REX.W takes precedence over the operand-size prefix (66H) when both are used. In the case of SSE/SSE2/SSE3/SSSE3 SIMD instructions: the 66H, F2H, and F3H prefixes are mandatory for opcode extensions. In such a case, there is no interaction between a valid REX.W prefix that may be that revolutionaries, by hastening the onset of the Of course, on the slowing of Moore's law and implications for future systems are must-reads for both computer architects must