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| <h3 class="section">4.1 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><a name="index-linker_002c-and-assembler-187"></a><a name="index-assembler_002c-and-linker-188"></a>The linker <code>ld</code> reads many object files (partial programs) and |
| combines their contents to form a runnable program. When <samp><span class="command">as</span></samp> |
| 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 <samp><span class="command">as</span></samp> 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 for the Renesas / SuperH SH, |
| <samp><span class="command">as</span></samp> pads sections if needed to |
| ensure they end on a word (sixteen bit) boundary. |
| |
| <p><a name="index-standard-assembler-sections-189"></a>An object file written by <samp><span class="command">as</span></samp> 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 or ELF output, |
| <samp><span class="command">as</span></samp> can also generate whatever other named sections you specify |
| using the ‘<samp><span class="samp">.section</span></samp>’ directive (see <a href="Section.html#Section"><code>.section</code></a>). |
| If you do not use any directives that place output in the ‘<samp><span class="samp">.text</span></samp>’ |
| or ‘<samp><span class="samp">.data</span></samp>’ sections, these sections still exist, but are empty. |
| |
| <p>When <samp><span class="command">as</span></samp> generates SOM or ELF output for the HPPA, |
| <samp><span class="command">as</span></samp> can also generate whatever other named sections you |
| specify using the ‘<samp><span class="samp">.space</span></samp>’ and ‘<samp><span class="samp">.subspace</span></samp>’ directives. See |
| <cite>HP9000 Series 800 Assembly Language Reference Manual</cite> |
| (HP 92432-90001) for details on the ‘<samp><span class="samp">.space</span></samp>’ and ‘<samp><span class="samp">.subspace</span></samp>’ |
| assembler directives. |
| |
| <p>Additionally, <samp><span class="command">as</span></samp> uses different names for the standard |
| text, data, and bss sections when generating SOM output. Program text |
| is placed into the ‘<samp><span class="samp">$CODE$</span></samp>’ section, data into ‘<samp><span class="samp">$DATA$</span></samp>’, and |
| BSS into ‘<samp><span class="samp">$BSS$</span></samp>’. |
| |
| <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, <samp><span class="command">as</span></samp> 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><a name="index-addresses_002c-format-of-190"></a><a name="index-section_002drelative-addressing-191"></a>In fact, every address <samp><span class="command">as</span></samp> ever uses is expressed as |
| <pre class="display"> (<var>section</var>) + (<var>offset into section</var>) |
| </pre> |
| <p class="noindent">Further, most expressions <samp><span class="command">as</span></samp> 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 <samp><span class="command">as</span></samp> and have no meaning except during assembly. |
| |
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