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<title>Using as: i386-Mnemonics</title>

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<a name="i386_002dMnemonics"></a>
<div class="header">
<p>
Next: <a href="i386_002dRegs.html#i386_002dRegs" accesskey="n" rel="next">i386-Regs</a>, Previous: <a href="i386_002dSyntax.html#i386_002dSyntax" accesskey="p" rel="prev">i386-Syntax</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> &nbsp; [<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>
</div>
<hr>
<a name="i386_002dMnemonics-1"></a>
<h4 class="subsection">9.15.4 i386-Mnemonics</h4>
<a name="Instruction-Naming"></a>
<h4 class="subsubsection">9.15.4.1 Instruction Naming</h4>

<a name="index-i386-instruction-naming"></a>
<a name="index-instruction-naming_002c-i386"></a>
<a name="index-x86_002d64-instruction-naming"></a>
<a name="index-instruction-naming_002c-x86_002d64"></a>

<p>Instruction mnemonics are suffixed with one character modifiers which
specify the size of operands.  The letters &lsquo;<samp>b</samp>&rsquo;, &lsquo;<samp>w</samp>&rsquo;, &lsquo;<samp>l</samp>&rsquo;
and &lsquo;<samp>q</samp>&rsquo; specify byte, word, long and quadruple word operands.  If
no suffix is specified by an instruction then <code>as</code> tries to
fill in the missing suffix based on the destination register operand
(the last one by convention).  Thus, &lsquo;<samp>mov %ax, %bx</samp>&rsquo; is equivalent
to &lsquo;<samp>movw %ax, %bx</samp>&rsquo;; also, &lsquo;<samp>mov $1, %bx</samp>&rsquo; is equivalent to
&lsquo;<samp>movw $1, bx</samp>&rsquo;.  Note that this is incompatible with the AT&amp;T Unix
assembler which assumes that a missing mnemonic suffix implies long
operand size.  (This incompatibility does not affect compiler output
since compilers always explicitly specify the mnemonic suffix.)
</p>
<p>Almost all instructions have the same names in AT&amp;T and Intel format.
There are a few exceptions.  The sign extend and zero extend
instructions need two sizes to specify them.  They need a size to
sign/zero extend <em>from</em> and a size to zero extend <em>to</em>.  This
is accomplished by using two instruction mnemonic suffixes in AT&amp;T
syntax.  Base names for sign extend and zero extend are
&lsquo;<samp>movs&hellip;</samp>&rsquo; and &lsquo;<samp>movz&hellip;</samp>&rsquo; in AT&amp;T syntax (&lsquo;<samp>movsx</samp>&rsquo;
and &lsquo;<samp>movzx</samp>&rsquo; in Intel syntax).  The instruction mnemonic suffixes
are tacked on to this base name, the <em>from</em> suffix before the
<em>to</em> suffix.  Thus, &lsquo;<samp>movsbl %al, %edx</samp>&rsquo; is AT&amp;T syntax for
&ldquo;move sign extend <em>from</em> %al <em>to</em> %edx.&rdquo;  Possible suffixes,
thus, are &lsquo;<samp>bl</samp>&rsquo; (from byte to long), &lsquo;<samp>bw</samp>&rsquo; (from byte to word),
&lsquo;<samp>wl</samp>&rsquo; (from word to long), &lsquo;<samp>bq</samp>&rsquo; (from byte to quadruple word),
&lsquo;<samp>wq</samp>&rsquo; (from word to quadruple word), and &lsquo;<samp>lq</samp>&rsquo; (from long to
quadruple word).
</p>
<a name="index-encoding-options_002c-i386"></a>
<a name="index-encoding-options_002c-x86_002d64"></a>

<p>Different encoding options can be specified via optional mnemonic
suffix.  &lsquo;<samp>.s</samp>&rsquo; suffix swaps 2 register operands in encoding when
moving from one register to another.  &lsquo;<samp>.d8</samp>&rsquo; or &lsquo;<samp>.d32</samp>&rsquo; suffix
prefers 8bit or 32bit displacement in encoding.
</p>
<a name="index-conversion-instructions_002c-i386"></a>
<a name="index-i386-conversion-instructions"></a>
<a name="index-conversion-instructions_002c-x86_002d64"></a>
<a name="index-x86_002d64-conversion-instructions"></a>
<p>The Intel-syntax conversion instructions
</p>
<ul>
<li> &lsquo;<samp>cbw</samp>&rsquo; &mdash; sign-extend byte in &lsquo;<samp>%al</samp>&rsquo; to word in &lsquo;<samp>%ax</samp>&rsquo;,

