100 S$ = "1100 + 1100 = " : GOSUB 400
110 S$ = "1100 + 1101 = " : GOSUB 400
120 S$ = "1100 + 1110 = " : GOSUB 400
130 S$ = "1100 + 1111 = " : GOSUB 400
140 S$ = "1101 + 0000 = " : GOSUB 400
150 S$ = "1101 + 0001 = " : GOSUB 400
160 S$ = "1101 + 0010 = " : GOSUB 400
170 S$ = "1101 + 0011 = " : GOSUB 400
180 END

400 A0 = VAL(MID$(S$, 4, 1))
410 A1 = VAL(MID$(S$, 3, 1))
420 A2 = VAL(MID$(S$, 2, 1))
430 A3 = VAL(MID$(S$, 1, 1))
440 B0 = VAL(MID$(S$, 11, 1))
450 B1 = VAL(MID$(S$, 10, 1))
460 B2 = VAL(MID$(S$, 9, 1))
470 B3 = VAL(MID$(S$, 8, 1))
480 GOSUB 600
490 PRINT S$;

REM 4 BIT PRINT
500 PRINT C;S3;S2;S1;S0
510 RETURN

REM 4 BIT ADD
REM  ADD A3 A2 A1 A0 TO B3 B2 B1 B0
REM  RESULT IN S3 S2 S1 S0
REM  CARRY IN C
600 C = 0
610 A = A0 : B = B0 : GOSUB 700 : S0 = S
620 A = A1 : B = B1 : GOSUB 700 : S1 = S
630 A = A2 : B = B2 : GOSUB 700 : S2 = S
640 A = A3 : B = B3 : GOSUB 700 : S3 = S
650 RETURN

REM FULL ADDER
REM  ADD A + B + C
REM  RESULT IN S
REM  CARRY IN C
700 BH = B : B = C : GOSUB 800 : C1 = C
710 A = S : B = BH : GOSUB 800 : C2 = C
720 C = C1 OR C2
730 RETURN

REM HALF ADDER
REM  ADD A + B
REM  RESULT IN S
REM  CARRY IN C
800 GOSUB 900 : S = C
810 C = A AND B
820 RETURN

REM XOR GATE
REM  A XOR B
REM  RESULT IN C
900 C = A AND NOT B
910 D = B AND NOT A
920 C = C OR D
930 RETURN