully, commands are then used to advance the state of the simulation. These Commands can be part of the Model Object, or they can be sent to the existing Simulation Object as interactions.

The Translator

The high performance Translator is that part of the GPSS World program which Creates the Simulation Objects. All Model Statements are Translated before they are incorporated into the simulation. Similarly, interactions are Translated in global scope before they are sent to an existing Simulation Object.

When errors occur which prevent a Simulation Object from being Created, a list of messages is placed in the Model Object. Then, you can move to the next error message by a mouse click in the Search / Next Error menu item or using accelerator keys. Each error is described in the Status Line at the bottom of the Main Window and the mouse cursor jumps to the location in the Model text where you can correct the error. Correcting and retranslating is quick and easy. There is a special command, Command / Retranslate, for retranslating the Model into an existing Simulation Object.

Report Objects

One of the strongest features of GPSS has always been its Standard Reports. With essentially no effort from the model developer, a simulation run automatically reports on all the GPSS Entities defined in the model when the simulation's Termination Count reaches zero. There are a variety of ways of customizing these reports by Editing the Settings in the Simulation Object.

Text Objects

A Text Object is simply a way to represent a plain text file in GPSS World. Their primary use is as the target of INCLUDE Commands. This allows a shared set of Model Statements to be accessed easily from a Model Object. You can even load an INCLUDE Statement into a function key. In that way, a complex Command list and/or PLUS Procedure in a text file can be sent to a Simulation with a single keystroke.

Text Objects turn up in other places, as well. Data Streams allow a Simulation to read and write data from text files, and to create Result Files for later analysis. As you might expect, these files are all represented as Text Objects.


    1.3. Architecture

At the interface level, GPSS World is an implementation of the Document/View architecture common to Windowstm applications. Objects can be opened with multiple views, modified, and saved to persistent memory according to highly intuitive action sequences on the part of the user. The familiar main window menu and the disabling of inappropriate menu commands guides the user to his/her objective with minimal distractions. Internally, GPSS World has been designed to take advantage of close interactivity even in a multitasking virtual memory environment. This is discussed next.


        1.3.1. Multitasking

The Multithreaded-design of GPSS World allows it to run multiple simulations and experiments simultaneously. Not only is screen update, user input, disk Input/Output, printing, and simulation done concurrently, but any number of simulations can be run at the same time, as well.


        1.3.2. Virtual Memory

Simulations are not directly limited to the size of the physical random access memory (RAM) where the Simulation Object is run. Using virtual memory, simulations can be up a gigabyte in size. No other direct entity limits are imposed, assuming adequate swap space is provided. For optimum performance, however, sufficient real memory must be available. All entity allocation and management is invisible to the user. Entities are created automatically, unless additional information is required. REALLOCATE statements are not used.


        1.3.3. Interactivity

GPSS World maintains a high level of interactivity with your simulations. You can send any Model Statement to an existing Simulation Object, using the Command item on the main menu of the Model Window, Accelerator Keys, or by using Model Settings to load function keys with your own customized Commands. You can use the Custom Command Dialog for statements not on the drop down menu, and using an INCLUDE command, you can send an interaction of any complexity to a simulation. Even INCLUDE Commands can be loaded into Function Keys, thereby supporting iterations of any complexity invoked by a single keystroke.

Block Statements, when used interactively, cause the Active Transaction to attempt to enter a temporary block, which is then discarded. This mode is called Manual Simulation Mode and it is useful during Experiments, providing all the functions available inside the simulation.

PLUS Procedure and Experiment Statements case a user-defined procedure or experiment to be registered in the simulationís Procedure Library. Any existing Procedure item of the same name and type is replaced.

Commands, except for HALT and SHOW, when received by an existing Simulation Object, are placed on the simulationís Command Queue to be executed in turn. HALT and SHOW are Immediate Commands, and are executed immediately when first received by the simulation. In addition, HALT removes all waiting Commands from the Command Queue. CONDUCT Commands initiate a PLUS Experiment that takes over control of the Simulation Object. Experiments can base decisions on results and invoke a statistical analysis, however interactivity is reduced during an Experiment.


