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NAME
ieee
 IEEE standard 754 for floatingpoint arithmetic
CONTENTS
Description
IEEE STANDARD 754 FloatingPoint Arithmetic
Data Formats
Additional Information Regarding Exceptions
See Also
Standards
DESCRIPTION
The IEEE Standard 754 for Binary FloatingPoint Arithmetic
defines representations of floatingpoint numbers and abstract
properties of arithmetic operations relating to precision,
rounding, and exceptional cases, as described below.
IEEE STANDARD 754 FloatingPoint Arithmetic
Radix: Binary.
Overflow and underflow:
Overflow goes by default to a signed oo.
Underflow is
gradual.
Zero is represented ambiguously as +0 or 0.
Its sign transforms correctly through multiplication or
division, and is preserved by addition of zeros
with like signs; but xx yields +0 for every
finite x.
The only operations that reveal zero’s
sign are division by zero and
copysign x ±0.
In particular, comparison (x > y, x >= y, etc.)
cannot be affected by the sign of zero; but if
finite x = y then oo = 1/(xy) != 1/(yx) = oo.
Infinity is signed.
It persists when added to itself
or to any finite number.
Its sign transforms
correctly through multiplication and division, and
(finite)/±oo = ±0
(nonzero)/0 = ±oo.
But
oooo, oo*0 and oo/oo
are, like 0/0 and sqrt(3),
invalid operations that produce NaN. ...
Reserved operands (NaNs):
An NaN is
( N ot a N umber).
Some NaNs, called Signaling NaNs, trap any floatingpoint operation
performed upon them; they are used to mark missing
or uninitialized values, or nonexistent elements
of arrays.
The rest are Quiet NaNs; they are
the default results of Invalid Operations, and
propagate through subsequent arithmetic operations.
If x != x then x is NaN; every other predicate
(x > y, x = y, x < y, ...) is FALSE if NaN is involved.
Rounding:
Every algebraic operation (+, , *, /,
v/)
is rounded by default to within half an
ulp,
and when the rounding error is exactly half an
ulp
then
the rounded value’s least significant bit is zero.
(An
ulp
is one
U nit
in the
L ast
P lace.)
This kind of rounding is usually the best kind,
sometimes provably so; for instance, for every
x = 1.0, 2.0, 3.0, 4.0, ..., 2.0**52, we find
(x/3.0)*3.0 == x and (x/10.0)*10.0 == x and ...
despite that both the quotients and the products
have been rounded.
Only rounding like IEEE 754 can do that.
But no single kind of rounding can be
proved best for every circumstance, so IEEE 754
provides rounding towards zero or towards
+oo or towards oo
at the programmer’s option.
Exceptions:
IEEE 754 recognizes five kinds of floatingpoint exceptions,
listed below in declining order of probable importance.
Exception Default Result
Invalid Operation NaN, or FALSE
Overflow ±oo
Divide by Zero ±oo
Underflow Gradual Underflow
Inexact Rounded value
NOTE: An Exception is not an Error unless handled
badly.
What makes a class of exceptions exceptional
is that no single default response can be satisfactory
in every instance.
On the other hand, if a default
response will serve most instances satisfactorily,
the unsatisfactory instances cannot justify aborting
computation every time the exception occurs.
Data Formats
Singleprecision:
Type name:
.Vt float
Wordsize: 32 bits.
Precision: 24 significant bits,
roughly like 7 significant decimals.
If x and x’ are consecutive positive singleprecision
numbers (they differ by 1
ulp),
then
5.9e08 < 0.5**24 < (x’x)/x <= 0.5**23 < 1.2e07.
Range: Overflow threshold = 2.0**128 = 3.4e38
Underflow threshold = 0.5**126 = 1.2e38
Underflowed results round to the nearest
integer multiple of
0.5**149 = 1.4e45.
Doubleprecision:
Type name:
.Vt double
(On some architectures,
.Vt long double
is the same as
.Vt double
)
Wordsize: 64 bits.
Precision: 53 significant bits,
roughly like 16 significant decimals.
If x and x’ are consecutive positive doubleprecision
numbers (they differ by 1
ulp),
then
1.1e16 < 0.5**53 < (x’x)/x <= 0.5**52 < 2.3e16.
Range: Overflow threshold = 2.0**1024 = 1.8e308
Underflow threshold = 0.5**1022 = 2.2e308
Underflowed results round to the nearest
integer multiple of
0.5**1074 = 4.9e324.
Extendedprecision:
Type name:
.Vt long double
(when supported by the hardware)
Wordsize: 96 bits.
Precision: 64 significant bits,
roughly like 19 significant decimals.
If x and x’ are consecutive positive extendedprecision
numbers (they differ by 1
ulp),
then
1.0e19 < 0.5**63 < (x’x)/x <= 0.5**62 < 2.2e19.
Range: Overflow threshold = 2.0**16384 = 1.2e4932
Underflow threshold = 0.5**16382 = 3.4e4932
Underflowed results round to the nearest
integer multiple of
0.5**16445 = 5.7e4953.
Quadextendedprecision:
Type name:
.Vt long double
(when supported by the hardware)
Wordsize: 128 bits.
Precision: 113 significant bits,
roughly like 34 significant decimals.
If x and x’ are consecutive positive quadextendedprecision
numbers (they differ by 1
ulp),
then
9.6e35 < 0.5**113 < (x’x)/x <= 0.5**112 < 2.0e34.
Range: Overflow threshold = 2.0**16384 = 1.2e4932
Underflow threshold = 0.5**16382 = 3.4e4932
Underflowed results round to the nearest
integer multiple of
0.5**16494 = 6.5e4966.
Additional Information Regarding Exceptions
For each kind of floatingpoint exception, IEEE 754
provides a Flag that is raised each time its exception
is signaled, and stays raised until the program resets
it.
Programs may also test, save and restore a flag.
Thus, IEEE 754 provides three ways by which programs
may cope with exceptions for which the default result
might be unsatisfactory:
 Test for a condition that might cause an exception
later, and branch to avoid the exception.
 Test a flag to see whether an exception has occurred
since the program last reset its flag.
 Test a result to see whether it is a value that only
an exception could have produced.
CAUTION: The only reliable ways to discover
whether Underflow has occurred are to test whether
products or quotients lie closer to zero than the
underflow threshold, or to test the Underflow
flag.
(Sums and differences cannot underflow in
IEEE 754; if x != y then xy is correct to
full precision and certainly nonzero regardless of
how tiny it may be.)
Products and quotients that
underflow gradually can lose accuracy gradually
without vanishing, so comparing them with zero
(as one might on a VAX) will not reveal the loss.
Fortunately, if a gradually underflowed value is
destined to be added to something bigger than the
underflow threshold, as is almost always the case,
digits lost to gradual underflow will not be missed
because they would have been rounded off anyway.
So gradual underflows are usually
provably
ignorable.
The same cannot be said of underflows flushed to 0.
At the option of an implementor conforming to IEEE 754,
other ways to cope with exceptions may be provided:
 ABORT.
This mechanism classifies an exception in
advance as an incident to be handled by means
traditionally associated with errorhandling
statements like "ON ERROR GO TO ...".
Different
languages offer different forms of this statement,
but most share the following characteristics:

