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 [4:0] z;
   always @(*)begin
   case (op)
    3'b000 :begin
         z= a + b;
         y=z[3:0];
         cf=z[4];
    end
    3'b001 :begin
         z= a + ~b + 1;
         y=z[3:0];
         cf=z[4];
    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=0;
    end
   endcase
   end 
endmodule