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    <p>Your code has two problems:</p> <p><strong>One</strong>: the indirect memory reference should be:</p> <pre><code>mov %0, ds : [ebx] </code></pre> <p>That is, with the <code>ds</code> out of the brackets.</p> <p><strong>Two</strong>: You are using memory constraint (<code>=m</code>) to refer to the <code>mem</code> variable. That is not possible because it is a local variable, and as such does not have a fixed address. Moreover, a single instruction cannot have both origin and destination in memory, you have to use a register. The easiest way would be to indicate <code>=g</code> that basically means <em>whatever</em>, but in your case it is not possible because <code>esp</code> cannot be moved directly to memory. You have to use <code>=r</code>.</p> <p><strong>Three</strong>: (?) You are clobbering the <code>ebx</code> register, so you should declare it as such, or else do not use it that way. That will not prevent compilation, but will make your code to behave erratically.</p> <p>In short:</p> <pre><code> unsigned int mem = 0; __asm volatile ( "mov ebx, esp\n\t" "mov %0, ds : [ebx]\n\t" : "=r"(mem) :: "ebx" ); </code></pre> <p>Or better not to force to use <code>ebx</code>, let instead the compiler decide:</p> <pre><code> unsigned int mem = 0, temp; __asm volatile ( "mov %1, esp\n\t" "mov %0, ds : [%1]\n\t" : "=r"(mem) : "r"(temp) ); </code></pre> <p>BTW, you don't need the <code>volatile</code> keyword in this code. That is used to avoid the assembler to be optimized away even if the output is not needed. If you write the code for the side-effect, add <code>volatile</code>, but if you write the code to get an output, do not add <code>volatile</code>. That way, if the optimizing compiler determines that the output is not needed, it will remove the whole block.</p>
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    1. COquite right, thanks.
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      1. USGary Yin
    2. COThe `volatile` keyword also prevents the compiler from moving or reordering the assembly code with respect to other code (e.g. hoisting it out of a loop).
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      1. USAdam Rosenfield
 

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