1. A data processing system wherein system operations are controlled by microinstructions in response to designated ones of system instructions each of which includes at least an operation code for designating a system operation to be executed and operand address data for specifying at least one operand which participates in the system operation to be executed, comprising: control storage means for storing the microinstructions for use in microinstruction sequences for executing each of the system instructions, said microinstructions including different initial microinstructions for use in different microinstruction sequences for system instructions having the same operation codes but containing different operand address data; control register means, connected to said control storage means, for receiving microinstructions from said control storage means; subsidiary address data generator means for generating different subsidiary address data in response to each of the combinations of said operation codes and said operand address data for the designated system instructions, said subsidiary address data enabling access to one of the different initial microinstructions for use in the different microinstruction sequences for each of the designated system instructions in combination with the operation code of each of the designated system instructions; indicating means for indicating that the initial microinstructions of the microinstruction sequences for the designated system instruction are to be accessed; and accessing means, connected to said subsidiary address data generator means and responsive to said indication by said indicating means, for accessing initial microinstructions from said control storage means at storage locations specified by each of the combinations of said operation codes and said subsidiary address data for respective designated system instructions, thereby accessing one of the different initial microinstructions for use in different microinstruction sequences for each of the designated system instructions.