The work memory of a programmable logic controller (PLC) is loaded with only one instruction at a time from the PLC's load memory which contains the entire instruction set to control a process. Each instruction has a format which contains at least one option code which must be evaluated to be true by the PLC interpreter before the corresponding instruction can be executed. If the interpreter evaluates the at least one option code to be false, the instruction is not executed and the next instruction is transferred from the load memory to the work memory and its corresponding option codes are then evaluated.