        1.3.4. Visualization

GPSS World excels in the visualization of running simulations. Consistent with the generality of the GPSS language, twenty different windows are available for observation of and interaction with your simulations. No additional effort beyond operating the windows is necessary to obtain, save, and print a visual representation of the state of the simulation. Whereas, Simulation Snapshots a one-time static picture of a quickly changing entity, Simulation Windows are updated online, dynamically changing to keep current with the changing state of the running simulation. Although the dynamic windows show the changing state of entities, they also cause simulations to run more slowly.

Many windows have a non-detailed view and a detailed view which show alternate sets of information. Others allow you to display a running numerical simulation clock at the bottom of the Main Window. You make the choices using the View item on the Main Window's menu bar.

The details on the appearance and operation of each of the windows is in Chapter 5.


Snapshots are available for:

 Current Events Chain

 Future Events Chain

 Individual Transactions

 Numeric Groups

 Transaction Groups

Dynamic Windows

Any number of online simulation windows can be opened on the following entity types:









The Table Window presents a dynamic histogram that is extremely useful for viewing the accumulation of values, looking for outliers, and observing the convergence to a parent probability distribution.

In addition, the Plot Window and the Expression Window can be used to follow values with meaning to your simulation. Any number of multicolored on-line plots can be viewed, saved, or printed by opening one or more Plot Windows. Each window can track the value of up to 8 general Expressions, including integration variables. Plots can be scrolled vertically and horizontally, and can be scaled in either direction, as well.

The Expression Window can be an extremely powerful microscope on your simulation. Using it, you can view the changing values of any number of PLUS Expressions, of your own creation. As with all Simulation Windows, you can open Expression Windows at any point in the simulation.

The Blocks Window shows the entry of Transactions into blocks. It allows you to set and remove breakpoints using the mouse or the keyboard and to visually step through the simulation.

Most Interactive Windows have a Debug Toolbar which allows a "Point and Shoot" debugging session using only the mouse. The Debug Toolbar is discussed more thoroughly in Chapter 5.

Simulation Clocks

Each Simulation Object has an internal toggle switch that causes it to print the changing value of its System Clock in the rightmost pane of the Status Line at the bottom of the Main Window. This provides an easy verification that the Simulation is progressing normally. When you click on any View of a Simulation Object, it will display its clock if you have set its "Show Clock" toggle switch. This is very useful for monitoring the status of Experiments.


A variety of animation possibilities exist in GPSS World. The level of realism varies from abstract visualization requiring no effort, to highly realistic dynamic representations involving complex user-drawn elements.

Abstract Animation

A comprehensive stylized animation capability is built into GPSS World. This facility is provided by a large set of Simulation Windows which show the GPSS Entities of the simulation dynamically, as they change state during the simulation. No additional effort is required to view such animations, other than merely opening the window. The graphical images may be saved for inclusion into reports, and/or printed, as well.

Post-processing Animation

GPSS World includes an external interface that can support simulation animation packages that are driven by trace data. Photorealistic animations can be developed in this manner. To use this interface, you construct an output Data Stream obeying the formatting and content rules of the third party animation package. Data Streams are discussed in the next section..

Online Animation

GPSS World includes set of dynamic call PLUS Procedures that allow you to invoke the functions in third party executable files. This can provide an online link to a third party animation package. The details are in Chapter 8.


    1.4. The Modeling Language

GPSS World contains an implementation of GPSS, the General Purpose Simulation System, enhanced by an embedded programming language PLUS, Programming Language Under Simulation.

The GPSS variant includes over 50 Block Statements and 25 Commands. It includes over 35 System Numeric Attributes, which provide instant state variables, accessible anywhere in the model.

PLUS is a small but powerful procedural programming language created out of only 12 statement types. Much of its power comes from a large procedure library containing mathematical and string manipulation functions, and a strong set of probability distributions.

In GPSS World, a Model is defined to be a sequence of Model Statements. These are either GPSS Statements, PLUS Procedure Statements, or PLUS Experiment Statements. GPSS Statements, in turn, are either Block Statements or Commands. Except for Function Follower Statements, all GPSS Statements must reside on a single text line of up to 250 characters. Any Model Statement can be part of the model in a Model File, or can be sent interactively to modify an existing simulation.

