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



   <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 <code>.code16</code> or

<code>.code16gcc</code> directive before the assembly language instructions to

be run in 16-bit mode.  You can switch <code>as</code> back to writing

normal 32-bit code with the <code>.code32</code> directive.



   <p><code>.code16gcc</code> provides experimental support for generating 16-bit

code from gcc, and differs from <code>.code16</code> in that <code>call</code>,

<code>ret</code>, <code>enter</code>, <code>leave</code>, <code>push</code>, <code>pop</code>,

<code>pusha</code>, <code>popa</code>, <code>pushf</code>, and <code>popf</code> 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. 

<code>.code16gcc</code> 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 <code>66 6a 04</code>, which pushes the

value <code>4</code> onto the stack, decrementing <code>%esp</code> by 2.



<pre class="smallexample">             pushw $4

     </pre>



   <p>The same code in a 16-bit code section would generate the machine

opcode bytes <code>6a 04</code> (ie. 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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