BINARY MOS RIPPLE CARRY PARALLEL ADDER/SUBTRACTOR AND APPROPRIATE ADDING/SUBTRACTING STAGE

摘要

The adder/subtracter disclosed sums a plurality of n-digit binary-coded numbers (A, B, C . . . Z) successively by forming corresponding partial sums (Sb, Sc . . . Sz) according to the following recursive formula: A+B+C . . . +Z=((A+B)+C) . . . +Z=(Sb+C) . . . +Z=Sc . . . +Z=Sz. The partial sums are formed by means of parallel adders/subtracters which, in turn, include adder/subtracter stages. Each of the stages is formed by a full adder and a switching section which forms the ones complement of the subtrahend in case of subtraction. The inputs of the parallel adder/subtracter for the first partial sum are preceded by series-connected like delay elements beginning with the second lowest weight and increasing by one from weight to weight, the delay provided by the delay elements being equal to the time required to generate the carry of the full adder. Beginning with the next to the last stage of the parallel adder/subtracter, additional like delay elements are connected in series between the output of the stages and the sum output terminal, which also increase by one from stage to stage. Additional delay elements and transfer stages may be placed between the switching section and the full adder, so that it is possible to multiply one of the addends by a power of two and then to form the sum. A circuit for the switching section is provided which is considerably simpler than the EXCLUSIVE-OR gate commonly used there.