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index.html
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<title>The Erlang Runtime System</title>
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<body class="book toc2 toc-left">
<div id="header">
<h1>The Erlang Runtime System</h1>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_preface">Preface</a>
<ul class="sectlevel2">
<li><a href="#who_is_this_book_for">About this book</a></li>
<li><a href="#_how_to_read_this_book">How to read this book</a></li>
<li><a href="#_erlang">Erlang</a></li>
<li><a href="#_acknowledgments">Acknowledgments</a></li>
</ul>
</li>
<li><a href="#P-ERTS">Understanding ERTS</a>
<ul class="sectlevel1">
<li><a href="#introduction">Introducing the Erlang Runtime System</a>
<ul class="sectlevel2">
<li><a href="#_erts_and_the_erlang_runtime_system">ERTS and the Erlang Runtime System</a></li>
<li><a href="#_how_to_read_this_book_2">How to read this book</a></li>
<li><a href="#ERTS">ERTS</a></li>
<li><a href="#Other_Erlang_Implementations">Other Erlang Implementations</a></li>
</ul>
</li>
<li><a href="#CH-Compiler">The Compiler</a>
<ul class="sectlevel2">
<li><a href="#_compiling_erlang">Compiling Erlang</a></li>
<li><a href="#_generating_intermediate_output">Generating Intermediate Output</a></li>
<li><a href="#_compiler_passes">Compiler Passes</a></li>
<li><a href="#_other_compiler_tools">Other Compiler Tools</a></li>
<li><a href="#_syntax_tools_and_merl">Syntax Tools and Merl</a></li>
<li><a href="#_compiling_elixir">Compiling Elixir</a></li>
</ul>
</li>
<li><a href="#CH-Processes">Processes</a>
<ul class="sectlevel2">
<li><a href="#_what_is_a_process">What is a Process?</a></li>
<li><a href="#_processes_are_just_memory">Processes Are Just Memory</a></li>
<li><a href="#_the_pcb">The PCB</a></li>
<li><a href="#_the_garbage_collector_gc">The Garbage Collector (GC)</a></li>
<li><a href="#_mailboxes_and_message_passing">Mailboxes and Message Passing</a></li>
<li><a href="#_lock_free_message_passing">Lock Free Message Passing</a></li>
<li><a href="#_the_process_dictionary">The Process Dictionary</a></li>
<li><a href="#_dig_in">Dig In</a></li>
</ul>
</li>
<li><a href="#CH-TypeSystem">The Erlang Type System and Tags</a>
<ul class="sectlevel2">
<li><a href="#_the_erlang_type_system">The Erlang Type System</a></li>
<li><a href="#_the_tagging_scheme">The Tagging Scheme</a></li>
</ul>
</li>
<li><a href="#CH-BEAM">The Erlang Virtual Machine: BEAM</a>
<ul class="sectlevel2">
<li><a href="#_working_memory_a_stack_machine_it_is_not">Working Memory: A stack machine, it is not</a></li>
<li><a href="#_dispatch_directly_threaded_code">Dispatch: Directly Threaded Code</a></li>
<li><a href="#_scheduling_non_preemptive_reduction_counting">Scheduling: Non-preemptive, Reduction counting</a></li>
<li><a href="#_memory_management_garbage_collecting">Memory Management: Garbage Collecting</a></li>
<li><a href="#_beam_it_is_virtually_unreal">BEAM: it is virtually unreal</a></li>
</ul>
</li>
<li><a href="#CH-beam_modules">Modules and The BEAM File Format (16p)</a>
<ul class="sectlevel2">
<li><a href="#modules">Modules</a></li>
<li><a href="#BEAM_files">The BEAM File Format</a></li>
</ul>
</li>
<li><a href="#CH-Instructions">Generic BEAM Instructions (25p)</a>
<ul class="sectlevel2">
<li><a href="#_instruction_definitions">Instruction definitions</a></li>
<li><a href="#_beam_code_listings">BEAM code listings</a></li>
<li><a href="#_calls">Calls</a></li>
<li><a href="#_stack_and_heap_management">Stack (and Heap) Management</a></li>
<li><a href="#_message_passing">Message Passing</a></li>
</ul>
</li>
<li><a href="#CH-Calls">Different Types of Calls, Linking and Hot Code Loading (5p)</a>
<ul class="sectlevel2">
<li><a href="#_hot_code_loading">Hot Code Loading</a></li>
<li><a href="#_code_loading">Code Loading</a></li>
</ul>
</li>
<li><a href="#CH-Beam_loader">The BEAM Loader</a>
<ul class="sectlevel2">
