Re: Endianness independance

[Jeffrey Carter <jrcarter@acm.org>]

Stephen Leake wrote:
> "Marin David Condic" <mcondic.auntie.spam@acm.org> writes:
> 
> 
>>Well, but if hardware is not standardized, then why would you expect
>>languages to be standardized? (With respect to data representations.)
> 
> 
> The point is that Ada provides a _standard_, _single_ representation
> clause that works for both kinds of hardware:
> 
> --  Abstract :
> --
> --  Define constants to reflect hardware bit and word endianness in
> --  record representation clauses. Obviously, this file is highly
> --  system-dependent.
> --
> with System;
> package SAL.Endianness is
>    pragma Pure;
> 
>    --  This is for gnat on a Mongoose processor
>    System_Name : constant System.Name := System.SYSTEM_NAME_GNAT;
> 
>    type Byte_Order_Type is (Big_Endian, Little_Endian);
>    Byte_Order : constant Byte_Order_Type := Big_Endian;
> 
>    --  The Mongoose documentation numbers bits with LSB = 0. However,
>    --  the byte order is Big_Endian, so the Ada compiler assumes bits
>    --  are numbered with MSB = 0, since that allows representation
>    --  clauses with fields that cross natural boundaries.
> 
>    Bit_Order      : constant := -1; -- 1 or -1
>    High_Bit_First : constant :=  1; -- 0 or  1
>    Low_Bit_First  : constant :=  0; -- 1 or  0
> 
>    --  Use one of these, corresponding to the natural size of the
>    --  record you are laying out.
>    LSBit_8        : constant :=  7; -- 0 or  7
>    LSBit_16       : constant := 15; -- 0 or 15
>    LSBit_32       : constant := 31; -- 0 or 31
>    LSBit_56       : constant := 55; -- 0 or 55
> 
> end SAL.Endianness;

You can make this system independent by using System.Default_Bit_Order, 
which, thanks to an AI (I don't remember which, but you can look it up) 
is static.

-- 
Jeff Carter
"If I could find a sheriff who so offends the citizens of Rock
Ridge that his very appearance would drive them out of town ...
but where would I find such a man? Why am I asking you?"
Blazing Saddles