3.6.8.8.2 Subnormals Operands

The FPU supports a Subnormal Operand Override mode, Subnormals-Are-Zero (SAZ), and its functionality is defined in Subnormal Operand Exception . The Subnormals-Are-Zero (SAZ) mode is enabled when FCR.SAZ is set.

If a subnormal operand is detected when SAZ mode is disabled, the subnormal exception will be signaled by setting FSR.SUBO (and FSR.SUBOS if not already set) during the WB-stage (i.e., at the same time as when all other exceptions are signaled). If SAZ mode is enabled, the subnormal exception will not be signaled.

Note: SAZ 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. In addition, SAZ mode is ignored by FTST such that a subnormal operand will always be recognized as such by the instruction, irrespective of the state of FCR.SAZ. However, SAZ mode can influence FF2LI/FF2DI operands. In these cases, subnormal or zero operands will write the same result (integer value of 0). But if the operand is subnormal and SAZ mode is disabled, a subnormal exception will also be signaled. Conversely, if the operand is subnormal and SAZ mode is enabled, a subnormal exception will not be signaled.