The inclusion of Expressions in PLUS Statements and Parenthesized Expressions in GPSS Statements is a strong enhancement to the modeling power and ease of use of the language. The use of the [*] character to denote multiplication has been retained as the default. This clearly distinguishes that arithmetic operation from the GPSS indirect addressing operation which is denoted by the [*] character. However, you can now switch the role of these operators if you prefer. This is done on Page 1 of the Settings Notebook of the Model Object.

Commands are discussed in Chapter 6, Block Statements in Chapter 7, and the PLUS Language, including the Procedure Library, in Chapter 8.


        1.4.1. Whatís New

Many changes have been made to the modeling language. The distinction of Control Statements has been removed. All GPSS Statements that are not Block Statements are now simply called Commands. Line numbers are ignored in GPSS World. If used, they must begin in the first column of the text line.

Automatic truncations have been removed from GPSS World. You must now explicitly use the INT( ) procedure, or some other method, if you wish intermediate numeric results to be truncated. This is true in all Expressions, even in VARIABLE and BVARIABLE Statements. Similarly, the data type returned by a System Numeric Attribute can now be integer, real, or string, depending on the SNA. Even those SNAs returning parts per thousand do so as a real number of double precision between 0 and 1000, inclusively. The old truncations and integer SNAs can be enforced by running a Commercial Version of GPSS World in GPSS/PC Compatibility Mode, which is discussed below.

PLUS Statements can span any number of text lines. Although GPSS Statements, except for Function Followers, must reside on a single text line, the maximum length is now 250 characters.

Parenthesized mathematical expressions can now be used nearly anywhere you can use an SNA. Expressions are an integral part of the PLUS Language, but they are also used to expand the power of operands in GPSS Statements. The permissible forms of each operand are given in the statement descriptions in Chapters 6 and 7. Many new statement types have been added, as well. These are discussed next.

Several new features are available in both Student and Commercial Version. Here's a quick overview:

 Automatic Optimizing Experiment Generator

 Automatic Screening Experiment Generator

 Snapshots available in Student Version

 Multiway 'ANOVA' Library Procedure

 Variable Argument count for 'PolyCatenate' Library Procedure

 'Call' Library Procedures to invoke external functions

 PLUS Trace Setting

 BATCH Mode Sessions

 EXIT Command with File Save Modes

 Block Creation Dialogs

 Adjustable Model Tabstops

 PLUS Data Stream Procedures

New GPSS Statements

Several new Blocks have been added.

New GPSS Blocks

 ADOPT - Change Assembly Set.

 DISPLACE - Change the Next Sequential Block of a given Transaction. Save FEC residual time. Can be used to reschedule events.

 PLUS - Evaluate PLUS Expression and save result in Parameter.

 INTEGRATION - Turn the integration of a User Variable ON or OFF.

 OPEN - Initialize a Data Stream for operation.

 CLOSE - End the Data Stream operation.

 READ - Bring the next line of data from a Data Stream into a parameter of the Current Transaction.

 WRITE - Send a line representing the value of a Transaction Parameter to the next slot in a Data Stream.

 SEEK - Change the Current Line Position in a Data Stream.

New Commands

Statements have been added to control the integration of continuous state variables, to provide for multiple Model Files and Procedure Libraries, and to invoke PLUS Experiments..

 CONDUCT -Begin a previously registered PLUS Experiment.

 EXIT - Terminate Session, optionally saving objects.

 INTEGRATE - Automatically integrate an Ordinary Differential Equation, with optional Transaction generation trigger values.

 INCLUDE - Insert a Text Object for Translation.


The new System Numeric Attribute A1 returns the Assembly Set of the Active Transaction. This is useful in conjunction with the new ADOPT Block which lets the Active Transaction change its Assembly Set.


        1.4.2. Language Elements

GPSS World provides for a range of language elements for convenience in the development of complex simulations.

Polymorphic Data Types

Variables can now take on values with one of four types. User controlled variables, such as Savevalues, Matrix Elements, Transaction Parameters, and User Variables can take on integer, real, string, or UNSPECIFIED values. Clock values are integer or real, in double precision.

