10-154r1
To: J3
From: Robert Corbett/Stan Whitlock
Subject: Interpretation Request: Can zero have more than one
				 bit sequence representation?
Date: 2010 June 15

NUMBER: F03/0007
TITLE: Can zero have more than one bit sequence representation?
KEYWORDS: zero, bits
DEFECT TYPE: Interpretation
STATUS: J3 consideration in progress

QUESTIONS:

Question (1):
Some processors provide two internal representations for the
integer value zero.  The ones' complement representation of signed
integers and the signed-magnitude representation of signed
integers both provide two representations for zero.  Must a
processor for such a machine use a bit sequence consisting of all
zero bits to represent zero, regardless of the internal
representation of zero?

Question (2):
The Fortran 2008 standard [13.3.1, 317:9] states

    The interpretation of a negative integer as a
    sequence of bits is processor dependent.

Could a standard-conforming processor interpret all negative
integers as a sequence of all zero bits?

ANSWERS:

Answer to question (1): Yes.  The standard does not allow for more
than one bit representation of zero.

Answer to question (2): No.  The standard specifies that the integer
value of a string of all zero bits is zero.  The standard does not
specify the integer values for bit strings when the leftmost bit is
one.  A processor could interpret all negative integers as a sequence
of a one followed by all zero bits.  It would not be a useful bit
representation but it is not disallowed.

EDITS to 10-007:

None.

SUBMITTED BY: Robert Corbett

HISTORY: 10-154    m192  F03/0007 submitted
         10-154r1  m192  Draft answer