1. An apparatus for operating on the contents of an X word having bits X.sub.i and a Y word having bits Y.sub.i to generate a result word having bits Z.sub.i, where i=0 to N-1, where Z.sub.0 is the least significant bit and Z.sub.N-1 is the most significant bit, said apparatus comprising: means for partitioning said X, Y and result words into a plurality of sub-words, there being one sub-word of said Y and result words corresponding to each sub-word of said X word; and means, responsive to a first instruction, for generating the sum of each X sub-word divided by 2.sup.m and the corresponding Y sub-word, the result thereof determining said corresponding sub-word of said result word, wherein m is an integer different from 0.