Abstract: In a computer system including a central processing unit, a program memory, a control program, a keyboard hardware interrupt handler routine and a single step interrupt handler routine, a method for replacing conventional data from a keyboard with user defined data including placing the central processing unit into single step mode, determining with the single step interrupt handler routine whether a keyboard hardware interrupt handler routine involves a keyboard data input instruction, and, if so, replacing the conventional data with user defined data, followed by either repeating the process or returning the system from single step mode to the main control program.