IEEE
[Revised: Wednesday, 10Nov1999 22:48:32 EST]
The IEEE standard for floating point arithmetic
The IEEE (Institute of Electrical and Electronics Engineers) has produced
a standard for floating point arithmetic. This standard specifies how single
precision (32 bit) and double precision (64 bit) floating point numbers
are to be represented, as well as how arithmetic should be carried out
on them.
At the PSC, the Ultrix
frontends use the IEEE format. The VMS
frontends and Cray C90
systems have their own vendorspecific formats for data.
Because many of our users may have occasion to transfer unformatted or
"binary" data between an IEEE machine and the Cray or the VAX/VMS, it is
worth noting the details of this format for comparison with the Cray and
VAX representations. The differences in the formats also affect the accuracy
of floating point computations.
Summary
Single Precision
The IEEE single precision floating point standard representation requires
a 32 bit word, which may be represented as numbered from 0 to 31, left
to right. The first bit is the sign bit, S, the next eight bits are the
exponent bits, 'E', and the final 23 bits are the fraction 'F':
S EEEEEEEE FFFFFFFFFFFFFFFFFFFFFFF
0 1 8 9 31
The value V represented by the word may be determined as follows:

If E=255 and F is nonzero, then V=NaN ("Not a number")

If E=255 and F is zero and S is 1, then V=Infinity

If E=255 and F is zero and S is 0, then V=Infinity

If 0<E<255 then V=(1)**S * 2 ** (E127) * (1.F) where "1.F" is intended
to represent the binary number created by prefixing F with an implicit
leading 1 and a binary point.

If E=0 and F is nonzero, then V=(1)**S * 2 ** (126) * (0.F) These are
"unnormalized" values.

If E=0 and F is zero and S is 1, then V=0

If E=0 and F is zero and S is 0, then V=0
In particular,
0 00000000 00000000000000000000000 = 0
1 00000000 00000000000000000000000 = 0
0 11111111 00000000000000000000000 = Infinity
1 11111111 00000000000000000000000 = Infinity
0 11111111 00000100000000000000000 = NaN
1 11111111 00100010001001010101010 = NaN
0 10000000 00000000000000000000000 = +1 * 2**(128127) * 1.0 = 2
0 10000001 10100000000000000000000 = +1 * 2**(129127) * 1.101 = 6.5
1 10000001 10100000000000000000000 = 1 * 2**(129127) * 1.101 = 6.5
0 00000001 00000000000000000000000 = +1 * 2**(1127) * 1.0 = 2**(126)
0 00000000 10000000000000000000000 = +1 * 2**(126) * 0.1 = 2**(127)
0 00000000 00000000000000000000001 = +1 * 2**(126) *
0.00000000000000000000001 =
2**(149) (Smallest positive value)
Double Precision
The IEEE double precision floating point standard representation requires
a 64 bit word, which may be represented as numbered from 0 to 63, left
to right. The first bit is the sign bit, S, the next eleven bits are the
exponent bits, 'E', and the final 52 bits are the fraction 'F':
S EEEEEEEEEEE FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
0 1 11 12 63
The value V represented by the word may be determined as follows:

If E=2047 and F is nonzero, then V=NaN ("Not a number")

If E=2047 and F is zero and S is 1, then V=Infinity

If E=2047 and F is zero and S is 0, then V=Infinity

If 0<E<2047 then V=(1)**S * 2 ** (E1023) * (1.F) where "1.F" is
intended to represent the binary number created by prefixing F with an
implicit leading 1 and a binary point.

If E=0 and F is nonzero, then V=(1)**S * 2 ** (1022) * (0.F) These are
"unnormalized" values.

If E=0 and F is zero and S is 1, then V=0

If E=0 and F is zero and S is 0, then V=0
Reference
ANSI/IEEE Standard 7541985,
Standard for Binary Floating Point Arithmetic
© Pittsburgh Supercomputing
Center (PSC), Carnegie Mellon University
URL: http://www.psc.edu/general/software/packages/ieee/ieee.html
