| <html lang="en"> |
| <head> |
| <title>i386-Prefixes - Using as</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Using as"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="i386_002dDependent.html#i386_002dDependent" title="i386-Dependent"> |
| <link rel="prev" href="i386_002dRegs.html#i386_002dRegs" title="i386-Regs"> |
| <link rel="next" href="i386_002dMemory.html#i386_002dMemory" title="i386-Memory"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU Assembler "as". |
| |
| Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, |
| 2000, 2001, 2002, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 |
| or any later version published by the Free Software Foundation; |
| with no Invariant Sections, with no Front-Cover Texts, and with no |
| Back-Cover Texts. A copy of the license is included in the |
| section entitled ``GNU Free Documentation License''. |
| |
| --> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="i386-Prefixes"></a> |
| <a name="i386_002dPrefixes"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="i386_002dMemory.html#i386_002dMemory">i386-Memory</a>, |
| Previous: <a rel="previous" accesskey="p" href="i386_002dRegs.html#i386_002dRegs">i386-Regs</a>, |
| Up: <a rel="up" accesskey="u" href="i386_002dDependent.html#i386_002dDependent">i386-Dependent</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">9.13.7 Instruction Prefixes</h4> |
| |
| <p><a name="index-i386-instruction-prefixes-936"></a><a name="index-instruction-prefixes_002c-i386-937"></a><a name="index-prefixes_002c-i386-938"></a>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><span class="samp">scas</span></samp>’ (scan string) instruction is |
| repeated with: |
| |
| <pre class="smallexample"> repne scas %es:(%edi),%al |
| </pre> |
| <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>Here is a list of instruction prefixes: |
| |
| <p><a name="index-section-override-prefixes_002c-i386-939"></a> |
| <ul> |
| <li>Section override prefixes ‘<samp><span class="samp">cs</span></samp>’, ‘<samp><span class="samp">ds</span></samp>’, ‘<samp><span class="samp">ss</span></samp>’, ‘<samp><span class="samp">es</span></samp>’, |
| ‘<samp><span class="samp">fs</span></samp>’, ‘<samp><span class="samp">gs</span></samp>’. These are automatically added by specifying |
| using the <var>section</var>:<var>memory-operand</var> form for memory references. |
| |
| <p><a name="index-size-prefixes_002c-i386-940"></a><li>Operand/Address size prefixes ‘<samp><span class="samp">data16</span></samp>’ and ‘<samp><span class="samp">addr16</span></samp>’ |
| change 32-bit operands/addresses into 16-bit operands/addresses, |
| while ‘<samp><span class="samp">data32</span></samp>’ and ‘<samp><span class="samp">addr32</span></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: |
| |
| <pre class="smallexample"> addr32 jmpl *(%ebx) |
| </pre> |
| <p><a name="index-bus-lock-prefixes_002c-i386-941"></a><a name="index-inhibiting-interrupts_002c-i386-942"></a><li>The bus lock prefix ‘<samp><span class="samp">lock</span></samp>’ inhibits interrupts during execution of |
| the instruction it precedes. (This is only valid with certain |
| instructions; see a 80386 manual for details). |
| |
| <p><a name="index-coprocessor-wait_002c-i386-943"></a><li>The wait for coprocessor prefix ‘<samp><span class="samp">wait</span></samp>’ waits for the coprocessor to |
| complete the current instruction. This should never be needed for the |
| 80386/80387 combination. |
| |
| <p><a name="index-repeat-prefixes_002c-i386-944"></a><li>The ‘<samp><span class="samp">rep</span></samp>’, ‘<samp><span class="samp">repe</span></samp>’, and ‘<samp><span class="samp">repne</span></samp>’ prefixes are added |
| to string instructions to make them repeat ‘<samp><span class="samp">%ecx</span></samp>’ times (‘<samp><span class="samp">%cx</span></samp>’ |
| times if the current address size is 16-bits). |
| <a name="index-REX-prefixes_002c-i386-945"></a><li>The ‘<samp><span class="samp">rex</span></samp>’ family of prefixes is used by x86-64 to encode |
| extensions to i386 instruction set. The ‘<samp><span class="samp">rex</span></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><span class="samp">rex</span></samp>’ prefixes directly. The ‘<samp><span class="samp">rex64xyz</span></samp>’ |
| instruction emits ‘<samp><span class="samp">rex</span></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. |
| </ul> |
| |
| </body></html> |
| |
