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| <title>Using the GNU Compiler Collection (GCC): Basic Asm</title> |
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| <a name="Basic-Asm"></a> |
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| Next: <a href="Extended-Asm.html#Extended-Asm" accesskey="n" rel="next">Extended Asm</a>, Up: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="u" rel="up">Using Assembly Language with C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Basic-Asm-_002d_002d_002d-Assembler-Instructions-Without-Operands"></a> |
| <h4 class="subsection">6.43.1 Basic Asm — Assembler Instructions Without Operands</h4> |
| <a name="index-basic-asm"></a> |
| <a name="index-assembly-language-in-C_002c-basic"></a> |
| |
| <p>A basic <code>asm</code> statement has the following syntax: |
| </p> |
| <div class="example"> |
| <pre class="example">asm <span class="roman">[</span> volatile <span class="roman">]</span> ( <var>AssemblerInstructions</var> ) |
| </pre></div> |
| |
| <p>The <code>asm</code> keyword is a GNU extension. |
| When writing code that can be compiled with <samp>-ansi</samp> and the |
| various <samp>-std</samp> options, use <code>__asm__</code> instead of |
| <code>asm</code> (see <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>). |
| </p> |
| <a name="Qualifiers-1"></a> |
| <h4 class="subsubheading">Qualifiers</h4> |
| <dl compact="compact"> |
| <dt><code>volatile</code></dt> |
| <dd><p>The optional <code>volatile</code> qualifier has no effect. |
| All basic <code>asm</code> blocks are implicitly volatile. |
| </p></dd> |
| </dl> |
| |
| <a name="Parameters"></a> |
| <h4 class="subsubheading">Parameters</h4> |
| <dl compact="compact"> |
| <dt><var>AssemblerInstructions</var></dt> |
| <dd><p>This is a literal string that specifies the assembler code. The string can |
| contain any instructions recognized by the assembler, including directives. |
| GCC does not parse the assembler instructions themselves and |
| does not know what they mean or even whether they are valid assembler input. |
| </p> |
| <p>You may place multiple assembler instructions together in a single <code>asm</code> |
| string, separated by the characters normally used in assembly code for the |
| system. A combination that works in most places is a newline to break the |
| line, plus a tab character (written as ‘<samp>\n\t</samp>’). |
| Some assemblers allow semicolons as a line separator. However, |
| note that some assembler dialects use semicolons to start a comment. |
| </p></dd> |
| </dl> |
| |
| <a name="Remarks"></a> |
| <h4 class="subsubheading">Remarks</h4> |
| <p>Using extended <code>asm</code> typically produces smaller, safer, and more |
| efficient code, and in most cases it is a better solution than basic |
| <code>asm</code>. However, there are two situations where only basic <code>asm</code> |
| can be used: |
| </p> |
| <ul> |
| <li> Extended <code>asm</code> statements have to be inside a C |
| function, so to write inline assembly language at file scope (“top-level”), |
| outside of C functions, you must use basic <code>asm</code>. |
| You can use this technique to emit assembler directives, |
| define assembly language macros that can be invoked elsewhere in the file, |
| or write entire functions in assembly language. |
| |
| </li><li> Functions declared |
| with the <code>naked</code> attribute also require basic <code>asm</code> |
| (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>). |
| </li></ul> |
| |
| <p>Safely accessing C data and calling functions from basic <code>asm</code> is more |
| complex than it may appear. To access C data, it is better to use extended |
| <code>asm</code>. |
| </p> |
| <p>Do not expect a sequence of <code>asm</code> statements to remain perfectly |
| consecutive after compilation. If certain instructions need to remain |
| consecutive in the output, put them in a single multi-instruction <code>asm</code> |
| statement. Note that GCC’s optimizers can move <code>asm</code> statements |
| relative to other code, including across jumps. |
| </p> |
| <p><code>asm</code> statements may not perform jumps into other <code>asm</code> statements. |
| GCC does not know about these jumps, and therefore cannot take |
| account of them when deciding how to optimize. Jumps from <code>asm</code> to C |
| labels are only supported in extended <code>asm</code>. |
| </p> |
| <p>Under certain circumstances, GCC may duplicate (or remove duplicates of) your |
| assembly code when optimizing. This can lead to unexpected duplicate |
| symbol errors during compilation if your assembly code defines symbols or |
| labels. |
| </p> |
| <p>Since GCC does not parse the <var>AssemblerInstructions</var>, it has no |
| visibility of any symbols it references. This may result in GCC discarding |
| those symbols as unreferenced. |
| </p> |
| <p>The compiler copies the assembler instructions in a basic <code>asm</code> |
| verbatim to the assembly language output file, without |
| processing dialects or any of the ‘<samp>%</samp>’ operators that are available with |
| extended <code>asm</code>. This results in minor differences between basic |
| <code>asm</code> strings and extended <code>asm</code> templates. For example, to refer to |
| registers you might use ‘<samp>%eax</samp>’ in basic <code>asm</code> and |
| ‘<samp>%%eax</samp>’ in extended <code>asm</code>. |
| </p> |
| <p>On targets such as x86 that support multiple assembler dialects, |
| all basic <code>asm</code> blocks use the assembler dialect specified by the |
| <samp>-masm</samp> command-line option (see <a href="x86-Options.html#x86-Options">x86 Options</a>). |
| Basic <code>asm</code> provides no |
| mechanism to provide different assembler strings for different dialects. |
| </p> |
| <p>Here is an example of basic <code>asm</code> for i386: |
| </p> |
| <div class="example"> |
| <pre class="example">/* Note that this code will not compile with -masm=intel */ |
| #define DebugBreak() asm("int $3") |
| </pre></div> |
| |
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| Next: <a href="Extended-Asm.html#Extended-Asm" accesskey="n" rel="next">Extended Asm</a>, Up: <a href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" accesskey="u" rel="up">Using Assembly Language with C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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