<li><a href="#_transforming_from_generic_to_specific_instructions">Transforming from Generic to Specific instructions</a></li>
<li><a href="#_understanding_ops_tab">Understanding ops.tab</a></li>
<li><a href="#_optimizations">Optimizations</a></li>
</ul>
</li>
<li><a href="#CH-Internal_instructions">BEAM Internal Instructions</a></li>
<li><a href="#CH-Scheduling">Scheduling</a>
<ul class="sectlevel2">
<li><a href="#_concurrency_parallelism_and_preemptive_multitasking">Concurrency, Parallelism, and Preemptive Multitasking</a></li>
<li><a href="#_preemptive_multitasking_in_erts_cooperating_in_c">Preemptive Multitasking in ERTS Cooperating in C</a></li>
<li><a href="#_reductions">Reductions</a></li>
<li><a href="#_the_process_state_or_em_status_em">The Process State (or <em>status</em>)</a></li>
<li><a href="#_process_queues">Process Queues</a></li>
<li><a href="#_ports">Ports</a></li>
<li><a href="#_reductions_2">Reductions</a></li>
<li><a href="#_the_scheduler_loop">The Scheduler Loop</a></li>
<li><a href="#_load_balancing">Load Balancing</a></li>
</ul>
</li>
<li><a href="#CH-Memory">The Memory Subsystem: Stacks, Heaps and Garbage Collection</a>
<ul class="sectlevel2">
<li><a href="#_the_memory_subsystem">The memory subsystem</a></li>
<li><a href="#SS-Memory_Allocators">Different type of memory allocators</a></li>
<li><a href="#_todo_system_flags_for_memory">TODO: system flags for memory</a></li>
<li><a href="#_process_memory">Process Memory</a></li>
<li><a href="#_other_interesting_memory_areas">Other interesting memory areas</a></li>
</ul>
</li>
<li><a href="#CH-DataStructures">Advanced data structures (ETS, DETS, Mnesia)</a></li>
<li><a href="#CH-IO">IO, Ports and Networking (10p)</a>
<ul class="sectlevel2">
<li><a href="#_standard_io">Standard IO</a></li>
<li><a href="#_ports_2">Ports</a></li>
<li><a href="#_distributed_erlang">Distributed Erlang</a></li>
<li><a href="#_sockets_udp_and_tcp">Sockets, UDP and TCP</a></li>
</ul>
</li>
<li><a href="#CH-Distribution">Distribution</a></li>
<li><a href="#CH-C">Interfacing C — BIFs NIFs and Linked in Drivers</a></li>
<li><a href="#CH-Native">Native Code</a></li>
</ul>
</li>
<li><a href="#P-Running">Running ERTS</a>
<ul class="sectlevel1">
<li><a href="#CH-Tracing">Tracing</a></li>
</ul>
</li>
<li><a href="#CH-Debugging">Debugging</a>
<ul class="sectlevel1">
<li><a href="#_preliminary_outline">Preliminary Outline</a></li>
<li><a href="#_introduction">Introduction</a></li>
<li><a href="#_debugger">debugger</a></li>
<li><a href="#_dbg">dbg</a></li>
<li><a href="#_redbug">Redbug</a>
<ul class="sectlevel2">
<li><a href="#_installing_redbug">Installing Redbug</a></li>
<li><a href="#_using_redbug">Using Redbug</a></li>
</ul>
</li>
<li><a href="#_crash_dumps">Crash Dumps</a></li>
<li><a href="#CH-Ops">Operation and Maintenance</a>
<ul class="sectlevel2">
<li><a href="#_connecting_to_the_system">Connecting to the System</a></li>
<li><a href="#_the_shell">The Shell</a></li>
</ul>
</li>
<li><a href="#CH-Tweak">Tweaking the Runtime System</a></li>
</ul>
</li>
<li><a href="#appendix">Appendices</a>
<ul class="sectlevel1">
<li><a href="#CH-BuildingERTS">Building the Erlang Runtime System</a>
<ul class="sectlevel2">
<li><a href="#_first_time_build">First Time Build</a></li>
<li><a href="#_getting_the_source">Getting the source</a></li>
<li><a href="#_building_with_kerl">Building with Kerl</a></li>
</ul>
</li>
<li><a href="#AP-Instructions">BEAM Instructions</a>
<ul class="sectlevel2">
<li><a href="#_functions_and_labels">Functions and Labels</a></li>
<li><a href="#_test_instructions">Test instructions</a></li>
<li><a href="#_function_calls">Function Calls</a></li>
<li><a href="#_stack_and_heap_management_2">Stack (and Heap) Management</a></li>
<li><a href="#_moving_extracting_modifying_data">Moving, extracting, modifying data</a></li>
<li><a href="#_building_terms">Building terms.</a></li>
<li><a href="#_binary_syntax">Binary Syntax</a></li>
<li><a href="#_floating_point_arithmetic">Floating Point Arithmetic</a></li>
