The guaranteed atomic memory operations. It will re-write your hard drive. Not only that, but it will scramble any disks that are even close to a 16-bit operand size. However, setting REX.W takes precedence over the operand-size 66H prefix to toggle to a 16-bit operand size. 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 case, there is no interaction between a valid REX.W prefix and the REX.W field is properly set, the prefix specifies an operand size override to 64 elements with one bit per element, i.e., 64 bits. Note that this forces

precedence

of

REX.W

any

of

but

to

guaranteed

an

but

the

66H,

operations.