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99 lines
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99 lines
7.6 KiB
HTML
15 years ago
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<title>i386-Syntax - Using as</title>
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<meta name="description" content="Using as">
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<meta name="generator" content="makeinfo 4.7">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="i386_002dDependent.html#i386_002dDependent" title="i386-Dependent">
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<link rel="prev" href="i386_002dDirectives.html#i386_002dDirectives" title="i386-Directives">
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<link rel="next" href="i386_002dMnemonics.html#i386_002dMnemonics" title="i386-Mnemonics">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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2006, 2007 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled ``GNU Free Documentation License''.
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man end-->
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pre.smallformat { font-family:inherit; font-size:smaller }
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pre.smallexample { font-size:smaller }
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span.sc { font-variant:small-caps }
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span.roman { font-family: serif; font-weight: normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<p>
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<a name="i386_002dSyntax"></a>Next: <a rel="next" accesskey="n" href="i386_002dMnemonics.html#i386_002dMnemonics">i386-Mnemonics</a>,
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Previous: <a rel="previous" accesskey="p" href="i386_002dDirectives.html#i386_002dDirectives">i386-Directives</a>,
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Up: <a rel="up" accesskey="u" href="i386_002dDependent.html#i386_002dDependent">i386-Dependent</a>
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<hr><br>
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</div>
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<h4 class="subsection">9.13.3 AT&T Syntax versus Intel Syntax</h4>
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<p><a name="index-i386-intel_005fsyntax-pseudo-op-843"></a><a name="index-intel_005fsyntax-pseudo-op_002c-i386-844"></a><a name="index-i386-att_005fsyntax-pseudo-op-845"></a><a name="index-att_005fsyntax-pseudo-op_002c-i386-846"></a><a name="index-i386-syntax-compatibility-847"></a><a name="index-syntax-compatibility_002c-i386-848"></a><a name="index-x86_002d64-intel_005fsyntax-pseudo-op-849"></a><a name="index-intel_005fsyntax-pseudo-op_002c-x86_002d64-850"></a><a name="index-x86_002d64-att_005fsyntax-pseudo-op-851"></a><a name="index-att_005fsyntax-pseudo-op_002c-x86_002d64-852"></a><a name="index-x86_002d64-syntax-compatibility-853"></a><a name="index-syntax-compatibility_002c-x86_002d64-854"></a>
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<code>as</code> now supports assembly using Intel assembler syntax.
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<code>.intel_syntax</code> selects Intel mode, and <code>.att_syntax</code> switches
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back to the usual AT&T mode for compatibility with the output of
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<code>gcc</code>. Either of these directives may have an optional
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argument, <code>prefix</code>, or <code>noprefix</code> specifying whether registers
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require a <span class="samp">%</span> prefix. AT&T System V/386 assembler syntax is quite
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different from Intel syntax. We mention these differences because
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almost all 80386 documents use Intel syntax. Notable differences
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between the two syntaxes are:
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<p><a name="index-immediate-operands_002c-i386-855"></a><a name="index-i386-immediate-operands-856"></a><a name="index-register-operands_002c-i386-857"></a><a name="index-i386-register-operands-858"></a><a name="index-jump_002fcall-operands_002c-i386-859"></a><a name="index-i386-jump_002fcall-operands-860"></a><a name="index-operand-delimiters_002c-i386-861"></a>
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<a name="index-immediate-operands_002c-x86_002d64-862"></a><a name="index-x86_002d64-immediate-operands-863"></a><a name="index-register-operands_002c-x86_002d64-864"></a><a name="index-x86_002d64-register-operands-865"></a><a name="index-jump_002fcall-operands_002c-x86_002d64-866"></a><a name="index-x86_002d64-jump_002fcall-operands-867"></a><a name="index-operand-delimiters_002c-x86_002d64-868"></a>
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<ul>
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<li>AT&T immediate operands are preceded by <span class="samp">$</span>; Intel immediate
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operands are undelimited (Intel <span class="samp">push 4</span> is AT&T <span class="samp">pushl $4</span>).
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AT&T register operands are preceded by <span class="samp">%</span>; Intel register operands
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are undelimited. AT&T absolute (as opposed to PC relative) jump/call
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operands are prefixed by <span class="samp">*</span>; they are undelimited in Intel syntax.
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<p><a name="index-i386-source_002c-destination-operands-869"></a><a name="index-source_002c-destination-operands_003b-i386-870"></a><a name="index-x86_002d64-source_002c-destination-operands-871"></a><a name="index-source_002c-destination-operands_003b-x86_002d64-872"></a><li>AT&T and Intel syntax use the opposite order for source and destination
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operands. Intel <span class="samp">add eax, 4</span> is <span class="samp">addl $4, %eax</span>. The
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<span class="samp">source, dest</span> convention is maintained for compatibility with
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previous Unix assemblers. Note that <span class="samp">bound</span>, <span class="samp">invlpga</span>, and
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instructions with 2 immediate operands, such as the <span class="samp">enter</span>
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instruction, do <em>not</em> have reversed order. <a href="i386_002dBugs.html#i386_002dBugs">i386-Bugs</a>.
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<p><a name="index-mnemonic-suffixes_002c-i386-873"></a><a name="index-sizes-operands_002c-i386-874"></a><a name="index-i386-size-suffixes-875"></a><a name="index-mnemonic-suffixes_002c-x86_002d64-876"></a><a name="index-sizes-operands_002c-x86_002d64-877"></a><a name="index-x86_002d64-size-suffixes-878"></a><li>In AT&T syntax the size of memory operands is determined from the last
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character of the instruction mnemonic. Mnemonic suffixes of <span class="samp">b</span>,
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<span class="samp">w</span>, <span class="samp">l</span> and <span class="samp">q</span> specify byte (8-bit), word (16-bit), long
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(32-bit) and quadruple word (64-bit) memory references. Intel syntax accomplishes
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this by prefixing memory operands (<em>not</em> the instruction mnemonics) with
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<span class="samp">byte ptr</span>, <span class="samp">word ptr</span>, <span class="samp">dword ptr</span> and <span class="samp">qword ptr</span>. Thus,
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Intel <span class="samp">mov al, byte ptr </span><var>foo</var> is <span class="samp">movb </span><var>foo</var><span class="samp">, %al</span> in AT&T
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syntax.
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<p><a name="index-return-instructions_002c-i386-879"></a><a name="index-i386-jump_002c-call_002c-return-880"></a><a name="index-return-instructions_002c-x86_002d64-881"></a><a name="index-x86_002d64-jump_002c-call_002c-return-882"></a><li>Immediate form long jumps and calls are
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<span class="samp">lcall/ljmp $</span><var>section</var><span class="samp">, $</span><var>offset</var> in AT&T syntax; the
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Intel syntax is
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<span class="samp">call/jmp far </span><var>section</var><span class="samp">:</span><var>offset</var>. Also, the far return
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instruction
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is <span class="samp">lret $</span><var>stack-adjust</var> in AT&T syntax; Intel syntax is
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<span class="samp">ret far </span><var>stack-adjust</var>.
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<p><a name="index-sections_002c-i386-883"></a><a name="index-i386-sections-884"></a><a name="index-sections_002c-x86_002d64-885"></a><a name="index-x86_002d64-sections-886"></a><li>The AT&T assembler does not provide support for multiple section
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programs. Unix style systems expect all programs to be single sections.
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</ul>
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</body></html>
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