input [3:0] A;
input [3:0] B;
input [1:0] S;

output [3:0] Result;
output reg Overflow;
output reg [6:0] Display;

reg [3:0] Result_reg;

wire [3:0] A;
wire [3:0] B;
wire [1:0] S;

assign Result = (Overflow) ? 4'bx : Result_reg;

always @(A,B) begin
    if (S == 0'b0)
        {Overflow, Result_reg} = A - B;
    else if (S == 1'b1)
        {Overflow, Result_reg} = A + B;
    end

always @(Overflow,Result_reg) begin
    case ({Overflow, Result_reg}) inside
        [5'b1000:5'b11111] : Display = 7'b0011101; 
        5'b00000: Display = 7'b1111110;//0
        5'b00001: Display = 7'b0110000;//1
        5'b00010: Display = 7'b1101101;//2
        5'b00011: Display = 7'b1111001;//3
        5'b00100: Display = 7'b0110011;//4
        5'b00101: Display = 7'b1011011;//5
        5'b00110: Display = 7'b1011111;//6
        5'b00111: Display = 7'b1110000;//7
        5'b01000: Display = 7'b1111111;//8
        5'b01001: Display = 7'b1111011;//9
        5'b01010: Display = 7'b1110111;//A
        5'b01011: Display = 7'b0011111;//B
        5'b01100: Display = 7'b1001110;//C
        5'b01101: Display = 7'b0111101;//D
        5'b01110: Display = 7'b1001111;//E
        5'b01111: Display = 7'b1000111;//F
        default: Display = 7'bx;
    endcase
end