A data processing apparatus, method and computer program are described that are capable of decoding instructions from different instruction sets. The method comprising: receiving an instruction; if an operation code of said instruction is an operation code of an instruction from a base set of instructions decoding said instruction according to decode rules for said base set of instructions; and if said operation code of said instruction is an operation code of an instruction from at least one further set of instructions decoding said instruction according to a set of decode rules determined by an indicator value indicating which of said at least one further set of instructions is currently to be decoded.