The most straightforward implementation of a parallel adder for two operands ?n_d, ?n_1 • • • , ?( and rn_d, rn_1 • • • , r( is through the use of z basic units called full adders. A full adder (FA) is a logical circuit that accepts two operand bits, say ?z and rz, and an incoming carry bit, denoted by •z , and then produces the corresponding sum bit, denoted by X
z , and an outgoing carry bit, denoted
by •zLd. As this notation suggests, the outgoing carry •zLd is also the incoming carry for the subsequent FA, which has ?