6.6.3.2.3 Subnormal Operand Exception

Should any affected instruction execute using a subnormal operand, and SAZ mode is disabled, the Subnormal Operand (FSR.SUBO) exception will be signaled. This provides a mechanism to indicate the use of a subnormal value without requiring the operand be tested (FTST).

Should SAZ mode be enabled and a subnormal operand is encountered (and changed to a 0 value), SUBO will not be signaled.

SAZ mode may be enabled irrespective of whether the SUBO exception is masked or not (though when enabled will never signal SUBO).