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| <h3 class="section">7.107 <code>.word </code><var>expressions</var></h3> |
| |
| <p><a name="index-g_t_0040code_007bword_007d-directive-520"></a>This directive expects zero or more <var>expressions</var>, of any section, |
| separated by commas. |
| |
| <p>The size of the number emitted, and its byte order, |
| depend on what target computer the assembly is for. |
| |
| <!-- on sparc the "special treatment to support compilers" doesn't --> |
| <!-- happen-32-bit addressability, period; no long/short jumps. --> |
| <p><a name="index-difference-tables-altered-521"></a><a name="index-altered-difference-tables-522"></a><blockquote> |
| <em>Warning: Special Treatment to support Compilers</em> |
| </blockquote> |
| |
| <p>Machines with a 32-bit address space, but that do less than 32-bit |
| addressing, require the following special treatment. If the machine of |
| interest to you does 32-bit addressing (or doesn't require it; |
| see <a href="Machine-Dependencies.html#Machine-Dependencies">Machine Dependencies</a>), you can ignore this issue. |
| |
| <p>In order to assemble compiler output into something that works, |
| <samp><span class="command">as</span></samp> occasionally does strange things to ‘<samp><span class="samp">.word</span></samp>’ directives. |
| Directives of the form ‘<samp><span class="samp">.word sym1-sym2</span></samp>’ are often emitted by |
| compilers as part of jump tables. Therefore, when <samp><span class="command">as</span></samp> assembles a |
| directive of the form ‘<samp><span class="samp">.word sym1-sym2</span></samp>’, and the difference between |
| <code>sym1</code> and <code>sym2</code> does not fit in 16 bits, <samp><span class="command">as</span></samp> |
| creates a <dfn>secondary jump table</dfn>, immediately before the next label. |
| This secondary jump table is preceded by a short-jump to the |
| first byte after the secondary table. This short-jump prevents the flow |
| of control from accidentally falling into the new table. Inside the |
| table is a long-jump to <code>sym2</code>. The original ‘<samp><span class="samp">.word</span></samp>’ |
| contains <code>sym1</code> minus the address of the long-jump to |
| <code>sym2</code>. |
| |
| <p>If there were several occurrences of ‘<samp><span class="samp">.word sym1-sym2</span></samp>’ before the |
| secondary jump table, all of them are adjusted. If there was a |
| ‘<samp><span class="samp">.word sym3-sym4</span></samp>’, that also did not fit in sixteen bits, a |
| long-jump to <code>sym4</code> is included in the secondary jump table, |
| and the <code>.word</code> directives are adjusted to contain <code>sym3</code> |
| minus the address of the long-jump to <code>sym4</code>; and so on, for as many |
| entries in the original jump table as necessary. |
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