Conversion between types is automatic. Procedures that require a specific data type as an argument will coerce the argument to the appropriate type. If you pass a string to a Procedure that requires a numeric argument, the numeric equivalent of the string is used. Similarly, if you attempt to WRITE a numeric value, it is converted to a line of text automatically.

String values have many uses. They can be used in Data Streams, to format reports and Result Files, and internally for direct access of data. String constants are denoted by enclosure of the string in a pair of double quotes. You can use two double quotes together to represent a single double quote internal to the string. For example, you would use a total of 6 double quote characters to represent a string within a string. The inner string would be "sandwiched" by two pairs of double quotes, and the overall string would be "sandwiched" by a double quote character on each end. Procedure Library contains many string procedures that can be used to create and manipulate strings.

You can now give the Data Type 'UNSPECIFIED' to Savevalue Entities, and Matrix Entities. The INITIAL Command allows you to assign this type to Savevalues, Matrix elements, and even whole Matrices at once. If an UNSPECIFIED datum is used in an operation that requires a value, an Error Stop will occur. The ANOVA Library Procedure has been upgraded to recognize UNSPECIFIED elements in a Result Matrix as unavailable experimental run results.

Initializing Data Structures

Variables and Matrix elements may be given initial values in several ways.

 The INITIAL Command can be used to give values to Logicswitch, Savevalue, and Matrix entities. PLUS Assignment Statements are used to assign values to higher dimension matrices.

 SAVEVALUE Block Statements assign values to Savevalue Entities.

 MSAVEVALUE Block Statements assign values to Matrix Entities.

 LOGIC Block Statements assign values to Logicswitch Entities.

 READ and ASSIGN Block Statements assign values to Transaction Parameters.

 PLUS Assignment Statements can be used to assign values to User Variables and Matrices.

 The CLEAR Command normally resets the values of Savevalues, Logicswitches, and Matrices to integer zero, This action can be suppressed by specifying OFF in the B Operand of CLEAR. Normally, you would use the OFF option in CLEAR Statements to be used in Experiments. Otherwise, the Result Matrix would not be preserved.

It is often useful to isolate initialization statements in a Text Object. Then, only an INCLUDE Command need be asserted to perform the complete initialization. This can be done interactively, as well.

PLUS Experiments can control initializations to any level of detail. Any initialization command can be invoked within an Experiment by use of the DoCommand Library Procedure.


Multidimensional Matrices

Matrices can be defined with up to 6 dimensions. Dynamic Matrix Windows view any cross-section of a Matrix, and there may be any number of Matrix Windows. Temporary matrices can be created for use during procedure invocation. PLUS assignment statements are used to initialize matrices of dimension 3, or more.


GPSS World supports the widespread use of expressions. They can appear in PLUS Procedures or, when parenthesized, in GPSS Statements. This means that a powerful level of computation can be achieved just in the Operands of Blocks and Commands. Expressions can do simple computations, invoke procedures that perform math or string operations, sample probability distributions, or execute user-defined algorithms, including file I/O.


        1.4.3. Embedded Programming Language

The Programming Language Under Simulation, PLUStm, is a simple, but powerful, embedded programming language that fills the detailed computational needs of users who require a fine level of control of data structures, computational algorithms, and ad hoc Block processing.

PLUS Procedure Statements can be used to define Procedures as part of the original model, or they can be sent to an existing simulation. This applies to PLUS Experiment Statements, as well.

A new GPSS Block, PLUS, allows users to invoke a PLUS Procedure as a Block routine. In effect, Users can create their own blocks, in as complex a manner as desired.

PLUS Procedures are easily created, can reside anywhere in the model, and can be INCLUDEd in a Procedure Library. They can also be redefined at any time, even interactively.

PLUS Statement types are discussed in Chapter 8. They include:

∑ Assignment BEGIN






∑ Procedure Call




        1.4.4. Procedure Libraries

GPSS World has a wide range of functions that can be accessed as procedure calls.

String Procedure Library

To facilitate manipulation of the new string data types, a built-in String Procedure library is available. It includes:

∑ Align - Return a copy of one string placed in another, right justified.

∑ Catenate - Return a copy of two strings combined into one.

∑ Copies - Create a string from many copies of a string.

