-- fa (a, b, ci, s, co);
b1_a0: fa PORT MAP (A(1) AND B(0), A(0) AND B(1), "0", P(1), C_b1(1));
b1_a1: fa PORT MAP (A(2) AND B(0), A(1) AND B(1), C_b1(1), PP1(2), C_b1(2));
b1_a2: fa PORT MAP (A(3) AND B(0), A(2) AND B(1), C_b1(2), PP1(3), C_b1(3));
b1_a3: fa PORT MAP ("0", A(3) AND B(1), C_b1(3), PP1(4), PP1(5));

-- fa (a, b, ci, s, co);
b2_a0: fa PORT MAP (PP1(2), A(0) AND B(2), "0", P(2), C_b2(1));
b2_a1: fa PORT MAP (PP1(3), A(1) AND B(2), C_b2(1), PP2(3), C_b2(2));
b2_a2: fa PORT MAP (PP1(4), A(2) AND B(2), C_b2(2), PP2(4), C_b2(3));
b2_a3: fa PORT MAP (PP1(5), A(3) AND B(2), C_b2(3), PP2(5), PP2(6));

-- fa (a, b, ci, s, co);
b3_a0: fa PORT MAP (PP2(3), A(0) AND B(3), "0", P(3), C_b3(1));
b3_a1: fa PORT MAP (PP2(4), A(1) AND B(3), C_b3(1), P(4), C_b3(2));
b3_a2: fa PORT MAP (PP2(5), A(2) AND B(3), C_b3(2), P(5), C_b3(3));
b3_a3: fa PORT MAP (PP2(6), A(3) AND B(3), C_b3(3), P(6), P(7));