Re: Bit operations in Ada (endianness)

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

Dennis Hoppe <dennis.hoppe@hoppinet.de> writes:

> Hi,
>
> I have another question regarding litte and big-endian. Is there
> any way to declare, that the type
>
>   type Bit_Field is array (Bit_Number) of Boolean;
>
> is represented by litte or big-endian? Bit_Number is a generic
> paremeter (mod <>). I figured out to set the endianness to
> record types, but it does not work for the above array type.

The arithmetic on modular types does what you need, so the problem
only arises when using the bit-array representation.  You need to
invert the index explicitly on little-endian machines.

> I also wonder, if I can query the operating system via Ada,
> which endianness it uses.

Look at System.Default_Bit_Order.

So a solution might look like:

generic
   type Bit_Number is mod <>;
package Bit_Fields is
   type Bit_Field is array (Bit_Number) of Boolean;
   function Index (N : Bit_Number) return Bit_Number;
end Bit_Numbers;

with System;
package body Bit_Numbers is
   function Index (N : Bit_Number) return Bit_Number is
   begin
      case System.Default_Bit_Order is
         when System.High_Order_First => return N;
         when System.Low_Order_First  => return Bit_Number'Last - N;
      end case;
   end Index;
end Bit_Numbers;

HTH

-- 
Ludovic Brenta.