No overflow flag when not in addition nor subtraction mode

src/alu/alu.v

@@ -23,12 +23,16 @@ module alu(
 wire [31:0] adder_out;
 
 begin
+  wire addition = op == 3'b000;
   wire subtraction = op == 3'b001;
   wire [31:0] adder_B = subtraction ? ~B : B;
   wire adder_cout;
 
   carry_select_adder a(A, adder_B, subtraction, adder_out, adder_cout);
-  assign Fflow = subtraction ? ~adder_cout : adder_cout;
+  assign Fflow =
+    addition ? adder_cout :
+    subtraction ? ~adder_cout :
+    0;
 end
 
 wire [31:0] mult_out_hi;