module bcd(IN,OUT);
       input [3:0] IN;
       output [6:0] OUT;
       
       assign OUT=IN[3]? (IN[0]? 7'b0010_000:7'b0000_000):
                          (IN[2]? (IN[1]? (IN[0]? 7'b1111_000:7'b0000_010):
                          (IN[0]? 7'b0010_010:7'b0011_001)):
                          (IN[1]? (IN[0]? 7'b0110_000:7'b0100_100):
                          (IN[0]? 7'b1111_001:7'b1000_000)));

endmodule

module mux_2to1 (IN1, IN2, S, OUT);
    input[3:0]IN1, IN2;
    input S;
    output [3:0]OUT;

    assign OUT=S? IN2:IN1;

endmodule

module BCD_ex(SW, D0, D1);
    input [3:0] SW;
    output [6:0] D0, D1;

    wire z;
    wire [4:0] A, V;
    wire [4:0] W;

    assign V=SW[3:0];
    assign z = V[4:0] > 9? 1:0;
    assign D1 =z? 7'b1111_001:7'b1000_000;

    assign A=V[3:0]-4'b1010;
    mux_2to1 M0 (V, A, z, W);
    bcd B0 (W, D0);
endmodule

module adder(SUM, COUT, A, B, CIN);
		input A, B, CIN;
		output SUM, COUT;
		
		wire x;
		
		xor x0(x, A, B);
		xor x1(SUM, x, CIN);
		
		assign COUT = x? CIN: B;
		
endmodule

module RippleCarryAdder(SUM, COUT, IN1, IN2, CIN);
		input [4:0] IN1, IN2;
		input CIN;
		output COUT;
		output [3:0] SUM;
		
		wire C0, C1, C2, C3;
		
		adder A0(SUM[0], C0, IN1[0], IN2[0], CIN);
		adder A1(SUM[1], C1, IN1[1], IN2[1], C0);
		adder A2(SUM[2], C2, IN1[2], IN2[2], C1);
		adder A3(SUM[3], C3, IN1[3], IN2[3], C2);
		
		assign COUT = C3;
		
endmodule

// if the inputs excede decimal value 9, result is unknown
module part4(SW, HEX0, HEX1);
		input [8:0] SW;
		output [6:0] HEX0, HEX1;
		
		wire COUT;
		wire [3:0] U;
		
		// Add the decimal number's 0-9
		RippleCarryAdder R0(U, COUT, SW[3:0], SW[7:4], SW[8]);
		// send the output along with carry to 7-segment display
		BCD_ex({COUT, U}, HEX0, HEX1);
endmodule