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    <p>The reason you are getting the bus error is because you are calling <code>ret</code> within your assembly code. <code>ret</code> causes the program control to transfer to the return address at the top of the stack, which you manipulate through use of <code>push</code> and <code>pop</code>. I'd highly suggest looking up what <code>ret</code> does in the Intel Instruction Set Reference.</p> <p>Below is code that I compiled, and runs successfully, on an iMac running Mac OS X 10.6.7.</p> <pre><code>#include &lt;stdio.h&gt; /*__declspec(naked)*/ void doStuff(unsigned long int val, unsigned long int flags, unsigned char *result) { __asm { push eax push ebx push ecx mov eax, dword ptr[ebp + 8] //val mov ebx, dword ptr[ebp + 12] //flags mov ecx, dword ptr[ebp + 16] //result and eax, ebx mov [ecx], eax pop ecx pop ebx pop eax } } int main(int argc, char *argv[]) { unsigned long val = 0xAA00A1F2; unsigned long flags = 0x00100002; unsigned char result = 0x0; doStuff(val, flags, &amp;result); printf("Result is: %2Xh\n", result); return 0; } </code></pre> <p>Notable changes are:</p> <ol> <li>Removal of <code>ret</code> in the inline assembly</li> <li>Use of register <code>ebp</code> instead of <code>esp</code> to reference the parameters to <code>doStuff</code></li> <li>Changing <code>flags</code> to be <code>0x00100002</code></li> </ol> <p>Change (1) fixes the bus error, (2) made referencing the parameters a little more consistent, and (3) is just a quick way to make sure the function is working as expected.</p> <p>Lastly, I highly recommend you become familiar with the GNU Debugger, GDB, if you haven't already. You can find more info about it at the project page <a href="http://www.gnu.org/software/gdb/" rel="nofollow">http://www.gnu.org/software/gdb/</a> as well as information about the Mac implementation and tutorial at <a href="http://developer.apple.com/library/mac/#documentation/DeveloperTools/gdb/gdb/gdb_toc.html" rel="nofollow">http://developer.apple.com/library/mac/#documentation/DeveloperTools/gdb/gdb/gdb_toc.html</a> .</p> <p><strong>EDIT:</strong> Added basic info/link to GDB, </p>
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    1. COThe use of `ret` works in Visual Studio and I thought it was required since a function call is supposed to push the return address on the stack and ret pops it off so it knows where to go after the function call. Also I have never seen ebp. All I found is ebp is different than esp because "the base pointer is manipulated only explicitly." Lastly, I have no idea what gdb is. Can you provide a link? Thanks.
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    2. COI just updated the answer with some links to GDB. I can't confirm this (I don't have Visual Studio on this machine), but my suspicion is that the reason using `ret` in VS worked was a result of how the compiler inserted your assembly into `doStuff()`. Try compiling your code with `gcc -S ...` to see what assembly the compiler generates. As you found out, `ebp` is the base pointer and it points to the bottom of the stack whereas `esp` points to the top. It doesn't really matter which one you use as long as you get the offsets right.
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