4-bit-pc.pl

and(A, A, A).
and(A, B, 0) :- A \= B.

or(1, 0, 1).
or(0, 1, 1).
or(1, 1, 1).
or(0, 0, 0).

xor(1, 0, 1).
xor(0, 1, 1).
xor(0, 0, 0).
xor(1, 1, 0).

not(1, 0).
not(0, 1).

half-adder(A, B, [S | Cout]) :- xor(A, B, S), and(A, B, Cout).

full-adder(A, B, Cin, [S | Cout]) :-
    half-adder(A, B, [Xs | Xc]),
    half-adder(Xs, Cin, [S | Yc]),
    xor(Xc, Yc, Cout).

add([X8, X4, X2, X1], [Y8, Y4, Y2, Y1], [S16, S8, S4, S2, S1]) :-
    full-adder(X1, Y1, 0, [S1 | C1]),
    full-adder(X2, Y2, C1, [S2 | C2]),
    full-adder(X4, Y4, C2, [S4 | C3]),
    full-adder(X8, Y8, C3, [S8 | S16]).

half-substractor(A, B, [D | Bout]) :- xor(A, B, D), not(A, NotA), and(NotA, B, Bout).

full-substractor(A, B, Bin, [D | Bout]) :-
    half-substractor(A, B, [Xd | Xb]),
    half-substractor(Xd, Bin, [D | Yb]),
    or(Xb, Yb, Bout).

substract([X8, X4, X2, X1], [Y8, Y4, Y2, Y1], [S8, S4, S2, S1]) :-
    full-substractor(X1, Y1, 0, [S1 | B1]),
    full-substractor(X2, Y2, B1, [S2 | B2]),
    full-substractor(X4, Y4, B2, [S4 | B3]),
    full-substractor(X8, Y8, B3, [S8 | _]).