Full adder subtractor

This circuit is a two-input Exclusive NOR (XNOR) gate built from Inverters, OR, and AND gates. The first stage of the circuit is supposedly logic splitting to produce complementary copies of the input signals. No buffers were utilized, however, and direct connections were used for uncomplemented copies of the input signals. The inverters produce the complements of the input signals A and B. The upper OR gate performs the OR logic function of the input signal A and the complement of the input signal B. The lower OR gate performs the OR logic function of the input signal B and the complement of the input signal A. The output of the two OR gates are ANDed together to produce the final output. The circuit is derived from Y = (A + B') · (A' + B) the input-output relation in product-of-sums form. Note: The 1 MΩ load resistor is not part of the XNOR circuit, its purpose is to provide a wiring point to the output of the AND gate so that the state of the output can be probed.