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The AIX operating system supports a 128-bit long double data type that provides greater precision than the default 64-bit long double data type. The 128-bit data type can handle up to 31 significant digits (compared to 17 handled by the 64-bit long double). However, while this data type can store numbers with more precision than the 64-bit data type, it does not store numbers of greater magnitude.

The following special issues apply to the use of the 128-bit long double data type:

**Compiling programs that use the 128-bit long double data type****Compliance with the IEEE 754 standard****Implementing the 128-bit long double format****Values of numeric macros**

To compile C programs that use
the 128-bit long double data type, use the **xlc128** command.
This command is an alias to the **xlc** command with support for the
128-bit data type. The **xlc** command supports only the 64-bit
long double data type.

The standard C library,
**libc.a** provides replacements for **libc.a**
routines which are implicitly sensitive to the size of long double.
Link with the **libc.a** library when compiling applications
that use the 64-bit long double data type. Link applications that use
128-bit long double values with both the **libc128.a** and
**libc.a** libraries. When linking, be sure to specify
the **libc128.a** library before the **libc.a**
library in the library search order.

The 64-bit implementation of the long double data type is fully compliant with the IEEE 754 standard, but the 128-bit implementation is not. Use the 64-bit implementation in applications that must conform to the IEEE 754 standard.

The 128-bit implementation differs from the IEEE standard for long double in the following ways:

**Supports only round-to-nearest mode. If the application changes the rounding mode, results are undefined.****Does not fully support the IEEE special numbers NaN and INF.****Does not support IEEE status flags for overflow, underflow, and other conditions. These flags have no meaning for the 128-bit long double inplementation.**

A 128-bit long double number consists of an ordered pair of 64-bit double-precision numbers. The first member of the ordered pair contains the high-order part of the number, and the second member contains the low-order part. The value of the long double quantity is the sum of the two 64-bit numbers.

Each of the two 64-bit numbers is itself a double-precision floating-point number with a sign, exponent, and significand. Typically the low-order member has a magnitude that is less than 0.5 units in the last place of the high part, so the values of the two 64-bit numbers do not overlap and the entire significand of the low-order number adds precision beyond the high-order number.

This representation results in several issues that must be considered in the use of these numbers:

**The exponent range is the same as that of double precision. Although the precision is greater, the magnitude of representable numbers is the same as 64-bit double precision.****As the absolute value of the magnitude decreases (near the denormal range), the additional precision available in the low-order part also decreases. When the value to be represented is in the denormal range, this representation provides no more precision than the 64-bit double-precision data type.****The actual number of bits of precision can vary. If the low-order part is much less than 1 ULP of the high-order part, significant bits (either all 0's or all 1's) are implied between the significands of the high-order and low-order numbers. Certain algorithms that rely on having a fixed number of bits in the significand can fail when using 128-bit long double numbers.**

Because of the storage method for
the long double data type, more than one number can satisfy certain values
that are available as macros.The representation of 128-bit long double
numbers means that the following macros required by standard C in the
**values.h** file do not have clear meaning:

**Number of bits in the mantissa (****LDBL_MANT_DIG**)**Epsilon (****LBDL_EPSILON**)**Maximum representable finite value (****LDBL_MAX**)

The number of bits in the
significand is not fixed, but for a correctly formatted number (except in the
denormal range) the minimum number available is 106. Therefore, the
value of the **LDBL_MANT_DIG** macro is 106.

The ANSI C standard defines the
value of epsilon as the difference between 1.0 and the least
representable value greater than 1.0, that is,
*b***(1-*p*), where *b* is the radix (2) and
*p* is the number of base *b* digits in the number.
This definition requires that the number of base *b* digits is fixed,
which is not true for 128-bit long double numbers.

The smallest representable value greater than 1.0 is this number:

0x3FF0000000000000, 0x0000000000000001

The difference between this value and 1.0 is this number:

0x0000000000000001, 0x0000000000000000 0.4940656458412465441765687928682213E-323

Because 128-bit numbers usually
provide at least 106 bits of precision, an appropriate minimum value for*
p* is 106. Thus, *b***(1-*p*) and 2**(-105)
yield this value:

0x3960000000000000, 0x0000000000000000 0.24651903288156618919116517665087070E-31

Both values satisfy the definition of epsilon according to standard C. The long double subroutines use the second value because it better characterizes the accuracy provided by the 128-bit implementation.

The value of the
**LDBL_MAX** macro is the largest 128-bit long double number that can
be multiplied by 1.0 and yield the original number. This value
is also the largest finite value that can be generated by primitive
operations, such as multiplication and division:

0x7FEFFFFFFFFFFFFF, 0x7C8FFFFFFFFFFFFF 0.1797693134862315807937289714053023E+309

The **cc** command.

List of 128-Bit Long Double Numerical Manipulation Subroutines.
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