71. Arithmetic Logic Unit

How do you plan to solve it?

/*Write your code here*/
module alu4(
    input [3:0]a,
    input [3:0]b,
    input [2:0]op,
    output reg [3:0]y,
    output reg cf
);
localparam [2:0]
 OP_ADD = 3'b000,
        OP_SUB = 3'b001,
        OP_AND = 3'b010,
        OP_OR  = 3'b011,
        OP_XOR = 3'b100;

wire[4:0]add5= {1'b0,a}+{1'b0,b};
wire[4:0]sub5= {1'b0,a}+{1'b0,~b}+5'b1;

always@(*) begin
    y=4'b0000;
    cf=1'b0;
    case(op)
    OP_ADD:begin
        y=add5[3:0];
        cf=add5[4];
    end

    OP_SUB:begin
        y=sub5[3:0];
        cf=~sub5[4];
    end

    OP_AND: y=a&b;
    OP_OR: y=a|b;
    OP_XOR:y=a^b;
    default:begin
        y=4'b0000;
        cf=1'b0;
    end
    endcase
end
endmodule