1. A method comprising: decoding an instruction of a first instruction format identifying a horizontal arithmetic operation, a first source having a first plurality of packed data elements including elements X 0 and X 1 and a second source having a second plurality of packed data elements including elements Y 0 and Y 1 ; executing the horizontal arithmetic operation on the first plurality of packed data elements and the second plurality of packed data elements to produce a first arithmetic result element from X 0 and X 1 and a second arithmetic result element from Y 0 and Y 1 ; and storing a third plurality of packed data elements including a first element to represent said first arithmetic result element, and a second element to represent said second arithmetic result element.