∑ Datatype - Return a string denoting the data type of the argument.

∑ Find - Return the offset of one string in another.

∑ Left - Return a copy of a substring starting on the left.

∑ Length - Return the count of characters in a string.

∑ Lowercase - Return the lowercase representation of a string.

∑ Place - Place one string in another. Left justify.

∑ PolyCatenate - Return a copy of one or more strings combined into one.

∑ Right - Return a copy of a substring starting on the right.

∑ String - Convert a data item to its string equivalent.

∑ StringCompare - Return an integer result if string comparison.

∑ Substring - Return a copy of a substring of the string argument.

∑ Trim - Remove leading and trailing white space.

∑ Uppercase - Return the uppercase equivalent of a string.

∑ Value - Return the numeric equivalent of a string.

∑ Word - Return a copy of one of the words in a string.

Math Procedure Library

Common Math library routines include

∑ ABS - Absolute Value

∑ ATN - Arctangent

∑ COS - Cosine

∑ INT - Truncate

∑ EXP - Power of e

∑ LOG - Natural Logarithm

∑ SIN - Sine

∑ SQR - Square Root

∑ TAN - Tangent

New Transaction Query Procedures

Transaction state query procedures now include:

∑ QueryXNExist - determine the existence of a Transaction.

∑ QueryXNParameter - retrieve the value of a Transaction Parameter.

∑ QueryXNAssemblySet - retrieve the Assembly Set of a Transaction.

∑ QueryXNPriority - retrieve the priority of a Transaction.

∑ QueryXNM1 - retrieve the mark time of a Transaction.

New Utility Procedures

New utility Procedures have been implemented for the support of Experiments:

∑ DoCommand - Translate a Command String in global context and send it to a Simulation Object

∑ ANOVA - Perform Analysis of Variance. The new multiway ANOVA Procedure now can handle replicates and limited factor interactions.

∑ Effects- Calculate Effects from a generated screening experiment.

∑ Exit- Terminate the GPSS World Session saving all objects, no objects, or inquiring User for each.

An new utility Procedure has been implemented to communicate with programs in external executable modules:

∑ Call- Invoke a function in an executable module. Pass no arguments. Optional return value is an integer.

∑ Call_Integer- Invoke a function in an executable module. Pass an integer argument. Optional return value is an integer.

∑ Call_Real- Invoke a function in an executable module. Pass a real argument. Optional return value is an integer.

∑ Call_String- Invoke a function in an executable module. Pass a string argument. Optional return value is an integer.

Over 20 built in probability distributions are available:

∑ Beta                                         ∑ LogLaplace

∑ Binomial                                  ∑ LogLogistic

∑ Discrete Uniform                      ∑ LogNormal

∑ Exponential                             ∑ Negative Binomial

∑ Extreme Value A                     ∑ Normal

∑ Extreme Value B                     ∑ Pareto

∑ Gamma                                   ∑ Pearson Type V

∑ Geometric                               ∑ Pearson Type VI

∑ Inverse Gaussian                     ∑ Poisson

∑ Inverse Weibull                       ∑ Triangular

∑ Laplace                                   ∑ Uniform

∑ Logistic                                    ∑ Weibull


        1.4.5. High Performance Translator

GPSS World features a new high performance model Translator that is hundreds of times faster than its predecessor. The largest models are Translated in no more than a few seconds. Any errors that are detected are placed in a circular error message queue so that they may be corrected quickly using the full screen editor in the Text View of the Model Object. Error correction is aided by automatic placement of the cursor next to the offending lexical element in the model. The error list is easily traversed by mouse or keystroke, and is saved with the Model Object.


        1.4.6. Block Creation Dialog

The creation of GPSS Blocks in GPSS World can optionally be guided by "fill-in-the-blanks" dialogs. You access these through the Edit / Insert Block menu item which opens the GPSS Block Menu Window. You can then open a Creation Dialog for any GPSS Block by clicking on the appropriate button. When you click the OK button after filling in the information needed by the Block, the resulting statement is checked for syntax errors and, if correct, is inserted into the Model just after the line containing the flashing insertion point.


