Original Publication Date: 1987-Nov-01
Included in the Prior Art Database: 2005-Feb-02
One way of forming the negative of a "twos complement" number is by inverting all bits of the number more significant than the least significant bit of value '1'. The bits are selected for inversion by means of an inversion mask which has the property of having '1's at all bit positions more significant than the least significant '1' and '0's elsewhere. When the inversion mask is combined with the number using the Boolean Exclusive- OR function, the '1's in the inversion mask cause the related bits of the number to be inverted. The '0's in the inversion mask cause the related bits of the number to remain unchanged. The logic diagram for the Selective Negator appears in Fig. 1. The diagram shows a basic n-bit implementation. Both the input W and the output N are "twos complement" numbers, which can be positive or negative.