Abstract: An apparatus for combining the contents of an X register, shifted by m places, with the contents of a Y register to generate a result Z. The functional unit can also be configured to perform parallel operations on sub-operands in the X and Y registers. The division of the apparatus into sub-operands is controlled by a mask which specifies the boundary of the sub-operands. The shifting operation is accomplished by multiplexers that connect the p.sup.th bit of the X register to the adder stage that operates on bit Y.sub.p-m of the Y register. Circuitry is provided at the boundary of the sub-operands to prevent the bit signals corresponding to the X register from being routed across a sub-operand boundary. Similarly, circuitry is provided for preventing the carry output of an adder stage that operates on one sub-operand from being propagated to an adder stage that operates on another sub-operand.