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119 lines
7.3 KiB
HTML
15 years ago
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<title>i386-Mnemonics - Using as</title>
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<meta name="description" content="Using as">
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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_002dSyntax.html#i386_002dSyntax" title="i386-Syntax">
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<link rel="next" href="i386_002dRegs.html#i386_002dRegs" title="i386-Regs">
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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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</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_002dMnemonics"></a>Next: <a rel="next" accesskey="n" href="i386_002dRegs.html#i386_002dRegs">i386-Regs</a>,
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Previous: <a rel="previous" accesskey="p" href="i386_002dSyntax.html#i386_002dSyntax">i386-Syntax</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">9.13.4 Instruction Naming</h4>
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<p><a name="index-i386-instruction-naming-887"></a><a name="index-instruction-naming_002c-i386-888"></a><a name="index-x86_002d64-instruction-naming-889"></a><a name="index-instruction-naming_002c-x86_002d64-890"></a>
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Instruction mnemonics are suffixed with one character modifiers which
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specify the size of operands. The letters <span class="samp">b</span>, <span class="samp">w</span>, <span class="samp">l</span>
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and <span class="samp">q</span> specify byte, word, long and quadruple word operands. If
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no suffix is specified by an instruction then <code>as</code> tries to
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fill in the missing suffix based on the destination register operand
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(the last one by convention). Thus, <span class="samp">mov %ax, %bx</span> is equivalent
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to <span class="samp">movw %ax, %bx</span>; also, <span class="samp">mov $1, %bx</span> is equivalent to
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<span class="samp">movw $1, bx</span>. Note that this is incompatible with the AT&T Unix
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assembler which assumes that a missing mnemonic suffix implies long
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operand size. (This incompatibility does not affect compiler output
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since compilers always explicitly specify the mnemonic suffix.)
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<p>Almost all instructions have the same names in AT&T and Intel format.
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There are a few exceptions. The sign extend and zero extend
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instructions need two sizes to specify them. They need a size to
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sign/zero extend <em>from</em> and a size to zero extend <em>to</em>. This
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is accomplished by using two instruction mnemonic suffixes in AT&T
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syntax. Base names for sign extend and zero extend are
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<span class="samp">movs...</span> and <span class="samp">movz...</span> in AT&T syntax (<span class="samp">movsx</span>
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and <span class="samp">movzx</span> in Intel syntax). The instruction mnemonic suffixes
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are tacked on to this base name, the <em>from</em> suffix before the
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<em>to</em> suffix. Thus, <span class="samp">movsbl %al, %edx</span> is AT&T syntax for
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“move sign extend <em>from</em> %al <em>to</em> %edx.” Possible suffixes,
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thus, are <span class="samp">bl</span> (from byte to long), <span class="samp">bw</span> (from byte to word),
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<span class="samp">wl</span> (from word to long), <span class="samp">bq</span> (from byte to quadruple word),
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<span class="samp">wq</span> (from word to quadruple word), and <span class="samp">lq</span> (from long to
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quadruple word).
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<p><a name="index-conversion-instructions_002c-i386-891"></a><a name="index-i386-conversion-instructions-892"></a><a name="index-conversion-instructions_002c-x86_002d64-893"></a><a name="index-x86_002d64-conversion-instructions-894"></a>The Intel-syntax conversion instructions
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<ul>
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<li><span class="samp">cbw</span> — sign-extend byte in <span class="samp">%al</span> to word in <span class="samp">%ax</span>,
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<li><span class="samp">cwde</span> — sign-extend word in <span class="samp">%ax</span> to long in <span class="samp">%eax</span>,
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<li><span class="samp">cwd</span> — sign-extend word in <span class="samp">%ax</span> to long in <span class="samp">%dx:%ax</span>,
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<li><span class="samp">cdq</span> — sign-extend dword in <span class="samp">%eax</span> to quad in <span class="samp">%edx:%eax</span>,
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<li><span class="samp">cdqe</span> — sign-extend dword in <span class="samp">%eax</span> to quad in <span class="samp">%rax</span>
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(x86-64 only),
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<li><span class="samp">cqo</span> — sign-extend quad in <span class="samp">%rax</span> to octuple in
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<span class="samp">%rdx:%rax</span> (x86-64 only),
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</ul>
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<p class="noindent">are called <span class="samp">cbtw</span>, <span class="samp">cwtl</span>, <span class="samp">cwtd</span>, <span class="samp">cltd</span>, <span class="samp">cltq</span>, and
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<span class="samp">cqto</span> in AT&T naming. <code>as</code> accepts either naming for these
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instructions.
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<p><a name="index-jump-instructions_002c-i386-895"></a><a name="index-call-instructions_002c-i386-896"></a><a name="index-jump-instructions_002c-x86_002d64-897"></a><a name="index-call-instructions_002c-x86_002d64-898"></a>Far call/jump instructions are <span class="samp">lcall</span> and <span class="samp">ljmp</span> in
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AT&T syntax, but are <span class="samp">call far</span> and <span class="samp">jump far</span> in Intel
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convention.
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<h4 class="subsection">9.13.5 AT&T Mnemonic versus Intel Mnemonic</h4>
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<p><a name="index-i386-mnemonic-compatibility-899"></a><a name="index-mnemonic-compatibility_002c-i386-900"></a>
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<code>as</code> supports assembly using Intel mnemonic.
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<code>.intel_mnemonic</code> selects Intel mnemonic with Intel syntax, and
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<code>.att_mnemonic</code> switches back to the usual AT&T mnemonic with AT&T
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syntax for compatibility with the output of <code>gcc</code>.
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Several x87 instructions, <span class="samp">fadd</span>, <span class="samp">fdiv</span>, <span class="samp">fdivp</span>,
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<span class="samp">fdivr</span>, <span class="samp">fdivrp</span>, <span class="samp">fmul</span>, <span class="samp">fsub</span>, <span class="samp">fsubp</span>,
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<span class="samp">fsubr</span> and <span class="samp">fsubrp</span>, are implemented in AT&T System V/386
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assembler with different mnemonics from those in Intel IA32 specification.
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<code>gcc</code> generates those instructions with AT&T mnemonic.
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</body></html>
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