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RE: Two technical questions on IEEE Std 754-2008



Chuck Stevens wrote:
I think COBOL needs separate arithmetic MODES for the two encodings
because, in COBOL terms, arithmetic includes "getting the data ready" for
the arithmetic operation, along with the arithmetic operation itself.

Ok, so from the language's point of view ALL data are what I call
"external", including what in other languages would be local variables.
At least that's the only way I can conceptualize this.

This is partially confirmed a bit later (I'll get back to something
mentioned in-between):

The only issue the language has to worry about is interchange formats,

Yes.  All we talk about is interchange formats.

and those have to address Endianness, character encoding, and other
details

I don't think the standard needs to address these; that's the
implementor's job.

Yes -- and so it is for BID vs DPD.  What COBOL needs to know is whether
the type is STANDARD-BINARY or STANDARD-DECIMAL (and what size thereof).

of the same nature as BID vs DPD for STANDARD-DECIMAL.

Well, they're similar -- but in a given implementation, I would
think the arithmetic ops would expect the values presented to it
to be encoded the same way.

No -- they are EXACTLY of the same nature.  The arithmetic ops are
not prepared to accept arguments of different Endianness either.

So if COBOL has a mechanism to deal with data imported from machines
with a different Endianness, it should be able to use the same mechanism
to deal with decimal encodings.  After all, BOTH issues involve nothing
more than a blind transformation of bits, knowing the size and logical
type of the item.  Ok -- I'll grant that there is one small additional
difference:  Endianness conversion only needs to know the size of the
type, but decimal re-encoding needs to know in addition that the field
holds a DFP item.  So yes, the support does not come entirely for free,
and I can understand that the initial version of new DFP support would
avoid the issue by only supporting DPD.  HOWEVER -- please make sure
that you don't encumber the LANGUAGE with additional types of arithmetic,
unless the language already has explicit Endianness declarations as well.

I'm actually trying to SIMPLIFY the job the COBOL standards committee has
to do.  Not knowing the details of how Endianness differences are handled
it is difficult to make specific suggestions.  So let me describe one way
to do it, and COBOL experts can perhaps redirect this to match reality.

    Suppose we are importing a record that contains both character fields
    and numeric fields of various sizes.  Character fields may be subject
    to Ascii/Ebcdic conversions, and numeric fields (other than perhaps
    packed decimal) may be subject to Endianness conversion.

    Therefore the interface handler must know, for each field, its logical
    type (character, packed decimal, or other numeric) as well as its size
    and offset, as there are three different applicable transformations:
    leave unchanged, translate characters, or reverse Endianness.

    What I'm suggesting is that there be a fourth logical type, namely DFP.
    There would then (for this purpose) be three numerical types instead
    of two: DFP (use DPD->BID if needed), BCD (leave alone), and OTHER (do
    Endianness conversion if needed).  The BFP types fall under OTHER for
    this purpose.

Imported files presumably have a magic number or other identifying tag to
record the origin's character set and Endianness.  If COBOL wanted to support
both BID and DPD encodings for DFP, it would need another indication for this
-- but if COBOL sticks with a prescribed interchange encoding, this would not
be necessary.

What am I missing here?  Perhaps the answer is in the "in-between" part I
promised to get back to:

I find it hard to envision that implementations conformant to IEEE 754
would allow different encodings for operands to a single arithmetic
operation -- on a theoretical fixed-word stack machine, Valuecall
(a-binary32), Valuecall (a-decimal128), ADD, for example.  If they're all
of the same basic type -- Valuecall (a-binary32), valuecall (a-binary128),
I'd expect the arithmetic instruction to handle that.

Well, although IEEE 754 discourages mixed-radix arithmetic it actually
requires mixed-size arithmetic with a single rounding per "formatOf"
operation.  Same-radix widening (which is what COBOL does) is in fact
not conforming (but then COBOL never said it would be) if the wide result
is then narrowed to the target format.  As for that hypothetical stack
machine:  the compiler knows the types, and would have an assortment of
different ADD_format1_to_format2 instructions all corresponding to the
generic ADD operation of the language.  Such languages exist.  It is of
course simpler if the computing model is that ALL operands are converted
to a single type -- preferably decimal128.

                                                       We in COBOL need to
know the form in which arithmetic operands are expected so we can put them
in that form before handing them off to the operations.  For a given mode
of arithmetic, all operands are converted to the same form, program-wide.

Right -- and here we come to the crux of the problem:  Do these operands
ALWAYS remain in the original encoding (possibly from an external source)?
If so, the conversion step I described above must indeed be carried out at
this point all the time, and there is no need to remember record layout
because individual fields are being presented.  These fields must however
be presented with size and type information -- and I claim it is sufficient
to identify DFP types as DFP: the implementation knows its own preferred
encoding, and (if COBOL requires a DPD-based interchange format) it knows
the source encoding.  (Here Endianness may be more difficult, conceptually,
because the implementation must somehow find out what the original encoding
is -- that magic source-id must be accessible.)

The is one situation where a choice of one or another flavour of DFP
arithmetic (DFP-DPD vs DFP-BID) makes sense:  when both are available
only as software libraries, and the compiler could call either one.
Perhaps that is the model that has been in everyone else's mind but
mine, because I'm familiar with machines that support DFP in hardware.
But since the two DFP arithmetics produce IDENTICAL results in all
circumstances (unlike BFP vs DFP vs "native"), I find it difficult to
understand why one would burden the LANGUAGE with the distinction, as
opposed to the installation or compilation options.

Michel.

P.S.  I think I'm running out of steam on this issue...

      It would perhaps help is somebody could tell me how COBOL
      implementations deal with Endianness and character-code
      issues, so I wouldn't have to guess like I did above.
---Sent: 2011-02-25 16:17:03 UTC


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