Abstract: A microinstruction control storage mechanism includes a read-only store (ROS), writeable control store (WCS), first cycle control store, and a reserved portion of main storage in a data processing system. The ROS stores frequently used sequences of microinstructions and is not altered during operation. Other sequences of microinstructions which are not frequently used are stored in the reserved portion of main storage. As required, blocks of microinstructions are paged into the WCS from the main storage. One cycle of execution is saved for each machine instruction by utilizing the operation code portion directly from the instruction register of the data processing system to access a microinstruction from the first cycle control store. An array of single-bit storage devices, accessed by microinstruction addresses also utilized to access microinstructions from the ROS, signal the existence of a faulty microinstruction from the ROS as determined by maintenance or design personnel.