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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 “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><span class="samp">.code16</span></samp>’ or |
| ‘<samp><span class="samp">.code16gcc</span></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><span class="samp">.code32</span></samp>’ directive or 64-bit code with the |
| ‘<samp><span class="samp">.code64</span></samp>’ directive. |
| |
| <p>‘<samp><span class="samp">.code16gcc</span></samp>’ provides experimental support for generating 16-bit |
| code from gcc, and differs from ‘<samp><span class="samp">.code16</span></samp>’ in that ‘<samp><span class="samp">call</span></samp>’, |
| ‘<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>’, |
| ‘<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 |
| 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><span class="samp">.code16gcc</span></samp>’ 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 ‘<samp><span class="samp">66 6a 04</span></samp>’, which pushes the |
| value ‘<samp><span class="samp">4</span></samp>’ onto the stack, decrementing ‘<samp><span class="samp">%esp</span></samp>’ by 2. |
| |
| <pre class="smallexample"> pushw $4 |
| </pre> |
| <p>The same code in a 16-bit code section would generate the machine |
| opcode bytes ‘<samp><span class="samp">6a 04</span></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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