1. Apparatus for processing data under control of program instructions from a first set of program instructions or program instructions from a second set of program instructions, said apparatus comprising: a software instruction interpreter operable to interpret a program instruction of said second set of program instructions as a sequence of program instructions of said first set of program instructions terminating with a sequence terminating instruction; and an instruction decoder responsive to said sequence terminating instruction: (i) if a hardware instruction execution unit for said second set of instructions is not available, then to initiate interpretation of a next program instruction of said second set of instructions using said software instruction interpreter; and (ii) if said hardware instruction execution unit for said second set of instructions is available, then to initiate execution of said next program instruction of said second set of instructions using said hardware instruction execution unit.