From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,b77305e4038ca5a X-Google-Attributes: gid103376,public From: "Pat Rogers" Subject: Re: Representation Clause Bit Ordering Date: 1999/09/08 Message-ID: X-Deja-AN: 522492894 References: <37860BA1.CF7E58B2@averstar.com> <7r5861$gob$1@infosun2.rus.uni-stuttgart.de> X-Priority: 3 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 X-Complaints-To: newsabuse@supernews.com Organization: Software Arts & Sciences X-MSMail-Priority: Normal Newsgroups: comp.lang.ada Date: 1999-09-08T00:00:00+00:00 List-Id: I have simple package supporting this approach for Ada 95 at http://www.classwide.com/products/freecode.htm Peter Hermann wrote in message news:7r5861$gob$1@infosun2.rus.uni-stuttgart.de... > Joe Wisniewski 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" > 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 > Message-ID: <325D1DFD.3DE1@watson.ibm.com> > References: <3259589E.1740@smtp.svl.trw.com> <325A510D.14E1@gsfc.nasa.gov> <53g3ag$1otk@info4.rus.uni-stuttgart.de> > Reply-To: ncohen@watson.ibm.com > NNTP-Posting-Host: socks1.watson.ibm.com > Mime-Version: 1.0 > Content-Type: text/plain; charset=us-ascii > Content-Transfer-Encoding: 7bit > X-Mailer: Mozilla 3.0 (Win95; I) > > 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 >