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<a class="dropdown-item" tabindex="-1" href="SysAndModels.html#"><b>Chapter 1</b></a>
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<a class="dropdown-item" href="IntroConcSysOverview.html"> 1.1. Introduction to Concurrent Systems</a>
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<a class="dropdown-item" href="SysAndModels.html"> 1.2. Systems and Models</a>
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<a class="dropdown-item" href="Themes.html"> 1.3. Themes and Guiding Principles</a>
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<a class="dropdown-item" href="Architectures.html"> 1.4. System Architectures</a>
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<a class="dropdown-item" href="StateModels.html"> 1.5. State Models in UML</a>
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<a class="dropdown-item" href="SequenceModels.html"> 1.6. Sequence Models in UML</a>
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<a class="dropdown-item" href="StateModelImplementation.html"> 1.7. Extended Example: State Model Implementation</a>
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<a class="dropdown-item disabled"><b>Chapter 2</b></a>
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<a class="dropdown-item" href="ProcessesOverview.html"> 2.1. Processes and OS Basics</a>
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<a class="dropdown-item" href="Multiprogramming.html"> 2.2. Processes and Multiprogramming</a>
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<a class="dropdown-item" href="KernelMechanics.html"> 2.3. Kernel Mechanics</a>
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<a class="dropdown-item" href="Syscall.html"> 2.4. System Call Interface</a>
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<a class="dropdown-item" href="ProcessCycle.html"> 2.5. Process Life Cycle</a>
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<a class="dropdown-item" href="EventsSignals.html"> 2.7. Events and Signals</a>
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<a class="dropdown-item" href="Extended2Processes.html"> 2.8. Extended Example: Listing Files with Processes</a>
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<a class="dropdown-item disabled"><b>Chapter 3</b></a>
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<a class="dropdown-item" href="IPCOverview.html"> 3.1. Concurrency with IPC</a>
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<a class="dropdown-item" href="IPCModels.html"> 3.2. IPC Models</a>
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<a class="dropdown-item" href="Pipes.html"> 3.3. Pipes and FIFOs</a>
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<a class="dropdown-item" href="POSIXvSysV.html"> 3.5. POSIX vs. System V IPC</a>
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<a class="dropdown-item" href="MQueues.html"> 3.6. Message Passing With Message Queues</a>
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<a class="dropdown-item" href="IPCSems.html"> 3.8. Semaphores</a>
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<a class="dropdown-item" href="Extended3Bash.html"> 3.9. Extended Example: Bash-lite: A Simple Command-line Shell</a>
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<a class="dropdown-item disabled"><b>Chapter 4</b></a>
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<a class="dropdown-item" href="SocketsOverview.html"> 4.1. Networked Concurrency</a>
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<a class="dropdown-item" href="FiveLayer.html"> 4.2. The TCP/IP Internet Model</a>
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<a class="dropdown-item" href="NetApps.html"> 4.3. Network Applications and Protocols</a>
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<a class="dropdown-item" href="Sockets.html"> 4.4. The Socket Interface</a>
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<a class="dropdown-item" href="InternetOverview.html"> 5.1. The Internet and Connectivity</a>
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<a class="dropdown-item" href="AppLayer.html"> 5.2. Application Layer: Overlay Networks</a>
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<a class="dropdown-item" href="ThreadsOverview.html"> 6.1. Concurrency with Multithreading</a>
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<a class="dropdown-item" href="ThreadArgs.html"> 6.5. Thread Arguments and Return Values</a>
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<a class="dropdown-item" href="ImplicitThreads.html"> 6.6. Implicit Threading and Language-based Threads</a>
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<a class="dropdown-item" href="Extended6Input.html"> 6.7. Extended Example: Keyboard Input Listener</a>
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<a class="dropdown-item" href="Extended6Primes.html"> 6.8. Extended Example: Concurrent Prime Number Search</a>
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<a class="dropdown-item" href="SynchOverview.html"> 7.1. Synchronization Primitives</a>
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<a class="dropdown-item" href="CritSect.html"> 7.2. Critical Sections and Peterson's Solution</a>
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<a class="dropdown-item" href="Locks.html"> 7.3. Locks</a>
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<a class="dropdown-item" href="Semaphores.html"> 7.4. Semaphores</a>
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<a class="dropdown-item" href="Barriers.html"> 7.5. Barriers</a>
