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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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<a name="i386-16bit"></a>
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<a name="i386_002d16bit"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="i386_002dArch.html#i386_002dArch">i386-Arch</a>,
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Previous: <a rel="previous" accesskey="p" href="i386_002dTBM.html#i386_002dTBM">i386-TBM</a>,
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Up: <a rel="up" accesskey="u" href="i386_002dDependent.html#i386_002dDependent">i386-Dependent</a>
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<h4 class="subsection">9.15.14 Writing 16-bit Code</h4>
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<p><a name="index-i386-16_002dbit-code-1147"></a><a name="index-g_t16_002dbit-code_002c-i386-1148"></a><a name="index-real_002dmode-code_002c-i386-1149"></a><a name="index-g_t_0040code_007bcode16gcc_007d-directive_002c-i386-1150"></a><a name="index-g_t_0040code_007bcode16_007d-directive_002c-i386-1151"></a><a name="index-g_t_0040code_007bcode32_007d-directive_002c-i386-1152"></a><a name="index-g_t_0040code_007bcode64_007d-directive_002c-i386-1153"></a><a name="index-g_t_0040code_007bcode64_007d-directive_002c-x86_002d64-1154"></a>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><span class="samp">.code16</span></samp>’ or
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‘<samp><span class="samp">.code16gcc</span></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><span class="samp">.code32</span></samp>’ directive or 64-bit code with the
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‘<samp><span class="samp">.code64</span></samp>’ directive.
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<p>‘<samp><span class="samp">.code16gcc</span></samp>’ provides experimental support for generating 16-bit
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code from gcc, and differs from ‘<samp><span class="samp">.code16</span></samp>’ in that ‘<samp><span class="samp">call</span></samp>’,
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‘<samp><span class="samp">ret</span></samp>’, ‘<samp><span class="samp">enter</span></samp>’, ‘<samp><span class="samp">leave</span></samp>’, ‘<samp><span class="samp">push</span></samp>’, ‘<samp><span class="samp">pop</span></samp>’,
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‘<samp><span class="samp">pusha</span></samp>’, ‘<samp><span class="samp">popa</span></samp>’, ‘<samp><span class="samp">pushf</span></samp>’, and ‘<samp><span class="samp">popf</span></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><span class="samp">.code16gcc</span></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>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>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><span class="samp">66 6a 04</span></samp>’, which pushes the
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value ‘<samp><span class="samp">4</span></samp>’ onto the stack, decrementing ‘<samp><span class="samp">%esp</span></samp>’ by 2.
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<pre class="smallexample"> pushw $4
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</pre>
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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><span class="samp">6a 04</span></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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