Re: Representation Clause Bit Ordering

["Pat Rogers" <progers@NOclasswideSPAM.com>]

I have simple package supporting this approach for Ada 95 at

http://www.classwide.com/products/freecode.htm


Peter Hermann <ica2ph@alpha1.csv.ica.uni-stuttgart.de> wrote in message
news:7r5861$gob$1@infosun2.rus.uni-stuttgart.de...
> Joe Wisniewski <wisniew@acm.org> wrote:
> > OK, those of good long-term memories.... was it in Ada Letters about
> > 4-5 years ago, there was a paper written about something very close
> > to this issue; or an endian-ness independent approach for rep-specs,
> > or something like this???????
>
> I dug out the following:
>
> --snip--snip--snip--snip--snip--snip--snip--snip--snip--snip--snip
>
> From: "Norman H. Cohen" <ncohen@watson.ibm.com>
> Newsgroups: comp.lang.ada
> Subject: Re: Rep Specs,endian,ncohen
> Date: Thu, 10 Oct 1996 12:02:05 -0400
> Organization: IBM Thomas J. Watson Research Center
> Lines: 83
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> Peter Hermann wrote:
>
> > I remember Norman H. Cohen has written an essay about the endian
> > /portability problem somewhere in AdaLetters(?).
>
> "Endian-Independent Record Representation Clauses", Ada Letters XIV,
No.
> 1 (January/February 1994), pp. 27-29
>
> Stephen Leake's solution appears to be an instance of the approach
> suggested in the article.  The article gives the general recipe for
> constructing the component clauses.
>
> Note that the problem being solved here is the porting of the source
> text for record-representation clauses, NOT the run-time translation
of
> a byte stream from one endianness to another.
>
> Please disregard the assertion in the article that this problem would
be
> solved in Ada 9X by an attribute definition clause for the 'Bit_Order
> attribute.  The implementation advice in RM95-13.5.3(8) allows a
> compiler to reject an attribute-definition clause for the nondefault
bit
> order in all the useful cases.
>
> Compilers are generally allowed to reject nondefault 'Bit_Order
> definitions because the designers of Ada 95 believed that component
> clauses interpreted according to the nondefault bit order could
specify
> noncontiguous bit fields, an implementation nightmare.  However, that
> depends on how you interpret the meaning of a bit offset in a
> nondefault-endian component clause.  If you insist that
>
>    at B range 10 .. 12
>
> be synonymous, assuming 8-bit bytes, with
>
>    at B+1 range 2 .. 4
>
> then you do indeed get noncontiguous bit fields.  But another
> interpretation is to look at the highest bit number, say b, specified
> with a given byte offset, and to view all component clauses with that
> byte offset as specifying a contiguous range of bits within the
> smallest  "loadable storage unit" (byte, halfword, word, or doubleword
> on a typical 32-bit or 64-bit machine) having at least b+1 bits.  For
> example, given the component clauses
>
>    A at 0 range 0 .. 5;
>    B at 0 range 6 .. 11;
>    C at 0 range 12 .. 15;
>
> and no other "at 0" component clauses, we would assume that A, B, and
C
> reside within a 16-bit loadable storage unit at offset 0, and that
when
> this 16-bit field is loaded into a register, B is a contiguous field
of
> bits somewhere in the middle of this 16-bit unit.  Whether A is at the
> high-order end or the low-order end of the 16-bit unit depends on
which
> bit ordering applies to the record type, but either way, there is a
> sensible interpretation with B specifying a field that is contiguous
> when loaded.
>
> Given this interpretation of bit offsets, there is a one-to-one
> correspondence between record representation clauses that specify a
> given layout when interpreted as big-endian clauses and other record
> representation clauses that specify the "same" layout when interpreted
> as little-endian representation clauses.  (I explain below what I mean
> when I say that layouts are the "same".)  Thus it would be practical
to
> require all compilers to support nondefault-endian
record-representation
> clauses, with an attribute-definition clause for T'Bit_Order
determining
> how the record-representation clause for type T is to be interpreted.
> This would solve the source portability problem by allowing the
> programmer to arbitrarily specify a layout in his favorite bit order,
> knowing that a compiler that is big-endian by default and a compiler
> that is little-endian by default will interpret the
> record-representation clause the "same" way.
>
> When I say that a layout on a big-endian machine and a layout on a
> little-endian machine are the "same", I mean that the layout is
divided
> into "loadable storage units" with the same sizes and byte offsets,
and
> that the left-to-right bit position of each record component within
> corresponding loadable storage units is the same.  These are the only
> properties of a layout that can be usefully preserved among
> opposite-endian machines.
>
> --
> Norman H. Cohen
> mailto:ncohen@watson.ibm.com
> http://www.research.ibm.com/people/n/ncohen
>