        1.4.7. Automatic Experiment Generators

GPSS World now has the ability to generate experiments written in the PLUS language and insert them into your Model Object. All you have to do is to fill in the blanks in a few dialog windows, accessible through the Main Window's Edit Command. You can create either screening or optimizing experiments automatically. Optionally, a CONDUCT Command will be loaded into a function key so that all you must do to run the experiment is to Create the Simulation Object and press the function key. All this, and more, is discussed in Chapter 13, and in Lessons 19 and 20 of the GPSS World Tutorial Manual.


    1.5. Compatibility

The following section is an extensive comparison between GPSS World, and its predecessor, GPSS/PC. You should skip this section if you have no experience with GPSS/PC.

GPSS World is compatible with GPSS/PC, and normally yields results that are statistically indistinguishable from it. This level of compatibility is available by simply correcting a few differences, and running the simulations.

In addition, a higher level of compatibility is available called GPSS/PC Compatibility Mode. In most cases, you can achieve precise duplication of results. However, GPSS World utilizes a new run time library. Its floating point round off differs slightly from that used in GPSS/PC. Even so, most GPSS/PC models can be modified slightly to achieve identical results when run under the Commercial Version of GPSS World in GPSS/PC Compatibility Mode. The procedure you should follow is detailed below, after the discussion of differences between GPSS World and GPSS/PC.


        1.5.1. Differences from GPSS/PC

GPSS World has many differences from GPSS/PC. GPSS World is based on the idea that a textual Model Object is built, and/or modified, and then Translated in order to create a Simulation Object. This differs from the GPSS/PC notions of Program File and Simulation.

In GPSS World, the model loading operation of GPSS/PC has been completely replaced. Instead of scanning each line, one at a time, GPSS World has a full-screen editor and a Create Simulation menu Command. Any errors detected during the Translation are saved in an error message queue, so they may be easily found and corrected. The speed of the Translator assures that the detection/correction of errors can proceed quickly.

GPSS World does not perform Keystroke Error Prevention, as did GPSS/PC. Instead, GPSS World uses the model Translator for creating the simulation. This improves the model load-time by a factor of hundreds. However, it changes the error detection mechanism. Now, errors are detected during Translation, and are corrected by Selecting "Next Error" from the Search submenu of the Main Window. The cursor then automatically moves to the error, and an error message is written in the status line at the bottom of the Main Window.

Perhaps the next most noticeable change from GPSS/PC is that line numbers are no longer necessary, and in fact, they are ignored by GPSS World. This means that the positioning of Blocks is no longer specified by the line number of the Block, only by its relative position in the Model File(s) given to the Translator. Since the new INCLUDE Statement allows Model Objects to contain other plain text files, it is the sequence of Block Statements read by the Translator that dictates the position of Blocks in the simulation. Although Blocks can no longer be inserted during a simulation, it turns out that the Translation time is so fast, for most purposes it will be as easy to retranslate the model when a change to the Block structure is desired. Even so, the high level of interactivity of "Manual Simulation" is retained. This means that any Model Statement can be used during a simulation as an Interactive Command.

The level of interactivity of GPSS World has been maintained. Any Model Statement can be passed to an existing simulation for execution. In fact, PLUS Procedures can be defined, or even redefined, on the fly. In the case of GPSS Commands, this usually redefines an entity, or controls the running of the simulation. In the case of a Block Statement, as in GPSS/PC, a "Manual Simulation" Block entry is attempted by the Active Transaction. All this is done using the Commands submenu, after the model has been Translated.

Automatic truncations have been removed from GPSS World. You must now explicitly use the INT( ) procedure, of some other method, if you wish intermediate numeric results to be truncated. This is true in all Expressions, even in VARIABLE and BVARIABLE Statements. Similarly, the data type returned by a System Numeric Attribute can now be either integer, real, or string, depending on the SNA. Even those SNAs returning parts per thousand do so as a real number of double precision between 0 and 1000, inclusively. The old truncations and integer SNAs can be enforced by running in GPSS/PC Compatibility Mode.

The multitasking architecture of GPSS World has led to other changes. To begin with, messages are now sent to represent Commands or status changes. Therefore, online window update is done via a queue of messages received from the simulation. Similarly, most Commands received by the Simulation Object are placed on the simulationís Command Queue before being executed one after the other. There are two exceptions: HALT and SHOW, which are done immediately when received. In addition, HALT deletes all remaining Commands on the queue.