<li><a href="#_pattern_matching">Pattern Matching</a></li>
<li><a href="#_exception_handling">Exception handling</a></li>
<li><a href="#_meta_instructions">Meta instructions</a></li>
<li><a href="#_generic_instructions">Generic Instructions</a></li>
<li><a href="#_specific_instructions">Specific Instructions</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_preface">Preface</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This book is not about how to write correct and
beautiful code, I am assuming that you already know how to do
that. This book isn’t really about profiling and performance tuning
either. Although, there is a chapter in this book on tracing and
profiling which can help you find bottlenecks and unnecessary usage of
resources. There also is a chapter on performance tuning.</p>
</div>
<div class="paragraph">
<p>These two chapters are the last chapters in the book, and the whole
book is building up to those chapters, but the real goal with this
book is to give you all the information, all the gory details, that
you need in order to really understand the performance of your Erlang
application.</p>
</div>
<div class="sect2">
<h3 id="who_is_this_book_for">About this book</h3>
<div class="paragraph">
<p>For anyone who: Want to tune an Erlang installation. Want to know how
to debug VM crashes. Want to improve performance of Erlang
applications. Want to understand how Erlang really works. Want to
learn how to build your own runtime environment.</p>
</div>
<div class="paragraph">
<p>If you want to debug the VM If you want to extend the VM If you want
to do performance tweaking—​jump to the last chapter … but to really
understand that chapter you need to read the book.</p>
</div>
</div>
<div class="sect2">
<h3 id="_how_to_read_this_book">How to read this book</h3>
<div class="paragraph">
<p>The Erlang RunTime System (ERTS) is a complex system with many
interdependent components. It is written in a very portable way so
that it can run on anything from a gum-stick computer to the largest
multicore system with terabytes of memory. In order to be able to
optimize the performance of such a system for your application, you
need to not only know your application, but you also need to have a
thorough understanding of ERTS itself.</p>
</div>
<div class="paragraph">
<p>With this knowledge of how ERTS works you will be able to understand
how your application behaves when running on ERTS, and you will also
be able to find and fix problems with the performance of your application.
In the second part of this book we will go through how you successfully
run, monitor and scale your ERTS application.</p>
</div>
<div class="paragraph">
<p>You don’t need to be an Erlang programmer to read this book, but you
will need some basic understanding of what Erlang is. This following
section will give you some Erlang background.</p>
</div>
</div>
<div class="sect2">
<h3 id="_erlang">Erlang</h3>
<div class="paragraph">
<p>In this section we will look at some basic Erlang concepts that
are vital to understanding the rest of the book.</p>
</div>
<div class="paragraph">
<p>Erlang has been called, especially by one of Erlang’s creators Joe
Armstrong, a concurrency oriented language. Concurrency is definitely
at the heart of Erlang, and to be able to understand how an Erlang
system works you need to understand the concurrency model of Erlang.</p>
</div>
<div class="paragraph">
<p>First of all we need to make a distinction between <em>concurrency</em> and
<em>parallelism</em>. In this book <em>concurrency</em> is the concept of having
two or more processes that <strong>can</strong> execute independently of each other,
this can be done by first executing one process then the other or by
interleaving the execution, or by executing the processes in
parallel. With <em>parallel</em> executions we mean that the processes
actually execute at the exact same time by using several physical
execution units. Parallelism can be achieved on different levels.
