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130 lines
6.5 KiB
HTML
130 lines
6.5 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU Assembler "as".
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Copyright (C) 1991-2019 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 1.3
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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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-->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>i386-16bit (Using as)</title>
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<meta name="description" content="i386-16bit (Using as)">
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<meta name="keywords" content="i386-16bit (Using as)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="i386_002dDependent.html#i386_002dDependent" rel="up" title="i386-Dependent">
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<link href="i386_002dArch.html#i386_002dArch" rel="next" title="i386-Arch">
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<link href="i386_002dTBM.html#i386_002dTBM" rel="prev" title="i386-TBM">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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div.example {margin-left: 3.2em}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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span.roman {font-family: initial; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="i386_002d16bit"></a>
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<div class="header">
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<p>
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Next: <a href="i386_002dArch.html#i386_002dArch" accesskey="n" rel="next">i386-Arch</a>, Previous: <a href="i386_002dTBM.html#i386_002dTBM" accesskey="p" rel="prev">i386-TBM</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> [<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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</div>
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<hr>
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<a name="Writing-16_002dbit-Code"></a>
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<h4 class="subsection">9.15.14 Writing 16-bit Code</h4>
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<a name="index-i386-16_002dbit-code"></a>
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<a name="index-16_002dbit-code_002c-i386"></a>
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<a name="index-real_002dmode-code_002c-i386"></a>
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<a name="index-code16gcc-directive_002c-i386"></a>
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<a name="index-code16-directive_002c-i386"></a>
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<a name="index-code32-directive_002c-i386"></a>
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<a name="index-code64-directive_002c-i386"></a>
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<a name="index-code64-directive_002c-x86_002d64"></a>
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<p>While <code>as</code> normally writes only “pure” 32-bit i386 code
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or 64-bit x86-64 code depending on the default configuration,
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it also supports writing code to run in real mode or in 16-bit protected
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mode code segments. To do this, put a ‘<samp>.code16</samp>’ or
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‘<samp>.code16gcc</samp>’ directive before the assembly language instructions to
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be run in 16-bit mode. You can switch <code>as</code> to writing
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32-bit code with the ‘<samp>.code32</samp>’ directive or 64-bit code with the
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‘<samp>.code64</samp>’ directive.
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</p>
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<p>‘<samp>.code16gcc</samp>’ provides experimental support for generating 16-bit
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code from gcc, and differs from ‘<samp>.code16</samp>’ in that ‘<samp>call</samp>’,
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‘<samp>ret</samp>’, ‘<samp>enter</samp>’, ‘<samp>leave</samp>’, ‘<samp>push</samp>’, ‘<samp>pop</samp>’,
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‘<samp>pusha</samp>’, ‘<samp>popa</samp>’, ‘<samp>pushf</samp>’, and ‘<samp>popf</samp>’ instructions
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default to 32-bit size. This is so that the stack pointer is
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manipulated in the same way over function calls, allowing access to
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function parameters at the same stack offsets as in 32-bit mode.
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‘<samp>.code16gcc</samp>’ also automatically adds address size prefixes where
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necessary to use the 32-bit addressing modes that gcc generates.
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</p>
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<p>The code which <code>as</code> generates in 16-bit mode will not
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necessarily run on a 16-bit pre-80386 processor. To write code that
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runs on such a processor, you must refrain from using <em>any</em> 32-bit
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constructs which require <code>as</code> to output address or operand
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size prefixes.
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</p>
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<p>Note that writing 16-bit code instructions by explicitly specifying a
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prefix or an instruction mnemonic suffix within a 32-bit code section
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generates different machine instructions than those generated for a
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16-bit code segment. In a 32-bit code section, the following code
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generates the machine opcode bytes ‘<samp>66 6a 04</samp>’, which pushes the
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value ‘<samp>4</samp>’ onto the stack, decrementing ‘<samp>%esp</samp>’ by 2.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> pushw $4
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</pre></div>
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<p>The same code in a 16-bit code section would generate the machine
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opcode bytes ‘<samp>6a 04</samp>’ (i.e., without the operand size prefix), which
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is correct since the processor default operand size is assumed to be 16
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bits in a 16-bit code section.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="i386_002dArch.html#i386_002dArch" accesskey="n" rel="next">i386-Arch</a>, Previous: <a href="i386_002dTBM.html#i386_002dTBM" accesskey="p" rel="prev">i386-TBM</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> [<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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</div>
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</body>
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</html>
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