Subtract the lower double-precision (64-bit) floating-point element in b from the lower double-precision (64-bit)
floating-point element in a, store that in the lower element of result, and copy the upper element from a to the
upper element of result.
Subtract the lower double-precision (64-bit) floating-point element in b from the lower double-precision (64-bit) floating-point element in a, store that in the lower element of result, and copy the upper element from a to the upper element of result.