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<a class="dropdown-item" href="SynchProblemsOverview.html"> 8.1. Synchronization Patterns and Problems</a>
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<a class="dropdown-item" href="SynchDesign.html"> 8.2. Basic Synchronization Design Patterns</a>
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<a class="dropdown-item" href="ProdCons.html"> 8.3. Producer-Consumer Problem</a>
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<a class="dropdown-item" href="ReadWrite.html"> 8.4. Readers-Writers Problem</a>
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<a class="dropdown-item" href="DiningPhil.html"> 8.5. Dining Philosophers Problem and Deadlock</a>
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<a class="dropdown-item" href="CigSmokers.html"> 8.6. Cigarette Smokers Problem and the Limits of Semaphores and Locks</a>
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<a class="dropdown-item" href="Extended8ModExp.html"> 8.7. Extended Example: Parallel Modular Exponentiation</a>
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<a class="dropdown-item" href="ParallelDistributedOverview.html"> 9.1. Parallel and Distributed Systems</a>
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<a class="dropdown-item" href="ParVConc.html"> 9.2. Parallelism vs. Concurrency</a>
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<a class="dropdown-item" href="ParallelDesign.html"> 9.3. Parallel Design Patterns</a>
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<a class="dropdown-item" href="Scaling.html"> 9.4. Limits of Parallelism and Scaling</a>
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<a class="dropdown-item" href="DistTiming.html"> 9.5. Timing in Distributed Environments</a>
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<a class="dropdown-item" href="DistDataStorage.html"> 9.6. Reliable Data Storage and Location</a>
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<a class="dropdown-item" href="DistConsensus.html"> 9.7. Consensus in Distributed Systems</a>
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<a class="dropdown-item" href="Extended9Blockchain.html"> 9.8. Extended Example: Blockchain Proof-of-Work</a>
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<a class="dropdown-item" href="CLangOverview.html"> A.1. C Language Reintroduction</a>
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<a class="dropdown-item" href="Debugging.html"> A.2. Documentation and Debugging</a>
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<a class="dropdown-item" href="BasicTypes.html"> A.3. Basic Types and Pointers</a>
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<a class="dropdown-item" href="Arrays.html"> A.4. Arrays, Structs, Enums, and Type Definitions</a>
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<script>ODSA.SETTINGS.DISP_MOD_COMP = true;ODSA.SETTINGS.MODULE_NAME = "SysAndModels";ODSA.SETTINGS.MODULE_LONG_NAME = "Systems and Models";ODSA.SETTINGS.MODULE_CHAPTER = "Introduction to Computer Systems"; ODSA.SETTINGS.BUILD_DATE = "2021-06-14 17:15:25"; ODSA.SETTINGS.BUILD_CMAP = false;JSAV_OPTIONS['lang']='en';JSAV_EXERCISE_OPTIONS['code']='java';</script><div class="section" id="systems-and-models">
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<h1>1.2. Systems and Models<a class="headerlink" href="SysAndModels.html#systems-and-models" title="Permalink to this headline">¶</a></h1>
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<p>Throughout this book, we are considering the idea of computer systems from a perspective that
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||
strives to apply insights from <a class="reference internal" href="Glossary.html#term-systems-theory"><span class="xref std std-term">systems theory</span></a>. From this view, a <a class="reference internal" href="Glossary.html#term-system"><span class="xref std std-term">system</span></a> is an
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||
integrated collection of entities and their interactions. The computer itself is a system. Computers
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||
are assembled from disparate hardware components that include a central processing unit (CPU),
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storage devices, input and output devices, random access memory (RAM) cards, and a printed circuit
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board (PCB) that links them all together. A software OS executes on top of these components, thus
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||
integrating their independent functionality into a more complex entity. To fully understand the
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||
nature of the computer as a whole, it’s important to understand the components’ independent
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functionality as well as how they work together.</p>
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<div class="figure mb-2 align-right" id="id1" style="width: 45%">
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||
<span id="sysinteractfig"></span><a class="reference internal image-reference" href="_images/CSF-Images.1.1.png"><img class="p-3 mb-2 align-center border border-dark rounded-lg" alt="A computer is a system of interacting hardware and software entities" src="_images/CSF-Images.1.1.png" style="width: 90%;" /></a>
