1. An apparatus for redefining an operation performed in response to an instruction, comprising: a register configured to store a field for a particular instruction; and a decode unit coupled to said register to receive said field, wherein said decode unit is configured to produce a first decoded instruction in response to said particular instruction if said field is encoded in a first state, and wherein said decode unit is configured to produce a second decoded instruction in response to said particular instruction if said field is encoded in a second state.