📄 secs-background.html

📁 gcc手册
💻 HTML
字号:
<html lang="en">

<head>

<title>Using as</title>

<meta http-equiv="Content-Type" content="text/html">

<meta name="description" content="Using as">

<meta name="generator" content="makeinfo 4.3">

<link href="http://www.gnu.org/software/texinfo/" rel="generator-home">

</head>

<body>

<div class="node">

<p>

Node:<a name="Secs%20Background">Secs Background</a>,

Next:<a rel="next" accesskey="n" href="Ld-Sections.html#Ld%20Sections">Ld Sections</a>,

Up:<a rel="up" accesskey="u" href="Sections.html#Sections">Sections</a>

<hr><br>

</div>



<h3 class="section">Background</h3>



   <p>Roughly, a section is a range of addresses, with no gaps; all data

"in" those addresses is treated the same for some particular purpose. 

For example there may be a "read only" section.



   <p>The linker <code>ld</code> reads many object files (partial programs) and

combines their contents to form a runnable program.  When <code>as</code>

emits an object file, the partial program is assumed to start at address 0. 

<code>ld</code> assigns the final addresses for the partial program, so that

different partial programs do not overlap.  This is actually an

oversimplification, but it suffices to explain how <code>as</code> uses

sections.



   <p><code>ld</code> moves blocks of bytes of your program to their run-time

addresses.  These blocks slide to their run-time addresses as rigid

units; their length does not change and neither does the order of bytes

within them.  Such a rigid unit is called a <em>section</em>.  Assigning

run-time addresses to sections is called <dfn>relocation</dfn>.  It includes

the task of adjusting mentions of object-file addresses so they refer to

the proper run-time addresses. 

For the H8/300 and H8/500,

and for the Hitachi SH,

<code>as</code> pads sections if needed to

ensure they end on a word (sixteen bit) boundary.



   <p>An object file written by <code>as</code> has at least three sections, any

of which may be empty.  These are named <dfn>text</dfn>, <dfn>data</dfn> and

<dfn>bss</dfn> sections.



   <p>When it generates COFF output,

<code>as</code> can also generate whatever other named sections you specify

using the <code>.section</code> directive (see <a href="Section.html#Section"><code>.section</code></a>). 

If you do not use any directives that place output in the <code>.text</code>

or <code>.data</code> sections, these sections still exist, but are empty.



   <p>When <code>as</code> generates SOM or ELF output for the HPPA,

<code>as</code> can also generate whatever other named sections you

specify using the <code>.space</code> and <code>.subspace</code> directives.  See

<cite>HP9000 Series 800 Assembly Language Reference Manual</cite>

(HP 92432-90001) for details on the <code>.space</code> and <code>.subspace</code>

assembler directives.



   <p>Additionally, <code>as</code> uses different names for the standard

text, data, and bss sections when generating SOM output.  Program text

is placed into the <code>$CODE$</code> section, data into <code>$DATA$</code>, and

BSS into <code>$BSS$</code>.



   <p>Within the object file, the text section starts at address <code>0</code>, the

data section follows, and the bss section follows the data section.



   <p>When generating either SOM or ELF output files on the HPPA, the text

section starts at address <code>0</code>, the data section at address

<code>0x4000000</code>, and the bss section follows the data section.



   <p>To let <code>ld</code> know which data changes when the sections are

relocated, and how to change that data, <code>as</code> also writes to the

object file details of the relocation needed.  To perform relocation

<code>ld</code> must know, each time an address in the object

file is mentioned:

     <ul>

<li>Where in the object file is the beginning of this reference to

an address? 

<li>How long (in bytes) is this reference? 

<li>Which section does the address refer to?  What is the numeric value of

     <pre class="display">          (<var>address</var>) - (<var>start-address of section</var>)?

          </pre>

     <li>Is the reference to an address "Program-Counter relative"? 

</ul>



   <p>In fact, every address <code>as</code> ever uses is expressed as

<pre class="display">     (<var>section</var>) + (<var>offset into section</var>)

     </pre>



<p>Further, most expressions <code>as</code> computes have this section-relative

nature. 

(For some object formats, such as SOM for the HPPA, some expressions are

symbol-relative instead.)



   <p>In this manual we use the notation {<var>secname</var> <var>N</var>} to mean "offset

<var>N</var> into section <var>secname</var>."



   <p>Apart from text, data and bss sections you need to know about the

<dfn>absolute</dfn> section.  When <code>ld</code> mixes partial programs,

addresses in the absolute section remain unchanged.  For example, address

<code>{absolute 0}</code> is "relocated" to run-time address 0 by

<code>ld</code>.  Although the linker never arranges two partial programs'

data sections with overlapping addresses after linking, <em>by definition</em>

their absolute sections must overlap.  Address <code>{absolute 239}</code> in one

part of a program is always the same address when the program is running as

address <code>{absolute 239}</code> in any other part of the program.



   <p>The idea of sections is extended to the <dfn>undefined</dfn> section.  Any

address whose section is unknown at assembly time is by definition

rendered {undefined <var>U</var>}--where <var>U</var> is filled in later. 

Since numbers are always defined, the only way to generate an undefined

address is to mention an undefined symbol.  A reference to a named

common block would be such a symbol: its value is unknown at assembly

time so it has section <em>undefined</em>.



   <p>By analogy the word <em>section</em> is used to describe groups of sections in

the linked program.  <code>ld</code> puts all partial programs' text

sections in contiguous addresses in the linked program.  It is

customary to refer to the <em>text section</em> of a program, meaning all

the addresses of all partial programs' text sections.  Likewise for

data and bss sections.



   <p>Some sections are manipulated by <code>ld</code>; others are invented for

use of <code>as</code> and have no meaning except during assembly.



   </body></html>
⌨️ 快捷键说明

复制代码 Ctrl + C
搜索代码 Ctrl + F
全屏模式 F11
切换主题 Ctrl + Shift + D
显示快捷键 ?
增大字号 Ctrl + =
减小字号 Ctrl + -