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(nasm.info)Section B.4.320

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B.4.320. `UD0', `UD1', `UD2': Undefined Instruction
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     UD0                           ; 0F FF                [186,UNDOC]
     UD1                           ; 0F B9                [186,UNDOC]
     UD2                           ; 0F 0B                [186]

   `UDx' can be used to generate an invalid opcode exception, for
testing purposes.

   `UD0' is specifically documented by AMD as being reserved for this
purpose.

   `UD1' is documented by Intel as being available for this purpose.

   `UD2' is specifically documented by Intel as being reserved for this
purpose. Intel document this as the preferred method of generating an
invalid opcode exception.

   All these opcodes can be used to generate invalid opcode exceptions
on all currently available processors.