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| <h4 class="subsection">9.24.2 MIPS ECOFF object code</h4> |
| |
| <p><a name="index-ECOFF-sections-1266"></a><a name="index-MIPS-ECOFF-sections-1267"></a>Assembling for a <span class="sc">mips</span> <span class="sc">ecoff</span> target supports some additional sections |
| besides the usual <code>.text</code>, <code>.data</code> and <code>.bss</code>. The |
| additional sections are <code>.rdata</code>, used for read-only data, |
| <code>.sdata</code>, used for small data, and <code>.sbss</code>, used for small |
| common objects. |
| |
| <p><a name="index-small-objects_002c-MIPS-ECOFF-1268"></a><a name="index-g_t_0040code_007bgp_007d-register_002c-MIPS-1269"></a>When assembling for <span class="sc">ecoff</span>, the assembler uses the <code>$gp</code> (<code>$28</code>) |
| register to form the address of a “small object”. Any object in the |
| <code>.sdata</code> or <code>.sbss</code> sections is considered “small” in this sense. |
| For external objects, or for objects in the <code>.bss</code> section, you can use |
| the <code>gcc</code> ‘<samp><span class="samp">-G</span></samp>’ option to control the size of objects addressed via |
| <code>$gp</code>; the default value is 8, meaning that a reference to any object |
| eight bytes or smaller uses <code>$gp</code>. Passing ‘<samp><span class="samp">-G 0</span></samp>’ to |
| <code>as</code> prevents it from using the <code>$gp</code> register on the basis |
| of object size (but the assembler uses <code>$gp</code> for objects in <code>.sdata</code> |
| or <code>sbss</code> in any case). The size of an object in the <code>.bss</code> section |
| is set by the <code>.comm</code> or <code>.lcomm</code> directive that defines it. The |
| size of an external object may be set with the <code>.extern</code> directive. For |
| example, ‘<samp><span class="samp">.extern sym,4</span></samp>’ declares that the object at <code>sym</code> is 4 bytes |
| in length, whie leaving <code>sym</code> otherwise undefined. |
| |
| <p>Using small <span class="sc">ecoff</span> objects requires linker support, and assumes that the |
| <code>$gp</code> register is correctly initialized (normally done automatically by |
| the startup code). <span class="sc">mips</span> <span class="sc">ecoff</span> assembly code must not modify the |
| <code>$gp</code> register. |
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