Size of 0..255 is not 8 bits?

[Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>]

Gnat-3.10p on Linux reads 2 bytes from a stream for an object of
type range 0..255 that was forced with a 'Size representation clause
to be 8 bits long.

Example:

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

procedure Strange2 is

   type Value is range 0..255;
   for Value'Size use 8;

   V: Value;

begin
   for X in 1..5 loop
      Value'Read(Stream(Standard_Input), V);
      Put(Integer(V));
      New_Line;
   end loop;
end Strange2;
-----------------------------------------------------------------

$ echo ABCDEFGHIJKLMNOP | ./strange2 
         65
         67
         69
         71
         73


Did I missunderstand something and is S'Size not usable for enforcing
the number of bits allocated for a type?
Is 0..255 a type for signed 16-bit arithmetic and not a type for
(as I had naturally assumed) 8-bit unsigned arithmetic?
Do I have to use Unsigned_8 instead if I want to have a guarantee
to get an 8-bit word (which doesn't provide arithmetic overflow
checks)?

Markus

-- 
Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK
email: mkuhn at acm.org,  home page: <http://www.cl.cam.ac.uk/~mgk25/>