quad precision question

[Nelson H. F. Beebe]

Stu Anderson <stu.andersonaboeing.com> writes on Wed, 27 Sep 2000
16:46:35 -0700 (PDT):

>> ...
>> When I run various Fortran 90/95 compilers with
>>    selected_real_kind(p=24),
>> I get the following results:
>>
>>               radix digits minexponent maxexponent
>>
>> HP              2    113      -16381       16384
>> Sun             2    113      -16381       16384
>>
>> RS6000          2    106        -968        1024
>> SGI             2    107        -915        1023
>>
>> Note: for IEEE double, the four numbers are (2 53 -1021 1024).
>>
>> The HP and the Sun are using what could be called IEEE Quad.
>> The RS6000 and the SGI are using two IEEE doubles.
>>
>> Why are the RS6000 and SGI numbers different?
>> ...

The reason is that Compaq/DEC Alpha, HP PA-RISC, IBM S/390, and Sun
SPARC all have true quadruple-precision arithmetic extending IEEE 754
arithmetic.

IBM Power and PowerPC, and SGI MIPS, each use a paired-double
representation of arithmetic, so their range is that same as that of
double-precision, and the precision is somewhat shorter than the other
vendors.

Only IBM S/390 on the recent G5 processors (announced in 1999), and
Hal/Fujitsu SPARC64-GP, implement the arithmetic entirely in hardware.
All other vendors implement it entirely in software.

A colleague and I recently submitted a long journal article analyzing
quadruple-precision arithmetic implementations with respect to
computation of certain functions, and found far too many
implementation deficiencies and errors in the more than 40 Fortran 77,
90, 95, and HPF compilers, and 55 C and C++ compilers, covering all
major desktop platforms in use today.  Java, regrettably, does not
even admit to the need for quadruple-precision arithmetic.



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