1. A microprocessor, comprising an execution unit; and a multiply unit, coupled to the execution unit, that includes a low-order register, a high-order register, and an extended register, wherein execution of a first instruction by the execution unit causes data stored in the extended register to be moved from the extended register to the high-order register, and wherein execution of a second instruction by the execution unit causes data stored in the high-order register to be moved from the high-order register to the extended register.