71. Arithmetic Logic Unit

How do you plan to solve it?

module alu4 (
    input [3:0] a,
    input [3:0] b,
    input [2:0] op,
    output reg [3:0] y,
    output reg       cf
);
    reg borrow;
    always @(a or b or op)
        case (op)
            3'b000: {cf, y}  = {1'b0, a} + {1'b0, b};
            3'b001: begin
                        {borrow, y} = {1'b0, a} + {1'b0, ~b} + 5'b1;
                        cf = ~borrow;
                    end
            3'b010: begin
                        y  = a & b;
                        cf = 0;
                    end
            3'b011: begin   
                        y  = a | b;
                        cf = 0;
                    end
            3'b100: begin   
                        y  = a ^ b;
                        cf = 0;
                    end
            default: begin  
                        y  = 4'b0000;
                        cf = 1'b0;
                    end 
        endcase
endmodule