The remaining changes will be listed here and discussed more fully, elsewhere in this manual.

∑ PLUS Statements can span any number of text lines. Although GPSS Statements, except for Function Followers, must reside on a single text line, the maximum length is now 250 characters.

∑ Parenthesized mathematical expressions can now be used nearly anywhere you can use an SNA.

∑ System Numeric Attributes now may return integer, real, or string values depending on the SNA. In GPSS/PC Compatibility Mode, SNAs return only integers, except as Function Modifiers.

∑ A new System Numeric Attribute, A1, has been created to return the Assembly Set of the Active Transaction.

∑ The HELP Block has been dropped. It is replace by the PLUS Block, which supports the inclusion of complete Procedures in a model, and by the Data Stream Blocks that support communication with external files and programs.

∑ The MICROWINDOW Control Statement has been replaced by the Expression Window, which is opened from the Window / Simulation Window submenu of the Main Window Menu.

∑ The PLOT Control Statement has been replaced by the Plot Window which is opened from the Window / Simulation Window submenu of the Main Window Menu.

∑ The END Control Statement has been replaced by EXIT, which can terminate a Session. END is now a keyword in the PLUS Language.

∑ The ANOVA Control Statement has been replaced by the ANOVA Library Procedure.

∑ The EVENTS Control Statement has been replaced by the FEC and CEC Snapshot Windows which is opened from the Window / Simulation Snapshot submenu of the Main Window Menu.

∑ The GROUPS Control Statement has been replaced by the Numeric and Transaction Groups Snapshot Windows which is opened from the Window / Simulation Window submenu of the Main Window Menu.

∑ The RESULT Control Statement has been replaced by the Stream IO Blocks which can automatically write to a Result File.

∑ The WINDOW Control Statement has been replaced by the Window submenu of the Model Window Menu.

∑ The USERCHAINS Control Statement has been replaced by the Userchain Snapshot Window which is opened from the Window / Simulation Snapshot submenu of the Main Window Menu.

∑ The ANITRACE Control Statement has been dropped. Animation in GPSS World is provided by Data Steams and third party postprocessors.

∑ The MOVE Block has been dropped. Animation in GPSS World is provided by Data Steams and third party postprocessors..

∑ The Positions Window and the POSITION.GPS file have been dropped. Animation in GPSS World is provided by Data Steams and third party animation packages.

∑ The @ file inclusion character has been replaced by INCLUDE, which takes a double quoted filespec as an operand.

∑ The Z1 System Numeric Attribute no longer represents the total amount of physical memory available. It now returns the value of the maximum amount of memory that can be allocated, as returned by the operating system.

∑ The following GPSS/PC source management functions have been replaced by the Graphical User Interface with full-screen editor and filing options: DELETE, DISPLAY, DOS, EDIT, RENUMBER, and SAVE.

∑ Integer, Real, or String Data types are supported in Savevalues, Matrices, Named Values, and Transaction Parameters. Clock values may be integer or real. The unlimited precision integer arithmetic of GPSS/PC has been replaced.

∑ REPORT Commands always operate in NOW mode. Operand A is no longer used, and must be Null.

∑ A library of PLUS Procedures is now available for use in PLUS Expressions. Procedures for manipulating String data types and over 20 built-in probability distributions are supported.

∑ GPSS Matrix entities can now have up to 6 dimensions. Any 2 D Cross Section can be viewed dynamically in a Matrix Window.

∑ Operator precedence has changed. The new precedence is discussed in Chapter 3. If there is any question, you should fully parenthesize expressions from GPSS/PC Program Files.


        1.5.2. Modifying Old GPSS/PC Programs

This section is for GPSS/PC users who wish to migrate existing GPSS/PC models to the GPSS World environment. It contains a list of changes you should make to your GPSS/PC Program Files. This will provide you with results that are not significantly different from those obtained form GPSS/PC simulations. You do not need GPSS/PC Compatibility Mode to achieve this level of correspondence.

However, if you want to obtain precise duplication of results, you must perform the procedure in the next section, as well.

