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 <a name="Basic-Script-Concepts"></a>
 <div class="header">
 <p>
 Next: <a href="Script-Format.html#Script-Format" accesskey="n" rel="next">Script Format</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
 </div>
 <hr>
 <a name="Basic-Linker-Script-Concepts"></a>
 <h3 class="section">3.1 Basic Linker Script Concepts</h3>
 <a name="index-linker-script-concepts"></a>
 <p>We need to define some basic concepts and vocabulary in order to
 describe the linker script language.
 </p>
 <p>The linker combines input files into a single output file.  The output
 file and each input file are in a special data format known as an
 <em>object file format</em>.  Each file is called an <em>object file</em>.
 The output file is often called an <em>executable</em>, but for our
 purposes we will also call it an object file.  Each object file has,
 among other things, a list of <em>sections</em>.  We sometimes refer to a
 section in an input file as an <em>input section</em>; similarly, a section
 in the output file is an <em>output section</em>.
 </p>
 <p>Each section in an object file has a name and a size.  Most sections
 also have an associated block of data, known as the <em>section
 contents</em>.  A section may be marked as <em>loadable</em>, which means that
 the contents should be loaded into memory when the output file is run.
 A section with no contents may be <em>allocatable</em>, which means that an
 area in memory should be set aside, but nothing in particular should be
 loaded there (in some cases this memory must be zeroed out).  A section
 which is neither loadable nor allocatable typically contains some sort
 of debugging information.
 </p>
 <p>Every loadable or allocatable output section has two addresses.  The
 first is the <em>VMA</em>, or virtual memory address.  This is the address
 the section will have when the output file is run.  The second is the
 <em>LMA</em>, or load memory address.  This is the address at which the
 section will be loaded.  In most cases the two addresses will be the
 same.  An example of when they might be different is when a data section
 is loaded into ROM, and then copied into RAM when the program starts up
 (this technique is often used to initialize global variables in a ROM
 based system).  In this case the ROM address would be the LMA, and the
 RAM address would be the VMA.
 </p>
 <p>You can see the sections in an object file by using the <code>objdump</code>
 program with the &lsquo;<samp>-h</samp>&rsquo; option.
 </p>
 <p>Every object file also has a list of <em>symbols</em>, known as the
 <em>symbol table</em>.  A symbol may be defined or undefined.  Each symbol
 has a name, and each defined symbol has an address, among other
 information.  If you compile a C or C++ program into an object file, you
 will get a defined symbol for every defined function and global or
 static variable.  Every undefined function or global variable which is
 referenced in the input file will become an undefined symbol.
 </p>
 <p>You can see the symbols in an object file by using the <code>nm</code>
 program, or by using the <code>objdump</code> program with the &lsquo;<samp>-t</samp>&rsquo;
 option.
 </p>
 <hr>
 <div class="header">
 <p>
 Next: <a href="Script-Format.html#Script-Format" accesskey="n" rel="next">Script Format</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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	<a name="Basic-Script-Concepts"></a>
	<div class="header">
	<p>
	Next: <a href="Script-Format.html#Script-Format" accesskey="n" rel="next">Script Format</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a>   [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
	</div>
	<hr>
	<a name="Basic-Linker-Script-Concepts"></a>
	<h3 class="section">3.1 Basic Linker Script Concepts</h3>
	<a name="index-linker-script-concepts"></a>
	<p>We need to define some basic concepts and vocabulary in order to
	describe the linker script language.
	</p>
	<p>The linker combines input files into a single output file. The output
	file and each input file are in a special data format known as an
	<em>object file format</em>. Each file is called an <em>object file</em>.
	The output file is often called an <em>executable</em>, but for our
	purposes we will also call it an object file. Each object file has,
	among other things, a list of <em>sections</em>. We sometimes refer to a
	section in an input file as an <em>input section</em>; similarly, a section
	in the output file is an <em>output section</em>.
	</p>
	<p>Each section in an object file has a name and a size. Most sections
	also have an associated block of data, known as the <em>section
	contents</em>. A section may be marked as <em>loadable</em>, which means that
	the contents should be loaded into memory when the output file is run.
	A section with no contents may be <em>allocatable</em>, which means that an
	area in memory should be set aside, but nothing in particular should be
	loaded there (in some cases this memory must be zeroed out). A section
	which is neither loadable nor allocatable typically contains some sort
	of debugging information.
	</p>
	<p>Every loadable or allocatable output section has two addresses. The
	first is the <em>VMA</em>, or virtual memory address. This is the address
	the section will have when the output file is run. The second is the
	<em>LMA</em>, or load memory address. This is the address at which the
	section will be loaded. In most cases the two addresses will be the
	same. An example of when they might be different is when a data section
	is loaded into ROM, and then copied into RAM when the program starts up
	(this technique is often used to initialize global variables in a ROM
	based system). In this case the ROM address would be the LMA, and the
	RAM address would be the VMA.
	</p>
	<p>You can see the sections in an object file by using the <code>objdump</code>
	program with the ‘<samp>-h</samp>’ option.
	</p>
	<p>Every object file also has a list of <em>symbols</em>, known as the
	<em>symbol table</em>. A symbol may be defined or undefined. Each symbol
	has a name, and each defined symbol has an address, among other
	information. If you compile a C or C++ program into an object file, you
	will get a defined symbol for every defined function and global or
	static variable. Every undefined function or global variable which is
	referenced in the input file will become an undefined symbol.
	</p>
	<p>You can see the symbols in an object file by using the <code>nm</code>
	program, or by using the <code>objdump</code> program with the ‘<samp>-t</samp>’
	option.
	</p>
	<hr>
	<div class="header">
	<p>
	Next: <a href="Script-Format.html#Script-Format" accesskey="n" rel="next">Script Format</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a>   [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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