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<p class="caption align-center px-3"><span class="caption-text"> Figure 1.2.1: A computer is a system of interacting hardware and software entities</span></p>
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</div>
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<p>The system is made more complex by adding application software to the mix. Each application is
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||
constructed from code modules, variables, and other run-time memory structures that are compiled and
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||
integrated to provide a single higher-level function. But software does not run itself; the software
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||
must be executed within the greater context of computing, as a sequence of steps performed by a
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||
physical computer with the help of an OS. Some applications run in isolation from one another, but
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||
they still require the OS and hardware. Others communicate with services that provide information or
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||
perform complimentary tasks, allowing the developer to build more powerful software without having
|
||
to start from scratch. <a href="SysAndModels.html#sysinteractfig">Figure 1.2.1</a> summarizes this view of the computer as
|
||
a system of interacting components.</p>
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||
<p>In short, all of computing is a system. As computer scientists continue to explore the power of
|
||
computing, we can take this claim even further to suggest that computing is made up of a system of
|
||
systems. In order to stream a movie over the Internet, the application software must integrate
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||
network connectivity and local file storage, probably with the help of a graphics processing unit
|
||
(GPU) that improves the visual quality of the experience. Each of these hardware and software
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||
components are systems, and the application is coordinating their interaction to serve the user. As
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||
another example, consider large-scale scientific projects that model changes in the climate or
|
||
advanced quantum mechanics phenomena. This type of work involves linking thousands of parallel
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||
computing nodes that coordinate their calculations and exchange data as needed, creating truly
|
||
complicated systems and interactions. Our goal is to understand the common principles and strategies
|
||
that make these systems work.</p>
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||
<div class="section" id="models-as-representations">
|
||
<h2>1.2.1. Models as Representations<a class="headerlink" href="SysAndModels.html#models-as-representations" title="Permalink to this headline">¶</a></h2>
|
||
<p>A first step toward understanding a system is constructing a <a class="reference internal" href="Glossary.html#term-model"><span class="xref std std-term">model</span></a> – a simplified
|
||
representation – of the system. <a href="SysAndModels.html#sysinteractfig">Figure 1.2.1</a> above is a model of a computer as a system. As a
|
||
simplification, models necessarily omit details that are present in the system itself. Some models
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||
have visual representations, facilitating human interpretation of the system. That is,
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||
<a class="reference internal" href="Glossary.html#term-visual-model"><span class="xref std std-term">visual models</span></a> are very good tools for people to make sense of the entities
|
||
and their interactions. System designers, developers, and users can use visual models to avoid
|
||
mistakes. Other models are <a class="reference internal" href="Glossary.html#term-formal-model"><span class="xref std std-term">formal</span></a>, written in a mathematical specification
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||
language. One way to characterize the differences between models is to describe their
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||
<a class="reference internal" href="Glossary.html#term-level-of-abstraction"><span class="xref std std-term">level of abstraction</span></a>. Visual models may have a relatively low level of abstraction,
|
||
including details that are aesthetically pleasing but not important. Formal models have a relatively
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||
high level of abstraction, omitting everything except the bare minimum.</p>
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||
<p>Consider the models shown in <a href="SysAndModels.html#sysphysmodel">Figure 1.2.2</a>. The model on the left has the lowest level of
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||
abstraction, as it includes details that show a yellow box on a grey ramp. This scenario is then
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||
converted into a free-body diagram in the middle. Free-body diagrams are used in physics to
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||
illustrate the forces acting on an object, while removing unneeded details such as the drawing of
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the box and the ramp. The equation on the right is the formal model that captures this information.