There are many features provided to ensure an easy move to the new system. All GPSS/PC users should note the small number of enhancements that may require changes to your existing Model Files. The other end of the spectrum is those users who wish to precisely duplicate the GPSS/PC results in order to establish the veracity of the model when running under GPSS World. After that, itís an easy matter to switch over to the new mode of operation, including the floating point clock, and the myriad of other features.

Line numbers are now ignored. Do not use old Program Files that replaced GPSS/PC Blocks by line number. If you use them as Include-files they will add to the size of the simulation, if you use them as interactive Commands they will be executed one at a time as Manual Simulation Statements.

You should make the following changes to your Program.

1. If you used Keystroke Error Prevention of GPSS/PC to finish keywords automatically, you must type in the remaining characters before Translating the model under GPSS World.

2. Replace @ with INCLUDE.

3. Filespecs must be in double quotes in INCLUDE.

4. Remove all REPORT Commands.

5. Remove END Commands and any labels, such as BEGIN, COUNT, NORMAL, etc. that now clash with GPSS World keywords.

6. Replace HELP Blocks with PLUS and/or Data Stream Blocks.

7. Remove MOVE Blocks. Positions Windows are no longer supported.

8. Fully parenthesize all Expressions in VARIABLE, FVARIABLE, and BVARIABLE Statements.

It is probably safest to remove all the old run commands from the old GPSS/PC Program File until you have tested your modified model. Do not use line number replacement of Blocks in the Savable Program. This no longer works.


        1.5.3. Strict Duplication of Results

Most GPSS/PC Program Files can be made to yield precisely the same results when run under GPSS World. However, because of differences in the rounding of real numbers in floating point calculations, you may need to make a few additional modifications to the GPSS/PC Program File, in addition to the ones in the previous section. If you only need results that will be statistically indistinguishable from those obtained from GPSS/PC, you do not need to do the following things using the Commercial Version of GPSS World.

1. This first step is to turn on GPSS/PC Compatibility Mode in the Model Settings. This causes GPSS World to use an integer clock and to truncate results, just as GPSS/PC did.

CHOOSE File / Open to open the Model Object

CHOOSE Edit / Settings

then on the first page check the checkbox labeled GPSS/PC Compatibility.

2. Now set the Random Number Streams of GENERATE, ADVANCE, and TRANSFER to correspond to the SETTINGS.GPS File you used with GPSS/PC. Next, set the Random Number Stream of Time Ties to 1.

CHOOSE      Edit / Settings

then select the Random Numbers page. Then set the entry fields of the Random Number Streams for GENERATE, ADVANCE, TRANSFER, and Time Ties.

3. Make sure all seeds in any RMULT Commands are less than 100,000,000.

4. Remove all dependencies on an identifier being given a default system value. For example, RN$IDENTIFIER seeds a random number stream with a distinct system default value. This entity specifier would need to be changed.

5. Do not call a Random Function from a Random Function. Replace any such statements.

6. Replace all fractions, occurring in TRANSFER Blocks, with parts-per-thousand integers.

7. Make sure Random GPSS Function Entities do not allow any Function Follower pair to be associated with a probability of 1/n, where n is the number of pairs. For example, if a Function is defined by 4 points, do not allow any probability to be .25. You can do this by adding additional points until 1/n is a repeating fraction, or is not associated with any of the Function Follower Pairs. FN$SNORM and FN$XPDIS do not have to be changed.

A special DOS program, named PCAID.EXE, has been provided to help you modify your GPSS/PC Program File. It will correct line numbers and remind you if you have GPSS FUNCTION Statements that need to be examined. You can start this program in a DOS Window.

In either case, keep in mind that Model Objects are not simple text files. They contain Settings, Bookmarks, and Result Lists that cannot be read by external word processing programs. For this reason, when you have finished modifying your GPSS/PC Program File, you should use a word processing program to copy it to the Windows clipboard. Then, in GPSS World, open a new Model Object

Select      File / New

and paste the text into place using

Select      Edit / Paste

This process creates a valid GPSS World Model Object based on your original model. You should now be able to get corresponding results for simulations run under GPSS/PC and GPSS World.


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