Through multiple execution units in the execution pipeline in one core,
in several cores on one CPU, by several CPUs in one machine or through
several machines.</p>
</div>
<div class="paragraph">
<p>Erlang uses processes to achieve concurrency. Conceptually Erlang
processes are similar to most OS processes, they execute in parallel
and can communicate through signals. In practice there is a huge
difference in that Erlang processes are much more lightweight than
most OS processes. Many other concurrent programming languages call
their equivalent to Erlang processes <em>agents</em>.</p>
</div>
<div class="paragraph">
<p>Erlang achieves concurrency by interleaving the execution of processes
on the Erlang virtual machine, the BEAM. On a multi-core processor the
BEAM can also achieve parallelism by running one scheduler per core and
executing one Erlang process per scheduler. The designer of an Erlang
system can achieve further parallelism by distributing the system on
several computers.</p>
</div>
<div class="paragraph">
<p>A typical Erlang system (a server or service built in Erlang) consists
of a number of Erlang <em>applications</em>, corresponding to a directory on disk.
Each application is made up of several Erlang <em>modules</em> corresponding to
files in the directory. Each module contains a number of <em>functions</em> and
each function is made up of <em>expressions</em>.</p>
</div>
<div class="paragraph">
<p>Since Erlang is a functional language it has no statements,
only expressions. Erlang expressions can be combined into an Erlang
function. A function takes a number of arguments and returns a
value. In <a href="#erlang_code_examples">Erlang Code Examples</a> we can see some examples of
Erlang expressions and functions.</p>
</div>
<div id="erlang_code_examples" class="listingblock">
<div class="title">Erlang Code Examples</div>
<div class="content">
<pre class="highlight"><code class="language-erlang" data-lang="erlang">%% Some Erlang expressions:
true.
1+1.
if (X > Y) -> X; true -> Y end.
%% An Erlang function:
max(X, Y) ->
if (X > Y) -> X;
true -> Y
end.</code></pre>
</div>
</div>
<div class="paragraph">
<p>Erlang has a number of <em>built in functions</em> (or <em>BIFs</em>) which are
implemented by the VM. This is either for efficiency reasons, like the
implementation of lists:append (which could be implemented in
Erlang). It could also be to provide some low level functionality
which would be hard or impossible to implement in Erlang itself, like
list_to_atom.</p>
</div>
<div class="paragraph">
<p>Since Erlang/OTP R13B03 you can also provide your own functions
implemented in C by using the <em>Native Implemented Functions</em> (<em>NIF</em>)
interface.</p>
</div>
</div>
<div class="sect2">
<h3 id="_acknowledgments">Acknowledgments</h3>
<div class="paragraph">
<p>A big thank you to everyone who has contributed in some way by checking in
something:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>Dmytro Lytovchenko
Kim Shrier
Andrea Leopardi
Anton N Ryabkov
Greg Baraghimian
Marc van Woerkom
Trevor Brown
Yves Müller
techgaun
Alex Fu
Alex Jiao
Amir Moulavi
Chris Yunker
Davide Bettio
Eric Yu
Kyle Baker
Luke Imhoff
Milton Inostroza
Ramkumar Rajagopalan
Yoshihiro Tanaka
tomdos</pre>
</div>
</div>
</div>
</div>
</div>
<h1 id="P-ERTS" class="sect0">Understanding ERTS</h1>
<div class="sect1">
<h2 id="introduction">Introducing the Erlang Runtime System</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The Erlang RunTime System (ERTS) is a complex system with many interdependent
components. It is written in a very portable way so that it can run on
anything from a gum stick computer to the largest multicore system
with terabytes of memory. In order to be able to optimize the
performance of such a system for your application, you need to not
only know your application, but you also need to have a thorough
understanding of ERTS itself.</p>
</div>
<div class="sect2">
<h3 id="_erts_and_the_erlang_runtime_system">ERTS and the Erlang Runtime System</h3>
<div class="paragraph">
<p>There is a difference between any Erlang Runtime System and a specific implementation of an Erlang Runtime
System. "Erlang/OTP" by Ericsson is the de facto standard
implementation of Erlang and the Erlang Runtime System. In this book I
will refer to this implementation as <em>ERTS</em> or spelled out <em>Erlang
RunTime System</em> with a capital T. (See <a href="#ERTS">ERTS</a> for a definition of
OTP).</p>
</div>
<div class="paragraph">
<p>There is no official definition of what an Erlang Runtime System is,
or what an Erlang Virtual Machine is. You could sort of imagine what
