A list of program instructions are mapped into memory addresses to form an executable program by simulating their execution in turn so as to determine a memory address of a next program instruction to be executed. That memory address is then examined to determine if a program instruction has already been mapped thereto. If the memory address is empty, then the next program instruction from the list is mapped to that empty memory address and the execution of that next program instruction is simulated and the process repeated. If the memory address is not empty, then the program instruction read from that memory address is simulated again and the process repeated.