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From: Simon Wright <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: Enumeration representation clause surprise.
Date: Fri, 13 Jun 2008 06:49:09 +0100
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Date: 2008-06-13T06:49:09+01:00
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Mike Silva <snarflemike@yahoo.com> writes:

> On Jun 12, 4:52�pm, Simon Wright <simon.j.wri...@mac.com> wrote:

>> The only reason I have ever written representation clauses is to
>> ensure that the contents of raw memory are what some external
>> hardware (or software, eg at the other end of a network connection)
>> requires them to be.
>
> OK, I understand that - so how would you pass this raw memory to the
> external hardware or software?

Obtain the address to which the hardware registers are mapped (talking
VME here) & convert to a pointer to a structure which describes how
the hardware expects to see memory, then write through that pointer.

Or, for example, function To_Stream_Element here:

http://coldframe.cvs.sourceforge.net/coldframe/adasntp/SNTP.impl/sntp_support.adb?revision=1.4&view=markup

where I'm packing an unrepresented Status into a single byte for
transmission.

Nowadays I might do this using shift-and-mask to avoid messing with
endianness (this code has to work with Ada 95, even if the new 05
features would help).

>                                 And in this case, how did the use of
> biased representations ever get into compilers at all?  Is there any
> possible use of biased representations that does not fail the
> requirement you have established?

If I don't care how the compiler wedges the data into the record so
long as it's as small as possible (pragma Pack) then a biased rep
would be quite reasonable.