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    <p>The easiest is to use the <a href="http://gcc.gnu.org/onlinedocs/gcc/Simple-Constraints.html#Simple-Constraints" rel="nofollow">constraint "m"</a>, to load from/save to a memory location:</p> <pre><code>#include &lt;stdio.h&gt; int main (void) { double a = 0.78539816339744; asm ("fldl %0;" "fsin;" "fstpl %0" : "+m"(a)); printf("sin(45°) = %f\n", a); return 0; } </code></pre> <p>The <a href="http://gcc.gnu.org/onlinedocs/gcc/Modifiers.html#Modifiers" rel="nofollow">modifier "+"</a> tells the compiler that you want your input in that memory location and to output to the same location.</p> <p>GCC is allowed to reorder your lines, but it will know the semantics of that line, and it will optimize accordingly.</p> <p>A more difficult example, employing loading from other variables than that are to be written to:</p> <pre><code>#include &lt;stdio.h&gt; int main (void) { double sin_a, cos_a, a = 0.5; asm ("fldl %2;" "fsincos;" "fstpl %1;" "fstpl %0;" : "=m"(sin_a), "=m"(cos_a) : "m"(a)); printf("sin(29°) = %f, cos(29°) = %f\n", sin_a, cos_a); return 0; } </code></pre> <p>Live demo: <a href="http://ideone.com/SuvpM" rel="nofollow">http://ideone.com/SuvpM</a>.</p> <p><s>PS: I could not get GCC to load a <strong>double</strong> value, though, so I don't know if my answer is any use to you. :-(</s></p> <h3>Update:</h3> <p>Got it: you need to append <code>s</code> (for single [32 bits], as in <code>flds</code>, floating-point load single), <code>l</code> (for double [64 bits]), or <code>t</code> (for long double [80 bits]) explicitely. My GCC 4.6.3 assumes a single value for every memory operand …</p>
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    1. COWow, thanks a lot! Now some how loading a double worked for me, at least I'm not getting any errors and the value can be used in subsequent assembly code.
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    2. CO@MarcelL I solved the double load/store problem. See my update.
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