pinning is similar to a 16-bit operand size. strtonum -- reliably convert string value to an integer. However, setting 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 changing