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<h4 class="subsection">9.13.13 Writing 16-bit Code</h4>

<p><a name="index-i386-16_002dbit-code-982"></a><a name="index-g_t16_002dbit-code_002c-i386-983"></a><a name="index-real_002dmode-code_002c-i386-984"></a><a name="index-g_t_0040code_007bcode16gcc_007d-directive_002c-i386-985"></a><a name="index-g_t_0040code_007bcode16_007d-directive_002c-i386-986"></a><a name="index-g_t_0040code_007bcode32_007d-directive_002c-i386-987"></a><a name="index-g_t_0040code_007bcode64_007d-directive_002c-i386-988"></a><a name="index-g_t_0040code_007bcode64_007d-directive_002c-x86_002d64-989"></a>While <code>as</code> normally writes only &ldquo;pure&rdquo; 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 &lsquo;<samp><span class="samp">.code16</span></samp>&rsquo; or
&lsquo;<samp><span class="samp">.code16gcc</span></samp>&rsquo; 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 &lsquo;<samp><span class="samp">.code32</span></samp>&rsquo; directive or 64-bit code with the
&lsquo;<samp><span class="samp">.code64</span></samp>&rsquo; directive.

   <p>&lsquo;<samp><span class="samp">.code16gcc</span></samp>&rsquo; provides experimental support for generating 16-bit
code from gcc, and differs from &lsquo;<samp><span class="samp">.code16</span></samp>&rsquo; in that &lsquo;<samp><span class="samp">call</span></samp>&rsquo;,
&lsquo;<samp><span class="samp">ret</span></samp>&rsquo;, &lsquo;<samp><span class="samp">enter</span></samp>&rsquo;, &lsquo;<samp><span class="samp">leave</span></samp>&rsquo;, &lsquo;<samp><span class="samp">push</span></samp>&rsquo;, &lsquo;<samp><span class="samp">pop</span></samp>&rsquo;,
&lsquo;<samp><span class="samp">pusha</span></samp>&rsquo;, &lsquo;<samp><span class="samp">popa</span></samp>&rsquo;, &lsquo;<samp><span class="samp">pushf</span></samp>&rsquo;, and &lsquo;<samp><span class="samp">popf</span></samp>&rsquo; 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. 
&lsquo;<samp><span class="samp">.code16gcc</span></samp>&rsquo; also automatically adds address size prefixes where
necessary to use the 32-bit addressing modes that gcc generates.

   <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>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 &lsquo;<samp><span class="samp">66 6a 04</span></samp>&rsquo;, which pushes the
value &lsquo;<samp><span class="samp">4</span></samp>&rsquo; onto the stack, decrementing &lsquo;<samp><span class="samp">%esp</span></samp>&rsquo; by 2.

<pre class="smallexample">             pushw $4
</pre>
   <p>The same code in a 16-bit code section would generate the machine
opcode bytes &lsquo;<samp><span class="samp">6a 04</span></samp>&rsquo; (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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