IEEE 754 Revision - Discussed But Not Included

Discussed But Not Included

Due to changes in CPU design and development, the 2008 IEEE floating point standard could be viewed as historical or outdated as the 1985 standard it replaced. There were many outside discussions and items not covered in the standardization process, the items below are the ones that became public knowledge

• Annex "L" recommended to language developers how to bind items in the standard to features in a language.

• Annex "U" provided guidance on the choice of numeric underflow definitions.

In 754 the definition of underflow was that the result is tiny and encounters a loss of accuracy.

Two definitions were allowed for the determination of the 'tiny' condition: before or after rounding the infinitely precise result to working precision, with unbounded exponent.

Two definitions of loss of accuracy were permitted: inexact result or loss due only to denormalization. No known hardware systems implemented the latter and it has been removed from the revised standard as an option.

Annex U of 754r recommended that only tininess after rounding and inexact as loss of accuracy be a cause for underflow signal.

• Annex "Z" introduced optional data types for supporting other fixed width floating point formats, as well as arbitrary precision formats (i.e., where the precision of representation and rounding is determined at execution time) – some of this material was moved into the body of the draft by generalizing section 5. Arbitrary precision was dropped.

• Inheritance and propagation of modes (exception handling, presubstitution, rounding) and flags (inexact, underflow, overflow, divide by zero, invalid). The desire is to have mode changes be able to be inherited by a callee, but not affect the caller. And have the flags propagate out to a caller.

• Interval and other arithmetics were discussed but not included as being outside scope (and a large piece of work in their own right). Work is starting in 2008 on a proposed IEEE standard for interval arithmetic.