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Opcode prefix can be duplicated. For example, let's take 8B opcode (mov) with operand size override prefix:

66 66 66 66 66 8B 00

visual studio disassembler's output (x64 mode):

mov ax, word ptr [rax]

Now, let's take this one:

66 66 66 66 67 8B 00

visual studio disassembler's output:

mov ax, word ptr [eax]

And this one:

66 66 66 67 /*it throws away left side*/ 67 8B 00

visual studio disassembler's output:

?? ?? ?? ?? mov eax, dword ptr [eax]

So, I want to ask why prefix 67 can't be duplicated. Is it visual studio specific processing that does not match hardware semantics or cpu itself does not allow this?

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  • Duplicating prefixes is undefined behavior. That said, I don't know why the VS disassemble does what it does. ndisasm and objdump, for example, decode that as expected. Commented Jun 5, 2015 at 10:26
  • "You could only form an infinitely long instruction by using redundant prefixes in front on the opcodes". "For example, you can take the innocuous looking instruction: 89 E5 mov %sp,%bp and turn it into a really long instruction: 66 66 66 66 … 66 66 89 E5 mov %sp,%bp" Commented Jun 5, 2015 at 11:09
  • @LưuVĩnhPhúc You can't have infinite prefixes. While I don't believe duplicate "classic" prefixes results in undefined behaviour, gas uses them for long NOPs, a total instruction length greater than 15 does. Commented Jun 5, 2015 at 15:46

1 Answer 1

Reset to default
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It seems VS simply does not support this. With ndisasm (the disassembler of nasm), it just works well, it is diassembled as:

6767668B00 mov ax,[eax]

The problem with this is however, in a normal 64 bit environment, addressing using only the lower 32 bit is normally not possible.

But I tested with:

 67674889D8 mov rax,rbx

and the CPU (i7-4770) executed it without an exception.

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