THREE INPUT ARITHMETIC LOGIC UNIT FORMING THE SUM OF A FIRST INPUT ANDED WITH A FIRST BOOLEAN COMBINATION OF A SECOND INPUT AND A THIRD INPUT PLUS A SECOND BOOLEAN COMBINATION OF THE SECOND AND THIRD INPUTS

A three input arithmetic logic unit (230) forms a mixed arithmetic and Boolean combination of three multibit input signals. The arithmetic logic unit (230) first forms a Boolean combination and then forms an arithmetic combination. The current instruction drives an instruction decoder (250, 245) that generates the functions signals F0-F7 which control the combination formed. The three input arithmetic logic unit (230) preferably employs a set of bit circuits (400), each forming carry propagate, generate and kill signals. These signals may employed with a multilevel logic tree circuit and a carry input to produce a bit resultant and a carry output to the next bit circuit. This structure permits formation of selected arithmetic, Boolean or mixed arithmetic and Boolean function of the three input signals based upon the current instruction. Selection of the function signals enables the combination to be insensitive to one of the input signals, thus performs a two input function of remaining input signals. The instruction itself may include the function signals and function modification bits, or the function signals and function modification signals may be stored in a special data register. Function modification signals cause modification of the function signals prior to use. The three input arithmetic logic unit (230) includes a least significant bit carry-in generator (246) supplying a carry input to the least significant bit. This carry input is determined by the combination being formed, and generally is "1" only during subtraction. The carry input may be specified in the special purpose data register (D0) for certain instructions. The combination formed is optionally modified dependent upon the sign bit of one of the inputs.