Re: Address and bit mask

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

milouz wrote on comp.lang.ada:
> Ok, thank you very much for all your answers, but I'm not really more
> advanced. Let me summarize :
>
> - my question was about address arithmetic (howto ?)
> - Some fellows gave me cues about the use of the
> System.Storage_Elements, but without gaving any concrete example.
> - Some others just wandered about the need of doing address
> arithmetic.
>
> Hum... well... I still do not know how to do some simple arithmetic
> with address. So, let me give you an example in C :
>
>   p = (char*) (((unsigned long)p + ALIGN_MASK) & ~ALIGN_MASK);
>
> How do you write that in Ada ?

You don't.  You declare an array of characters, specify that each
character takes up ALIGN_MASK*Storage_Unit bits and that the array is
aligned on an ALIGN_MASK boundary.  No need for address arithmetic or
bitwise operations.

declare
  Alignment : constant := 2**3; -- bytes
  type P_T is array (Positive range <>) of Character;
  for P_T'Alignment use Alignment; -- storage_elements
  for P_T'Component_Size use Alignment * System.Storage_Unit; -- bits

  P : P_T (1 .. Number_Of_Elements_In_The_Array);
  -- if P starts at a specific address, you say so:
  for P'Address use The_Starting_Address_Of_P;
begin
  for Index in P'Range loop
     P (Index) := Foo;
  end loop;
end;

The above is not strictly equivalent to your C example; it is much
better:
- it says what it does (as opposed to how it does it);
- it has bounds checking on the array such that you cannot read or
write outside the array;
- it allows you to pass the array as a parameter to subprograms, as
opposed to a pointer to the array (worry not, the compiler will pass
by address behind the scenes)

If the extremely rare case where you really, really must do address
arithmetic, you still don't need bitwise operations on addresses.  You
simply use System.Storage_Elements like so:

function Succ (A : in System.Address) return System.Address is
   Alignment : constant Storage_Offset := 2**3;
   use System.Storage_Elements;
begin
   return (A + Alignment) mod Alignment;
   -- note: "+" and "mod" are in System.Storage_Elements
end Increment;

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Ludovic Brenta.
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