Numerical exceptions not raised properly?

[arnold@nospam.com (Stephen Arnold)]

Howdy folks:

I'm pretty new to Ada, as well as GNAT, so this could be due to my own 
ignorance (rather than a GNAT problem), but it seems that certain 
exceptions are not being raised properly.

Setup:  Win95b with latest GNAT 3.12p package.

When I compile and run some simple code (I'm doing the Lovelace 
tutorial):

with Ada.Text_IO, Ada.Integer_Text_IO;
use Ada.Text_IO, Ada.Integer_Text_IO;

procedure Compute is

 procedure Double(Item : in out Integer) is
 begin -- procedure Double.
   Item := Item + 2;
 end Double;

 X : Integer := 1;   -- Local variable X of type Integer.

begin -- procedure Compute
 loop
  Put_Line("X = ");
  Put(X);
  New_Line;
  Double(X);
 end loop;
end Compute;

It doesn't seem to raise an exception when X gets too big (ie, you can 
see it wrap negative, then it goes to zero and stays there).  When I try 
dividing by zero, the exception is raised correctly.

Is there something I'm missing, or?

Thanks in advance, Steve Arnold