No means is provided to substitute a value for
the offending operation’s result and resume
computation from what may be the middle of an
expression.
An exceptional result is abandoned.


In a subprogram that lacks an errorhandling
statement, an exception causes the subprogram to
abort within whatever program called it, and so
on back up the chain of calling subprograms until
an errorhandling statement is encountered or the
whole task is aborted and memory is dumped.


 STOP.
This mechanism, requiring an interactive
debugging environment, is more for the programmer
than the program.
It classifies an exception in
advance as a symptom of a programmer’s error; the
exception suspends execution as near as it can to
the offending operation so that the programmer can
look around to see how it happened.
Quite often
the first several exceptions turn out to be quite
unexceptionable, so the programmer ought ideally
to be able to resume execution after each one as if
execution had not been stopped.
 ... Other ways lie beyond the scope of this document.
Ideally, each
elementary function should act as if it were indivisible, or
atomic, in the sense that ...
 No exception should be signaled that is not deserved by
the data supplied to that function.
 Any exception signaled should be identified with that
function rather than with one of its subroutines.
 The internal behavior of an atomic function should not
be disrupted when a calling program changes from
one to another of the five or so ways of handling
exceptions listed above, although the definition
of the function may be correlated intentionally
with exception handling.
The functions in
libm
are only approximately atomic.
They signal no inappropriate exception except possibly ...
Over/Underflow
 
when a result, if properly computed, might have lain barely within range, and

Inexact in
cabs,
cbrt,
hypot,
log10
and
pow
 
when it happens to be exact, thanks to fortuitous cancellation of errors.


Otherwise, ...
Invalid Operation is signaled only when
 
any result but NaN would probably be misleading.

Overflow is signaled only when
 
the exact result would be finite but beyond the overflow threshold.

DividebyZero is signaled only when
 
a function takes exactly infinite values at finite operands.

Underflow is signaled only when
 
the exact result would be nonzero but tinier than the underflow threshold.

Inexact is signaled only when
 
greater range or precision would be needed to represent the exact result.


SEE ALSO
fenv(3),
ieee_test(3),
math(3)
An explanation of IEEE 754 and its proposed extension p854
was published in the IEEE magazine MICRO in August 1984 under
the title "A Proposed Radix and Wordlengthindependent
Standard for Floatingpoint Arithmetic" by
.An W. J. Cody
et al.
The manuals for Pascal, C and BASIC on the Apple Macintosh
document the features of IEEE 754 pretty well.
Articles in the IEEE magazine COMPUTER vol. 14 no. 3 (Mar.
1981), and in the ACM SIGNUM Newsletter Special Issue of
Oct. 1979, may be helpful although they pertain to
superseded drafts of the standard.
STANDARDS
ieee754
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