6.6.8.8.1 Subnormal Results
The FPU supports a subnormal result override mode, Flush-To-Zero (FTZ), the functionality of which is defined in Flush-To-Zero (FTZ). Flush-To-Zero (FTZ) is enabled when both FCR.FTZ and FCR.UDFM are set. Should the underflow exception be unmasked (FCR.UDFM = 0), then the FCR.FTZ bit will have no effect.
This mode is implemented within the WB-stage such 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 such that it may 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 0 but never subnormal.