Abstract: A system for stopping the operation of a data processing system executing control words stored in a read/write control word storage device. A special control word is stored at the control storage address where it is desired to stop operation of the processor and the regular control word at that address is stored in a service processor memory. When the control storage address containing the special control word is reached, the special control word causes the system clock to be inhibited, thereby stopping the execution of control words, an indication that the processor has been stopped is generated, and the regular control word is restored to the control word storage device from the service processor memory for resumption of system operation.