71. Arithmetic Logic Unit

How do you plan to solve it?

/*Write your code here*/
module alu4(
    input [3:0]a,b,
    input [2:0]op,
    output reg [4:0]y,
    output reg cf
);
    localparam [2:0]
        ADD = 3'b000,
        SUB  = 3'b001,
        AND  = 3'b010,
        OR  = 3'b011,
        XOR  = 3'b100;
    
    wire [4:0] add5 = {1'b0, a} + {1'b0, b};         // ADD path
    wire [4:0] sub5 = {1'b0, a} + {1'b0, ~b} + 5'b1; // SUB path


    always @* begin
        y  = 4'b0000;
        cf = 1'b0;
        case (op)
            ADD: begin 
                y=add5[3:0];
                cf=add5[4];
            end                                        // pass-through
            SUB : begin
                y=sub5[3:0];
                cf = ~sub5[4];
            end
                                        // logical left
            AND :begin
                y=a&b;
                cf=1'b0;
            end                              // logical right
            OR :begin
                y=a|b;
                cf=1'b0;
            end
                                    // arithmetic right
            XOR : begin
                y=a^b;
                cf=1'b0;
            end    // rotate left
               // rotate right
            default:begin
                y=4'b0000;
                cf=1'b0;
            end                                        // reserved → PASS
        endcase
    end
endmodule