1. In a data processing system requiring the generation of microinstructions for performing data processing operations and including a a central processor unit having means responsive to macroinstructions for providing decoded macroinstructions for use in generating one or more microinstructions, said central processor unit further including first control store means for providing one or more microinstructions in response to identification signals, one or more selected fields of each of said microinstructions being capable of modification; second control store means for providing a plurality of microinstruction portions, one or more of said second plurality of microinstruction portions providing modifier fields containing information usable for modifying said one or more selected fields of said one or more microinstructions, said second control store means being responsive to a said decoded macroinstruction for producing said plurality of microinstruction portions; and modification means responsive to said one or more microinstructions provided by said first control store means and to the modifier fields of said one or more microinstruction portions provided by said second control store means for providing modified microinstructions for performing said data processing operation.