J3/05-152

Date: 4 Feb 2005
To: J3
From: John Reid
Subject: Interpretation request re EXPONENT, FRACTION, and SET_EXPONENT

NUMBER: F03/00xx
TITLE: Denormal inputs to EXPONENT, FRACTION, and SET_EXPONENT
KEYWORDS: denormal, intrinsic, EXPONENT, FRACTION, SET_EXPONENT
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

What results do EXPONENT, FRACTION, and SET_EXPONENT return if
the value of X is denormal?

If a denorm is loaded into the cpu in extended precision, it
becomes a normal value with changed fractional and exponent parts.
Which of these values should be taken?

In the definition of EXPONENT, the words 'of the model representation
for the value of X' appear; but in the definition of FRACTION and
SET_EXPONENT, the words are 'for the model representation of X'.
Was this difference intended?


ANSWER:

In all three cases, the intention was to treat the value as if it
were held according to the model in 13.14 with the same radix b,
but with an expanded exponent range. This is why the words 'for the
value of X' were used in EXPONENT. It has to be this way for a
denormal value since otherwise it does not lie within the model.
Edits are provided to make the intention clear.


DISCUSSION:

This interpretation is satisfactory from a mathematical point of
view since the values of EXPONENT(X) and EXPONENT(DBLE(X)) will
be the same when X is of type default real and has a denormal value.
Similar properties holds for the other two functions.


EDITS:

Page and line numbers refer to J3/04-007.

[316:5-6] Subclause 13.7.37, Result Value, lines 1-2. Replace
"model ... X" by "representation for the value of X in the model
(13.4) that has the radix of X but no limits on exponent values".

[317:8] Subclause 13.7.40, Result Value, line 2. Replace
"model ... X" by "representation for the value of X in the model
that has the radix of X but no limits on exponent values".

[351:5] Subclause 13.7.107, Result Value, line 2. Replace
"model ... X" by "representation for the value of X in the model
that has the radix of X but no limits on exponent values".

SUBMITTED BY: John Reid

HISTORY: J3/05-152 at J3 meeting 171