Abstract: A data processing system is used to evaluate a data processing function by executing a sequence of program instructions including an intermediate value generating instruction Inst0 and an intermediate value consuming instruction Inst1. In dependence upon one or more input operands to the evaluation, an embedded opcode within the intermediate value passed between the intermediate value generating instruction and the intermediate value consuming instruction may be set to have a value indicating that a substitute instruction should be used in place of the intermediate value consuming instruction. The instructions may be floating point instructions, such as a floating point power instruction evaluating the data processing function ab.