J3/04-305

Date: 5-May-2004
To:   J3
From: Interop/interp/Bill Long
Subj: Answer to F90 interpretation # F90/000207

NUMBER: F90/000207
TITLE: Integer bit-model inconsistency
KEYWORDS: Bit manipulation procedures, models for integer data
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION: Is the following observation true?

[F90 185:last sentence of 13.5.7] and [WG5/N1176, 219:22+] says ``In
particular, whereas the models are identical for w_{z-1}=0, they do
not correspond for w_{z-1}=1 and ...''

This statement assumes r==2 in the model for integer data defined in
 13.7.1, it is not true for general r. The above sentence should be
 changed to something like

``In particular, whereas the models are identical for r=2 and
  w_{z-1}=0, they do not correspond for r/=2 or w_{z-1}=1, and ...''

ANSWER: Yes.

DISCUSSION:

The integer model in 13.7.1 describes integer values using a
signed-magnitude, base r representation. This model is intended to
provide definitions for the parameters q and r that are used in the
computations of the results for certain numeric inquiry functions.
The representation is not intended to describe how integers are stored
in memory and, indeed, no common implementation uses a
signed-magnitude representation.

The bit model in 13.5.7 describes a method for characterizing a
sequence of bits by specifying an ordering and position number (the
index k) and the number of bits (z) in the sequence. The bit position
numbering and the sequence size parameter are used in the description
of the bit manipulation and BIT_SIZE intrinsics.

The INTEGER data type is overloaded in Fortran to specify both
numerical integers and bit sequences. The consequential attempt to
relate the integer model to the bit model leads to the mismatched text
quoted.  This attempt, which exists in both f95 and f03, is
fundamentally flawed. Rather than patching over this mismatch with
qualifications, a future version of the standard should correct the
underlying defect of specifing the INTEGER type for the arguments that
represent bit sequences in the descriptions of the bit manipulation
and BIT_SIZE intrinsics.

EDITS: none

SUBMITTED BY: Michael Hennecke

HISTORY: submitted Mar. 12, 1996      (first appeared in 96-006r2)
         WG5/N1404         Draft answer
         00-260      m154  Passed by J3 meeting
         00-329      m155  Failed J3 letter ballot
         WG5/N1452         Suggested revision
         01-292      m158  Passed by J3 meeting 11-1
         01-380      m159  Failed J3 letter ballot
         04-305      m168  Submitted for J3 meeting vote