IBM Floating Point Architecture - Definition and Overview

IBM System/360 computers, and subsequent machines based on that architecture (mainframes), support a hexadecimal floating-point format. The format is used by SAS Transport files as required by the FDA for New Drug Application (NDA) study submissions. See TS-140 [1] (http://support.sas.com/techsup/technote/ts140.html).

Hexadecimal floating-point uses a similar approach to IEEE 754 binary floating-point, but with many differences. The significand is longer, and the exponent is shorter:

 1     7                               56 bits
+-+-----------+----------------------------------------------------+
|S|  Exp      |  Fraction                                          |
+-+-----------+----------------------------------------------------+
63 62       56 55                                                 0

The bias is 64 because the exponent is smaller. Even though the base is 16, the exponent in this form is slightly smaller than the equivalent in IEEE 754.

Note that in this format the initial bit is not suppressed, and the radix point is set to the left of the mantissa.

See, for example: Schwarz, CMOS floating-point unit for the S/390 Parallel Enterprise Server G4 (http://www.research.ibm.com/journal/rd/414/schwarz.html)

Since 1998, IBM mainframes have also included binary floating-point units which conform to IEEE 754. When decimal floating-point is added later this decade (200x), each will have at least three floating-point units.