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||
The net force F is the sum of the forces created by gravity (mg), the normal force N, and the force
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||
of friction <span class="math notranslate nohighlight">\(F_f\)</span>. All three models convey the same information, but they provide different
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||
levels of abstraction.</p>
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||
<div class="figure mb-2 align-center" id="id2">
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||
<span id="sysphysmodel"></span><a class="reference internal image-reference" href="_images/CSF-Images.1.2.png"><img class="p-3 mb-2 align-center border border-dark rounded-lg" alt="Three models for the same physical phenomenon" src="_images/CSF-Images.1.2.png" style="width: 70%;" /></a>
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||
<p class="caption align-center px-3"><span class="caption-text"> Figure 1.2.2: Three models for the same physical phenomenon</span></p>
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||
</div>
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||
<p>A <a class="reference internal" href="Glossary.html#term-static-model"><span class="xref std std-term">static (or structural) model</span></a> describes the fixed, unchanging features of a
|
||
system, while omitting explanations of how the system or entities change. Readers with a background
|
||
in object-oriented programming may be familiar with class diagrams created using
|
||
<a class="reference internal" href="Glossary.html#term-unified-modeling-language"><span class="xref std std-term">unified modeling language (UML)</span></a>. These models describe features
|
||
like class inheritance and sub-typing, while omitting characteristics like the values of an object’s
|
||
variables or the messages it uses to communicate with other objects. Throughout this text, we will
|
||
focus repeatedly on one particular type of static model, the system architecture.</p>
|
||
<p>In contrast, a <a class="reference internal" href="Glossary.html#term-dynamic-model"><span class="xref std std-term">dynamic (or behavioral) model</span></a> focuses on changes to the
|
||
system. These models are particularly important to understanding systems, because many of their key
|
||
features are <a class="reference internal" href="Glossary.html#term-emergent-property"><span class="xref std std-term">emergent properties</span></a>. An emergent property is one that
|
||
results from the dynamic features of the system and is not intuitive from static models alone. We
|
||
will be using two dynamic UML models throughout this book. A <a class="reference internal" href="Glossary.html#term-state-model"><span class="xref std std-term">state model</span></a> describes the
|
||
system’s possible states – defined by meaningful combinations of system parameters – and the possible
|
||
transitions between them. For instance, a streaming video service may characterize different states
|
||
to represent “requesting the video,” “buffering received data,” “pausing the video,” and “playing
|
||
the video.” A <a class="reference internal" href="Glossary.html#term-sequence-model"><span class="xref std std-term">sequence model</span></a> illustrates the order of messages and key events that a system
|
||
experiences over time. One use of a sequence model is to describe a specific order in which state
|
||
transitions occur. These models provide a clear illustration of network communication protocols and
|
||
messages to coordinate parallel and distributed computation.</p>
|
||
</div>
|
||
<div class="section" id="from-models-to-implementations">
|
||
<h2>1.2.2. From Models to Implementations<a class="headerlink" href="SysAndModels.html#from-models-to-implementations" title="Permalink to this headline">¶</a></h2>
|
||
<p>Models are useful tools for analyzing and illustrating the behavior of a system. Well-constructed
|
||
models of the natural world can help scientists develop insight into and explain real-world
|
||
phenomena, such as the chemical reactions that release energy stored in food, the wave-like behavior
|
||
of electrons in an atom, or the fractal designs in crystals. User manuals for appliances, vehicles,
|
||
children’s toys, etc., use illustrations as models to convey information to consumers who are not
|
||
necessarily experts in the field.</p>
|
||
<p>In contrast to many other fields, the field of computer systems places a strong emphasis on turning
|
||
models into implementations. That is, our goal is not simply to build and interpret models, but to
|
||
turn these models into working artifacts. This implementation process can be quite challenging and
|
||
creates many opportunities for errors; mistakes that cause the system to behave differently from the
|
||
model can render the model useless or, even worse, increase confusion. As such, much of our focus
|
||
throughout this book is on successfully turning models into concrete, executable implementations.</p>
|
||
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|
||
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|
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|
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|
||
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|
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|
||
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|
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|
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|
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«  <a id="prevmod1" href="IntroConcSysOverview.html">1.1. Introduction to Concurrent Systems</a>
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<a id="nextmod1" href="Themes.html">1.3. Themes and Guiding Principles</a>  »
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