Subnormal Override Functions

Although not IEEE 754 compliant, subnormal operands and/or results may be overridden to improve the performance of some applications that do not require subnormal number precision. Use of the subnormal override function:

  • Avoids the consequences of processing or having to deal with subnormal datum.
  • Handles result underflows when a result is subnormal, negating the need to handle an underflow exception.

The subnormal override functions consist of two parts, one to flush subnormal input operands to zero (referred to as Subnormals-Are-Zeros, or SAZ mode), and the other to remove subnormal results (referred to as Flush-To-Zero, or FTZ mode).

Note: Subnormal override modes are not applicable to FCPS/FCPQ (no result to override), FAND, FIOR, FTST, FMOV, FMOVC or any CPU to/from FPU data move instruction.