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| <title>Using as: i386-Prefixes</title> |
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| <a name="i386_002dPrefixes"></a> |
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| Next: <a href="i386_002dMemory.html#i386_002dMemory" accesskey="n" rel="next">i386-Memory</a>, Previous: <a href="i386_002dRegs.html#i386_002dRegs" accesskey="p" rel="prev">i386-Regs</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Instruction-Prefixes"></a> |
| <h4 class="subsection">9.15.6 Instruction Prefixes</h4> |
| |
| <a name="index-i386-instruction-prefixes"></a> |
| <a name="index-instruction-prefixes_002c-i386"></a> |
| <a name="index-prefixes_002c-i386"></a> |
| <p>Instruction prefixes are used to modify the following instruction. They |
| are used to repeat string instructions, to provide section overrides, to |
| perform bus lock operations, and to change operand and address sizes. |
| (Most instructions that normally operate on 32-bit operands will use |
| 16-bit operands if the instruction has an “operand size” prefix.) |
| Instruction prefixes are best written on the same line as the instruction |
| they act upon. For example, the ‘<samp>scas</samp>’ (scan string) instruction is |
| repeated with: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> repne scas %es:(%edi),%al |
| </pre></div> |
| |
| <p>You may also place prefixes on the lines immediately preceding the |
| instruction, but this circumvents checks that <code>as</code> does |
| with prefixes, and will not work with all prefixes. |
| </p> |
| <p>Here is a list of instruction prefixes: |
| </p> |
| <a name="index-section-override-prefixes_002c-i386"></a> |
| <ul> |
| <li> Section override prefixes ‘<samp>cs</samp>’, ‘<samp>ds</samp>’, ‘<samp>ss</samp>’, ‘<samp>es</samp>’, |
| ‘<samp>fs</samp>’, ‘<samp>gs</samp>’. These are automatically added by specifying |
| using the <var>section</var>:<var>memory-operand</var> form for memory references. |
| |
| </li><li> <a name="index-size-prefixes_002c-i386"></a> |
| Operand/Address size prefixes ‘<samp>data16</samp>’ and ‘<samp>addr16</samp>’ |
| change 32-bit operands/addresses into 16-bit operands/addresses, |
| while ‘<samp>data32</samp>’ and ‘<samp>addr32</samp>’ change 16-bit ones (in a |
| <code>.code16</code> section) into 32-bit operands/addresses. These prefixes |
| <em>must</em> appear on the same line of code as the instruction they |
| modify. For example, in a 16-bit <code>.code16</code> section, you might |
| write: |
| |
| <div class="smallexample"> |
| <pre class="smallexample"> addr32 jmpl *(%ebx) |
| </pre></div> |
| |
| </li><li> <a name="index-bus-lock-prefixes_002c-i386"></a> |
| <a name="index-inhibiting-interrupts_002c-i386"></a> |
| The bus lock prefix ‘<samp>lock</samp>’ inhibits interrupts during execution of |
| the instruction it precedes. (This is only valid with certain |
| instructions; see a 80386 manual for details). |
| |
| </li><li> <a name="index-coprocessor-wait_002c-i386"></a> |
| The wait for coprocessor prefix ‘<samp>wait</samp>’ waits for the coprocessor to |
| complete the current instruction. This should never be needed for the |
| 80386/80387 combination. |
| |
| </li><li> <a name="index-repeat-prefixes_002c-i386"></a> |
| The ‘<samp>rep</samp>’, ‘<samp>repe</samp>’, and ‘<samp>repne</samp>’ prefixes are added |
| to string instructions to make them repeat ‘<samp>%ecx</samp>’ times (‘<samp>%cx</samp>’ |
| times if the current address size is 16-bits). |
| </li><li> <a name="index-REX-prefixes_002c-i386"></a> |
| The ‘<samp>rex</samp>’ family of prefixes is used by x86-64 to encode |
| extensions to i386 instruction set. The ‘<samp>rex</samp>’ prefix has four |
| bits — an operand size overwrite (<code>64</code>) used to change operand size |
| from 32-bit to 64-bit and X, Y and Z extensions bits used to extend the |
| register set. |
| |
| <p>You may write the ‘<samp>rex</samp>’ prefixes directly. The ‘<samp>rex64xyz</samp>’ |
| instruction emits ‘<samp>rex</samp>’ prefix with all the bits set. By omitting |
| the <code>64</code>, <code>x</code>, <code>y</code> or <code>z</code> you may write other |
| prefixes as well. Normally, there is no need to write the prefixes |
| explicitly, since gas will automatically generate them based on the |
| instruction operands. |
| </p></li></ul> |
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