3.6.3.2.2 Flush-To-Zero (FTZ)

Flush-To-Zero (FTZ) mode 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. Should a floating-point operation generate an infinitely precise result that is less than the smallest possible subnormal number, then the functional block will round this to a result of 0 with the same sign as the subnormal value. This will occur irrespective of whether FTZ mode is enabled or not. Both Underflow (FSR.UDF) and Inexact (FSR.INX) will be signaled (if not already set, sticky status FSR.UDFS and FSR.INXS will also be set). Should a floating-point result be a subnormal number (that the functional block has not rounded up to the smallest magnitude normal number), and FTZ mode is enabled, the result will be replaced with 0 of the same sign as the subnormal value it is replacing. Again, both Underflow (FSR.UDF) and Inexact (FSR.INX) will be signaled (if not already set, sticky status FSR.UDFS and FSR.INXS will also be set), though the Underflow exception has to be masked (in order to enable FTZ mode), so no interrupt will be issued. Forcing the result to 0 allows the user to ignore underflows (though at the expense of some accuracy).

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. For example, the following code sequence will only apply the FTZ function to the FSUB instruction:

 * Assume FCR.FTZ=0 && FCR.UDFM=1 at entry
ADD.s F0, F1,F2                 ; add without FTZ
IOR.s #0x0400, FCR              ; set FCR.FTZ
SUB.s F3, F4,F5                 ; sub with FTZ
AND.s #0xFBFF, FCR              ; clear FCR.FTZADD.s F2,F6,F7                  ;add without FTZ