such an ideal Platonic system would look like by taking ERTS and
removing all the implementation specific details. This is
unfortunately a circular definition, since you need to know the
general definition to be able to identify an implementation specific
detail. In the Erlang world we are usually too pragmatic to worry about
this.</p>
</div>
<div class="paragraph">
<p>We will try to use the term <em>Erlang Runtime System</em> to refer to the
general idea of any Erlang Runtime System as opposed to the specific
implementation by Ericsson which we call the Erlang RunTime System
or usually just ERTS.</p>
</div>
<div class="paragraph">
<p><strong>Note</strong> This book is mostly a book about ERTS in particular and only to
a small extent about any general Erlang Runtime System. If you assume
that we talk about the Ericsson implementation unless it is clearly
stated that we are talking about a general principle you will probably
be right.</p>
</div>
</div>
<div class="sect2">
<h3 id="_how_to_read_this_book_2">How to read this book</h3>
<div class="paragraph">
<p>In <a href="#P-Running">Running ERTS</a> of this book we will look at how to tune the
runtime system for your application and how to profile and debug
your application and the runtime system. In order to really know
how to tune the system you also need to know the system. In
<a href="#P-ERTS">Understanding ERTS</a> of this book you will get a deep understanding of
how the runtime system works.</p>
</div>
<div class="paragraph">
<p>The following chapters of <a href="#P-ERTS">Understanding ERTS</a> will go over each
component of the system by itself.
You should be able to read any one of these
chapters without having a full understanding of how the other
components are implemented, but you will need a basic understanding of
what each component is. The rest of this introductory chapter should
give you enough basic understanding and vocabulary to be able to jump
between the rest of the chapters in part one in any order you like.</p>
</div>
<div class="paragraph">
<p>However, if you have the time,
read the book in
order the first time. Words that are specific to Erlang and ERTS or
used in a specific way in this book are usually explained at their
first occurrence. Then, when you know the vocabulary, you can come
back and use Part I as a reference whenever you have a problem with a
particular component.</p>
</div>
</div>
<div class="sect2">
<h3 id="ERTS">ERTS</h3>
<div class="paragraph">
<p>In this there is a basic overview of the main components of
ERTS and some vocabulary needed to understand the more
detailed descriptions of each component in the following chapters.</p>
</div>
<div class="sect3">
<h4 id="_the_erlang_node_erts">The Erlang Node (ERTS)</h4>
<div class="paragraph">
<p>When you start an Elixir or Erlang application or system, what you
really start is an Erlang node. The node runs the Erlang
RunTime System and the virtual machine BEAM. (or possibly another
implementation of Erlang (see <a href="#Other_Erlang_Implementations">Other Erlang Implementations</a>)).</p>
</div>
<div class="paragraph">
<p>Your application code will run in an Erlang node, and all the layers
of the node will affect the performance of your application. We will
look at the stack of layers that makes up a node. This will help you
understand your options for running your system in different
environments.</p>
</div>
<div class="paragraph">
<p>In OO terminology one could say that an Erlang node is an object of
the Erlang Runtime System class. The equivalent in the Java world is a
JVM instance.</p>
</div>
<div class="paragraph">
<p>All execution of Elixir/Erlang code is done within a node. An Erlang node
runs in one OS process, and you can have several Erlang nodes
running on one machine.</p>
</div>
<div class="paragraph">
<p>To be completely correct according to the Erlang OTP documentation a
node is actually an executing runtime system that has been given a
name. That is, if you start Elixir without giving a name through one
of the command line switches <code>--name NAME@HOST</code> or <code>--sname NAME</code> (or
<code>-name</code> and <code>-sname</code> for an Erlang runtime.) you will have a runtime
but not a node. In such a system the function <code>Node.alive?</code>
(or in Erlang <code>is_alive()</code>) returns false.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ iex
Erlang/OTP 19 [erts-8.1] [source-0567896] [64-bit] [smp:4:4]
[async-threads:10] [hipe] [kernel-poll:false]
Interactive Elixir (1.4.0) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Node.alive?
