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From: Peter Vogelsanger <czvop@OCAG.CH>
Subject: NVRAM or how can I enforce a range check in Ada83.
Date: 1996/11/15
Message-ID: <9611150709.AA09539@algol.ocag.ch>#1/1
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sender: Ada programming language <INFO-ADA@LISTSERV.NODAK.EDU>
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comments: cc: czvop@hydra.ocag.ch
newsgroups: comp.lang.ada
Date: 1996-11-15T00:00:00+00:00
List-Id: <comp.lang.ada>


Hello Ada people (or fans ;-))

We are useing a Non Volatile RAM (EEPROM) in our project. The NVRAM driver
accesses the hardware by word operations. Now we've programmed a generic driver
which transform the generic type to a byte or word array. Because of the
possibility of an hardware error, we have to check the read values from the
NVRAM. We use an unchecked_conversion to transform from byte array to the
generic type. We have got no informations about the type inside this generic
procedure.

Code:

  generic
     type Elements is private;
  procedure Read (Item : out Elements);

  procedure Read (Item : out Elements) is
     function Convert is new Unchecked_Conversion (Source => <<byte array>>,
                                                   Target => Elements);
  begin
     Item := Convert (<<Nvram_Byte_Array_Of_Element_Size>>);
  end Read;

The Unchecked_Conversion does not make any checks, therefore the name unchecked.
We've tried to use a temporary variable to assigne the unchecked_converion
value and then assigne our parameter with this temporary variable and hoped
that a check would be performed.

   begin
     Temp := Convert(<<Nvram_Byte_Array_Of_Element_Size>>);
     Item := Temp;
   exception
     when constraint_error =>
         << some critical action>>
   end Read;

But as I've read later the Reference Manual, I've seen that no range check are
done by an assignment.

Now is there a construct that enforce the compiler to make a range check? But
remember in this generic procedure we haven't got any type information.
Also important: we use Ada83.

Has somebody any idea about this problem?

Best Regards

Peter
>