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| <title>Using as: i386-16bit</title> |
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| <a name="i386_002d16bit"></a> |
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| <p> |
| Next: <a href="i386_002dBugs.html#i386_002dBugs" accesskey="n" rel="next">i386-Bugs</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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| <a name="Writing-16_002dbit-Code"></a> |
| <h4 class="subsection">9.15.15 Writing 16-bit Code</h4> |
| |
| <a name="index-i386-16_002dbit-code"></a> |
| <a name="index-16_002dbit-code_002c-i386"></a> |
| <a name="index-real_002dmode-code_002c-i386"></a> |
| <a name="index-code16gcc-directive_002c-i386"></a> |
| <a name="index-code16-directive_002c-i386"></a> |
| <a name="index-code32-directive_002c-i386"></a> |
| <a name="index-code64-directive_002c-i386"></a> |
| <a name="index-code64-directive_002c-x86_002d64"></a> |
| <p>While <code>as</code> normally writes only “pure” 32-bit i386 code |
| or 64-bit x86-64 code depending on the default configuration, |
| it also supports writing code to run in real mode or in 16-bit protected |
| mode code segments. To do this, put a ‘<samp>.code16</samp>’ or |
| ‘<samp>.code16gcc</samp>’ directive before the assembly language instructions to |
| be run in 16-bit mode. You can switch <code>as</code> to writing |
| 32-bit code with the ‘<samp>.code32</samp>’ directive or 64-bit code with the |
| ‘<samp>.code64</samp>’ directive. |
| </p> |
| <p>‘<samp>.code16gcc</samp>’ provides experimental support for generating 16-bit |
| code from gcc, and differs from ‘<samp>.code16</samp>’ in that ‘<samp>call</samp>’, |
| ‘<samp>ret</samp>’, ‘<samp>enter</samp>’, ‘<samp>leave</samp>’, ‘<samp>push</samp>’, ‘<samp>pop</samp>’, |
| ‘<samp>pusha</samp>’, ‘<samp>popa</samp>’, ‘<samp>pushf</samp>’, and ‘<samp>popf</samp>’ instructions |
| default to 32-bit size. This is so that the stack pointer is |
| manipulated in the same way over function calls, allowing access to |
| function parameters at the same stack offsets as in 32-bit mode. |
| ‘<samp>.code16gcc</samp>’ also automatically adds address size prefixes where |
| necessary to use the 32-bit addressing modes that gcc generates. |
| </p> |
| <p>The code which <code>as</code> generates in 16-bit mode will not |
| necessarily run on a 16-bit pre-80386 processor. To write code that |
| runs on such a processor, you must refrain from using <em>any</em> 32-bit |
| constructs which require <code>as</code> to output address or operand |
| size prefixes. |
| </p> |
| <p>Note that writing 16-bit code instructions by explicitly specifying a |
| prefix or an instruction mnemonic suffix within a 32-bit code section |
| generates different machine instructions than those generated for a |
| 16-bit code segment. In a 32-bit code section, the following code |
| generates the machine opcode bytes ‘<samp>66 6a 04</samp>’, which pushes the |
| value ‘<samp>4</samp>’ onto the stack, decrementing ‘<samp>%esp</samp>’ by 2. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> pushw $4 |
| </pre></div> |
| |
| <p>The same code in a 16-bit code section would generate the machine |
| opcode bytes ‘<samp>6a 04</samp>’ (i.e., without the operand size prefix), which |
| is correct since the processor default operand size is assumed to be 16 |
| bits in a 16-bit code section. |
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