3.6.8.9.1 Subnormal Results
The FPU supports a Subnormal Result Override mode, Flush-To-Zero (FTZ), and its functionality is defined in Flush-To-Zero (FTZ) . Flush-To-Zero (FTZ) is enabled when both FCR.FTZ and FCR.UDFM are set. If the underflow exception is unmasked (FCR.UDFM = 0), then the FCR.FTZ bit will have no effect.
This mode is implemented within the WB-stage so that results written to the destination register (and those forwarded) will be adjusted accordingly if FTZ mode is enabled. The FCR.FTZ bit is only examined during the WB-stage of an instruction, allowing it to be modified as late as the cycle before the instruction enters the WB-stage.
Note: FTZ mode is not applicable to
FAND,
FIOR, FMOV, FMOVC, or any CPU
to/from FPU data move instruction, none of which can modify any FPU status. It is also
not applicable to FTST because the FSR is the only possible destination for this
operation. In addition, FTZ mode will have no effect on FF2LI/FF2DI and FLI2F/FDI2F
instruction results because FF2LI/FF2DI results are integers, and FLI2F/FDI2F
destination data may be zero but never subnormal.