Abstract: A system for generating an early indication of a fault in the operation of a computer program is described. In one embodiment, each location in a computer's memory space is assigned a code indicating the conditions under which the programmer expects the memory location to be accessed during the program's execution. When a memory location is accessed, the control/status signals generated by the CPU are compared to the assigned code. If the type of access, as indicated by the control/status signal, does not conform to the expected type of access, as indicated by the assigned code, an output is generated to indicate that a fault has occurred in the operation of the program. In another embodiment, a truncated numerical difference between the present memory address and the immediately previous memory address is generated. This difference is used along with the assigned code and the control/status signal to determine whether access to the present memory address is unexpected.