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

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<a name="i386_002dVariations"></a>
<div class="header">
<p>
Next: <a href="i386_002dChars.html#i386_002dChars" accesskey="n" rel="next">i386-Chars</a>, Up: <a href="i386_002dSyntax.html#i386_002dSyntax" accesskey="u" rel="up">i386-Syntax</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="AT_0026T-Syntax-versus-Intel-Syntax"></a>
<h4 class="subsubsection">9.15.3.1 AT&amp;T Syntax versus Intel Syntax</h4>

<a name="index-i386-intel_005fsyntax-pseudo-op"></a>
<a name="index-intel_005fsyntax-pseudo-op_002c-i386"></a>
<a name="index-i386-att_005fsyntax-pseudo-op"></a>
<a name="index-att_005fsyntax-pseudo-op_002c-i386"></a>
<a name="index-i386-syntax-compatibility"></a>
<a name="index-syntax-compatibility_002c-i386"></a>
<a name="index-x86_002d64-intel_005fsyntax-pseudo-op"></a>
<a name="index-intel_005fsyntax-pseudo-op_002c-x86_002d64"></a>
<a name="index-x86_002d64-att_005fsyntax-pseudo-op"></a>
<a name="index-att_005fsyntax-pseudo-op_002c-x86_002d64"></a>
<a name="index-x86_002d64-syntax-compatibility"></a>
<a name="index-syntax-compatibility_002c-x86_002d64"></a>

<p><code>as</code> now supports assembly using Intel assembler syntax.
<code>.intel_syntax</code> selects Intel mode, and <code>.att_syntax</code> switches
back to the usual AT&amp;T mode for compatibility with the output of
<code>gcc</code>.  Either of these directives may have an optional
argument, <code>prefix</code>, or <code>noprefix</code> specifying whether registers
require a &lsquo;<samp>%</samp>&rsquo; prefix.  AT&amp;T System V/386 assembler syntax is quite
different from Intel syntax.  We mention these differences because
almost all 80386 documents use Intel syntax.  Notable differences
between the two syntaxes are:
</p>
<a name="index-immediate-operands_002c-i386"></a>
<a name="index-i386-immediate-operands"></a>
<a name="index-register-operands_002c-i386"></a>
<a name="index-i386-register-operands"></a>
<a name="index-jump_002fcall-operands_002c-i386"></a>
<a name="index-i386-jump_002fcall-operands"></a>
<a name="index-operand-delimiters_002c-i386"></a>

<a name="index-immediate-operands_002c-x86_002d64"></a>
<a name="index-x86_002d64-immediate-operands"></a>
<a name="index-register-operands_002c-x86_002d64"></a>
<a name="index-x86_002d64-register-operands"></a>
<a name="index-jump_002fcall-operands_002c-x86_002d64"></a>
<a name="index-x86_002d64-jump_002fcall-operands"></a>
<a name="index-operand-delimiters_002c-x86_002d64"></a>
<ul>
<li> AT&amp;T immediate operands are preceded by &lsquo;<samp>$</samp>&rsquo;; Intel immediate
operands are undelimited (Intel &lsquo;<samp>push 4</samp>&rsquo; is AT&amp;T &lsquo;<samp>pushl $4</samp>&rsquo;).
AT&amp;T register operands are preceded by &lsquo;<samp>%</samp>&rsquo;; Intel register operands
are undelimited.  AT&amp;T absolute (as opposed to PC relative) jump/call
operands are prefixed by &lsquo;<samp>*</samp>&rsquo;; they are undelimited in Intel syntax.

</li><li> <a name="index-i386-source_002c-destination-operands"></a>
<a name="index-source_002c-destination-operands_003b-i386"></a>
<a name="index-x86_002d64-source_002c-destination-operands"></a>
<a name="index-source_002c-destination-operands_003b-x86_002d64"></a>
AT&amp;T and Intel syntax use the opposite order for source and destination
operands.  Intel &lsquo;<samp>add eax, 4</samp>&rsquo; is &lsquo;<samp>addl $4, %eax</samp>&rsquo;.  The
&lsquo;<samp>source, dest</samp>&rsquo; convention is maintained for compatibility with
previous Unix assemblers.  Note that &lsquo;<samp>bound</samp>&rsquo;, &lsquo;<samp>invlpga</samp>&rsquo;, and
instructions with 2 immediate operands, such as the &lsquo;<samp>enter</samp>&rsquo;
instruction, do <em>not</em> have reversed order.  <a href="i386_002dBugs.html#i386_002dBugs">i386-Bugs</a>.

</li><li> <a name="index-mnemonic-suffixes_002c-i386"></a>
<a name="index-sizes-operands_002c-i386"></a>
<a name="index-i386-size-suffixes"></a>
<a name="index-mnemonic-suffixes_002c-x86_002d64"></a>
<a name="index-sizes-operands_002c-x86_002d64"></a>
<a name="index-x86_002d64-size-suffixes"></a>
In AT&amp;T syntax the size of memory operands is determined from the last
character of the instruction mnemonic.  Mnemonic suffixes of &lsquo;<samp>b</samp>&rsquo;,
&lsquo;<samp>w</samp>&rsquo;, &lsquo;<samp>l</samp>&rsquo; and &lsquo;<samp>q</samp>&rsquo; specify byte (8-bit), word (16-bit), long
(32-bit) and quadruple word (64-bit) memory references.  Intel syntax accomplishes
this by prefixing memory operands (<em>not</em> the instruction mnemonics) with
&lsquo;<samp>byte ptr</samp>&rsquo;, &lsquo;<samp>word ptr</samp>&rsquo;, &lsquo;<samp>dword ptr</samp>&rsquo; and &lsquo;<samp>qword ptr</samp>&rsquo;.  Thus,
Intel &lsquo;<samp>mov al, byte ptr <var>foo</var></samp>&rsquo; is &lsquo;<samp>movb <var>foo</var>, %al</samp>&rsquo; in AT&amp;T
syntax.

<p>In 64-bit code, &lsquo;<samp>movabs</samp>&rsquo; can be used to encode the &lsquo;<samp>mov</samp>&rsquo;
instruction with the 64-bit displacement or immediate operand.
</p>
</li><li> <a name="index-return-instructions_002c-i386"></a>
<a name="index-i386-jump_002c-call_002c-return"></a>
<a name="index-return-instructions_002c-x86_002d64"></a>
<a name="index-x86_002d64-jump_002c-call_002c-return"></a>
Immediate form long jumps and calls are
&lsquo;<samp>lcall/ljmp $<var>section</var>, $<var>offset</var></samp>&rsquo; in AT&amp;T syntax; the
Intel syntax is
&lsquo;<samp>call/jmp far <var>section</var>:<var>offset</var></samp>&rsquo;.  Also, the far return
instruction
is &lsquo;<samp>lret $<var>stack-adjust</var></samp>&rsquo; in AT&amp;T syntax; Intel syntax is
&lsquo;<samp>ret far <var>stack-adjust</var></samp>&rsquo;.

</li><li> <a name="index-sections_002c-i386"></a>
<a name="index-i386-sections"></a>
<a name="index-sections_002c-x86_002d64"></a>
<a name="index-x86_002d64-sections"></a>
The AT&amp;T assembler does not provide support for multiple section
programs.  Unix style systems expect all programs to be single sections.
</li></ul>

<hr>
<div class="header">
<p>
Next: <a href="i386_002dChars.html#i386_002dChars" accesskey="n" rel="next">i386-Chars</a>, Up: <a href="i386_002dSyntax.html#i386_002dSyntax" accesskey="u" rel="up">i386-Syntax</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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