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POHow to use address constants in GCC x86 inline assembly
 

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  1. POHow to use address constants in GCC x86 inline assembly
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    <p>The GCC toolchain uses AT&amp;T assembler syntax by default, but support for Intel syntax is available via the <code>.intel_syntax</code> directive.</p> <p>Additionally, both AT&amp;T and Intel syntax are available in a <code>prefix</code> and a <code>noprefix</code> version, which differ in whether or not they require to prefix register names with a <code>%</code> sigil.</p> <p>Depending on which directives are present, the format for address constants changes.</p> <p>Let's consider the following C code</p> <pre><code>*(int *)0xdeadbeef = 0x1234; </code></pre> <p>Using <code>objdump -d</code>, we find that it's compiled to the following assembler instruction</p> <pre><code>movl $0x1234,0xdeadbeef </code></pre> <p>As there are no registers involved, this is the correct syntax for both <code>.att_syntax prefix</code> and <code>.att_syntax noprefix</code>, ie. embedded in C code, they look like this</p> <pre><code>__asm__(".att_syntax prefix"); __asm__("movl $0x1234,0xdeadbeef"); __asm__(".att_syntax noprefix"); __asm__("movl $0x1234,0xdeadbeef"); </code></pre> <p>You can optionally surround the address constant with parentheses, ie.</p> <pre><code>__asm__("movl $0x1234,(0xdeadbeef)"); </code></pre> <p>will work as well.</p> <p>When adding a sigil to a plain address constant, the code will fail to copile</p> <pre><code>__asm__("movl $0x1234,$0xdeadbeef"); // won't compile </code></pre> <p>When surrounding this expression with paranthesis, the compiler will emit wrong code without warning, ie</p> <pre><code>__asm__("movl $0x1234,($0xdeadbeef)"); // doesn't warn, but doesn't work! </code></pre> <p>This will incorrectly emit the instruction</p> <pre><code>movl $0x1234,0x0 </code></pre> <p>In Intel mode, an address constant has to be prefixed with a segment register as well as the operand size and the <code>PTR</code> flag if ambiguity is possible. On my machine (an Intel dual core laptop with Windows XP and current MinGW and Cygwin GCC versions), the register <code>ds</code> is used by default.</p> <p>Square brackets around the constant are optional. The address constant is also correctly recognized if the segment register is omitted, but the brackets are present. Omitting the register emits a warning on my system, though.</p> <p>In <code>prefix</code> mode, the segment register has to be prefixed with <code>%</code>, but only using brackets will still work. These are the different ways to generate the correct instruction:</p> <pre><code>__asm__(".intel_syntax noprefix"); __asm__("mov DWORD PTR ds:0xdeadbeef,0x1234"); __asm__("mov DWORD PTR ds:[0xdeadbeef],0x1234"); __asm__("mov DWORD PTR [0xdeadbeef],0x1234"); // works, but warns! __asm__(".intel_syntax prefix"); __asm__("mov DWORD PTR %ds:0xdeadbeef,0x1234"); __asm__("mov DWORD PTR %ds:[0xdeadbeef],0x1234"); __asm__("mov DWORD PTR [0xdeadbeef],0x1234"); // works, but warns! </code></pre> <p>Omitting both segment register and brackets will fail to compile</p> <pre><code>__asm__("mov DWORD PTR 0xdeadbeef,0x1234"); // won't compile </code></pre> <p>I'll mark this question as <em>community wiki</em>, so if you have anything useful to add, feel free to do so.</p>
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    1. USChris Peterson
    1. USChristoph
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    1. COIt's obviously not *just* AS in the toolchain here; which version(s) of gcc are involved? Have you checked the output of gcc -S to validate that what's being fed into GNU as is what you expect?
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      1. POHow to use address constants in GCC x86 inline assembly
      1. USCharles Duffy
    2. COok, it was me who messed up - at least partially; replacing `($0xdeadbeaf)` with `0xdeadbeaf` seems to work; I'll edit the question after testing this...
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      1. USChristoph
    3. CO@Charles: yes, `gcc -S` yields the expected results
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