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From: lutz@iks-jena.de (Lutz Donnerhacke)
Newsgroups: comp.lang.ada
Subject: Re: Query on portable bit extraction
Date: Fri, 26 Oct 2001 17:08:31 +0000 (UTC)
Organization: IKS GmbH Jena
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Date: 2001-10-26T17:08:31+00:00
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* Mark Johnson wrote:
>We have code that we want to run on both a big (sgi) and little (pc)
>endian machine. As an example, the layout of the data in memory is...
>[big_endian bit order]
>Byte  0 1 2 3 4 5 6 7
>21 -  X X X Y Y Y Y Y
>22 -  Y Y Z Z Z Z Z Z
>where we need to extract XXX as the "wait code", YYYYYYY as the "value",
>and ZZZZZZ is a pad area (don't care).

>Thinking of records & representation specifications, I'd like to be able
>to do something like this...
>  subtype Wv is Natural range 0..7;
>  subtype Vv is Natural range 0..127;
>  subtype Pv is Natural range 0..63;
>  type Wait_Value is
>    record
>      Wait : Wv;
>      Value : Vv;
>      Pad : Pv;
>    end record;
>  for Wait_Value use
>    record
>      Wait at 0 range 0..2;
>      Value at 0 range 3..9;
>      Pad at 1 range 2..7;
>    end record;
>  for Wait_Value'Bit_Order use System.High_Order_First;

Incorrect code: Value does cross a Storage_Element boundary. Split it into
two parts and combine them later. Example:
http://www.iks-jena.de/mitarb/lutz/ada/net/ --> i.e. IPV4 Header
 
>which looks pretty reasonable until I get the following GNAT error
>messages on the PC...
> error: attempt to specify non-contiguous field not permitted
> error: (caused by non-standard Bit_Order specified)

Correct. for xx'Bit_Order use ... does only define the direction of counting.
It does not swap or correct alignments.

>somewhere, I recode the representation specification to be...
>  for Wait_Value use
>    record
>      Wait at 1 range 5..7;
>      Value at 0 range 6..12;
>      Pad at 0 range 0..5;
>    end record;
>(assuming System.Low_Order_First bit order) which works as expected.

On both systems? Very strange.