There is disclosed a computer and a method of operating a computer to allow combination of data values in the context of the execution of so-called "packed instructions". A data string comprising a certain number of sub-strings representing discrete data values and which are not independently addressable is held in a source register store. A combining instruction which operates to carry out a polyadic operation on at least some of the sub-strings to generate a result sub-string is then executed. A result data string comprising said result sub-string is loaded into a destination register. These "combining" instructions have the advantage that they are general purpose instructions which can be used in a plurality of different situations. The instructions are particularly useful in a packed arithmetic environment.