false
iex(2)></pre>
</div>
</div>
<div class="paragraph">
<p>The runtime system itself is not that strict in its use
of the terminology. You can ask for the name of the node even
if you didn’t give it a name. In Elixir you use the function
<code>Node.list</code> with the argument <code>:this</code>, and in Erlang you
call <code>nodes(this).</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>iex(2)> Node.list :this
[:nonode@nohost]
iex(3)></pre>
</div>
</div>
<div class="paragraph">
<p>In this book we will use the term node for any running instance
of the runtime whether it is given a name or not.</p>
</div>
</div>
<div class="sect3">
<h4 id="_layers_in_the_execution_environment">Layers in the Execution Environment</h4>
<div class="paragraph">
<p>Your program (application) will run in one or more nodes, and the
performance of your program will depend not only on your application
code but also on all the layers below your code in the <em>ERTS
stack</em>. In <a href="#the_erts_stack">ERTS Stack</a> you can see the ERTS Stack
illustrated with two Erlang nodes running on one machine.</p>
</div>
<div id="the_erts_stack" class="imageblock">
<div class="content">
<img src="diag-23ae168750d38078baa5a26c03ad58e1.png" alt="ERTS Stack" width="200" height="340">
</div>
<div class="title">Figure 1. ERTS Stack</div>
</div>
<div class="paragraph">
<p>If you are using Elixir there is yet another layer to the stack.</p>
</div>
<div id="the_elixir_stack" class="imageblock">
<div class="content">
<img src="diag-cf0bdec078872559c8e8cf768e835b11.png" alt="Elixir Stack" width="200" height="380">
</div>
<div class="title">Figure 2. Elixir Stack</div>
</div>
<div class="paragraph">
<p>Let’s look at each layer of the stack and see how you can tune them
to your application’s need.</p>
</div>
<div class="paragraph">
<p>At the bottom of the stack there is the hardware you are running
on. The easiest way to improve the performance of your app is probably
to run it on better hardware. You might need to start exploring higher
levels of the stack if economical or physical constraints or
environmental concerns won’t let you upgrade your hardware.</p>
</div>
<div class="paragraph">
<p>The two most important choices for your hardware
is whether it is multicore and whether it is 32-bit or 64-bit. You
need different builds of ERTS depending on whether you want to use
multicore or not and whether you want to use 32-bit or 64-bit.</p>
</div>
<div class="paragraph">
<p>The second layer in the stack is the OS level. ERTS runs on most
versions of Windows and most POSIX "compliant" operating systems,
including Linux, VxWorks, FreeBSD, Solaris, and Mac OS X. Today most of
the development of ERTS is done on Linux and OS X, and you can expect
the best performance on these platforms. However, Ericsson has been
using Solaris internally in many projects and ERTS has been tuned for
Solaris for many years. Depending on your use case you might actually
get the best performance on a Solaris system. The OS choice is usually
not based on performance requirements, but is restricted by other
factors. If you are building an embedded application you might be
restricted to Raspbian or VxWork, and if you for some reason are
building an end user or client application you might have to use
Windows. The Windows port of ERTS has so far not had the highest
priority and might not be the best choice from a performance or
maintenance perspective. If you want to use a 64-bit ERTS you of
course need to have both a 64-bit machine and a 64-bit OS. We will not
cover many OS specific questions in this book, and most examples will
be assuming that you run on Linux.</p>
</div>
<div class="paragraph">
<p>The third layer in the stack is the Erlang Runtime System. In our case
this will be ERTS. This and the fourth layer, the Erlang Virtual Machine
(BEAM), is what this book is all about.</p>
</div>
<div class="paragraph">
<p>The fifth layer, OTP, supplies the Erlang standard libraries. OTP
originally stood for "Open Telecom Platform" and was a number of