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| <h3 class="section">4.4 Sub-Sections</h3> |
| |
| <p><a name="index-numbered-subsections-204"></a><a name="index-grouping-data-205"></a>Assembled bytes |
| conventionally |
| fall into two sections: text and data. |
| You may have separate groups of |
| data in named sections |
| that you want to end up near to each other in the object file, even though they |
| are not contiguous in the assembler source. <samp><span class="command">as</span></samp> allows you to |
| use <dfn>subsections</dfn> for this purpose. Within each section, there can be |
| numbered subsections with values from 0 to 8192. Objects assembled into the |
| same subsection go into the object file together with other objects in the same |
| subsection. For example, a compiler might want to store constants in the text |
| section, but might not want to have them interspersed with the program being |
| assembled. In this case, the compiler could issue a ‘<samp><span class="samp">.text 0</span></samp>’ before each |
| section of code being output, and a ‘<samp><span class="samp">.text 1</span></samp>’ before each group of |
| constants being output. |
| |
| <p>Subsections are optional. If you do not use subsections, everything |
| goes in subsection number zero. |
| |
| <p>Each subsection is zero-padded up to a multiple of four bytes. |
| (Subsections may be padded a different amount on different flavors |
| of <samp><span class="command">as</span></samp>.) |
| |
| <p>Subsections appear in your object file in numeric order, lowest numbered |
| to highest. (All this to be compatible with other people's assemblers.) |
| The object file contains no representation of subsections; <code>ld</code> and |
| other programs that manipulate object files see no trace of them. |
| They just see all your text subsections as a text section, and all your |
| data subsections as a data section. |
| |
| <p>To specify which subsection you want subsequent statements assembled |
| into, use a numeric argument to specify it, in a ‘<samp><span class="samp">.text |
| </span><var>expression</var></samp>’ or a ‘<samp><span class="samp">.data </span><var>expression</var></samp>’ statement. |
| When generating COFF output, you |
| can also use an extra subsection |
| argument with arbitrary named sections: ‘<samp><span class="samp">.section </span><var>name</var><span class="samp">, |
| </span><var>expression</var></samp>’. |
| When generating ELF output, you |
| can also use the <code>.subsection</code> directive (see <a href="SubSection.html#SubSection">SubSection</a>) |
| to specify a subsection: ‘<samp><span class="samp">.subsection </span><var>expression</var></samp>’. |
| <var>Expression</var> should be an absolute expression |
| (see <a href="Expressions.html#Expressions">Expressions</a>). If you just say ‘<samp><span class="samp">.text</span></samp>’ then ‘<samp><span class="samp">.text 0</span></samp>’ |
| is assumed. Likewise ‘<samp><span class="samp">.data</span></samp>’ means ‘<samp><span class="samp">.data 0</span></samp>’. Assembly |
| begins in <code>text 0</code>. For instance: |
| <pre class="smallexample"> .text 0 # The default subsection is text 0 anyway. |
| .ascii "This lives in the first text subsection. *" |
| .text 1 |
| .ascii "But this lives in the second text subsection." |
| .data 0 |
| .ascii "This lives in the data section," |
| .ascii "in the first data subsection." |
| .text 0 |
| .ascii "This lives in the first text section," |
| .ascii "immediately following the asterisk (*)." |
| </pre> |
| <p>Each section has a <dfn>location counter</dfn> incremented by one for every byte |
| assembled into that section. Because subsections are merely a convenience |
| restricted to <samp><span class="command">as</span></samp> there is no concept of a subsection location |
| counter. There is no way to directly manipulate a location counter—but the |
| <code>.align</code> directive changes it, and any label definition captures its |
| current value. The location counter of the section where statements are being |
| assembled is said to be the <dfn>active</dfn> location counter. |
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