
A microprocessor-based system (10) is tested by a testing device (32) that substitutes instructions from a substitute memory (38) for those in the main memory (28) of the system (10) under test. When the processor (12) of the system under test attempts to read a location in its memory (28), it places signals on bus address lines (22) to designate the memory location and places a data-direction signal on a control line (50) to indicate that the memory (28) is to transmit rather than receive infor...