</li><li> &lsquo;<samp>cwde</samp>&rsquo; &mdash; sign-extend word in &lsquo;<samp>%ax</samp>&rsquo; to long in &lsquo;<samp>%eax</samp>&rsquo;,

</li><li> &lsquo;<samp>cwd</samp>&rsquo; &mdash; sign-extend word in &lsquo;<samp>%ax</samp>&rsquo; to long in &lsquo;<samp>%dx:%ax</samp>&rsquo;,

</li><li> &lsquo;<samp>cdq</samp>&rsquo; &mdash; sign-extend dword in &lsquo;<samp>%eax</samp>&rsquo; to quad in &lsquo;<samp>%edx:%eax</samp>&rsquo;,

</li><li> &lsquo;<samp>cdqe</samp>&rsquo; &mdash; sign-extend dword in &lsquo;<samp>%eax</samp>&rsquo; to quad in &lsquo;<samp>%rax</samp>&rsquo;
(x86-64 only),

</li><li> &lsquo;<samp>cqo</samp>&rsquo; &mdash; sign-extend quad in &lsquo;<samp>%rax</samp>&rsquo; to octuple in
&lsquo;<samp>%rdx:%rax</samp>&rsquo; (x86-64 only),
</li></ul>

<p>are called &lsquo;<samp>cbtw</samp>&rsquo;, &lsquo;<samp>cwtl</samp>&rsquo;, &lsquo;<samp>cwtd</samp>&rsquo;, &lsquo;<samp>cltd</samp>&rsquo;, &lsquo;<samp>cltq</samp>&rsquo;, and
&lsquo;<samp>cqto</samp>&rsquo; in AT&amp;T naming.  <code>as</code> accepts either naming for these
instructions.
</p>
<a name="index-jump-instructions_002c-i386"></a>
<a name="index-call-instructions_002c-i386"></a>
<a name="index-jump-instructions_002c-x86_002d64"></a>
<a name="index-call-instructions_002c-x86_002d64"></a>
<p>Far call/jump instructions are &lsquo;<samp>lcall</samp>&rsquo; and &lsquo;<samp>ljmp</samp>&rsquo; in
AT&amp;T syntax, but are &lsquo;<samp>call far</samp>&rsquo; and &lsquo;<samp>jump far</samp>&rsquo; in Intel
convention.
</p>
<a name="AT_0026T-Mnemonic-versus-Intel-Mnemonic"></a>
<h4 class="subsubsection">9.15.4.2 AT&amp;T Mnemonic versus Intel Mnemonic</h4>

<a name="index-i386-mnemonic-compatibility"></a>
<a name="index-mnemonic-compatibility_002c-i386"></a>

<p><code>as</code> supports assembly using Intel mnemonic.
<code>.intel_mnemonic</code> selects Intel mnemonic with Intel syntax, and
<code>.att_mnemonic</code> switches back to the usual AT&amp;T mnemonic with AT&amp;T
syntax for compatibility with the output of <code>gcc</code>.
Several x87 instructions, &lsquo;<samp>fadd</samp>&rsquo;, &lsquo;<samp>fdiv</samp>&rsquo;, &lsquo;<samp>fdivp</samp>&rsquo;,
&lsquo;<samp>fdivr</samp>&rsquo;, &lsquo;<samp>fdivrp</samp>&rsquo;, &lsquo;<samp>fmul</samp>&rsquo;, &lsquo;<samp>fsub</samp>&rsquo;, &lsquo;<samp>fsubp</samp>&rsquo;,
&lsquo;<samp>fsubr</samp>&rsquo; and &lsquo;<samp>fsubrp</samp>&rsquo;,  are implemented in AT&amp;T System V/386
assembler with different mnemonics from those in Intel IA32 specification.
<code>gcc</code> generates those instructions with AT&amp;T mnemonic.
</p>
<hr>
<div class="header">
<p>
Next: <a href="i386_002dRegs.html#i386_002dRegs" accesskey="n" rel="next">i386-Regs</a>, Previous: <a href="i386_002dSyntax.html#i386_002dSyntax" accesskey="p" rel="prev">i386-Syntax</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> &nbsp; [<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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