An ISA-defined instruction includes an immediate field having a first and second portions specifying first and second values, which instructs the microprocessor to perform an operation using a constant value as one of its source operands. The constant value is the first value rotated/shifted by a number of bits based on the second value. An instruction translator translates the instruction into one or more microinstructions. An execution pipeline executes the microinstructions generated by the instruction translator. The instruction translator, rather than the execution pipeline, generates the constant value for the execution pipeline as a source operand of at least one of the microinstructions for execution by the execution pipeline. Alternatively, if the immediate field value is not within a predetermined subset of values known by the instruction translator, the instruction translator generates, rather than the constant, a second microinstruction for execution by the execution pipeline to generate the constant.