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Section 3 Floating-Point Unit (FPU)
R01UH0134EJ0400 Rev. 4.00 Page 105 of 2108
Sep 24, 2014
SH7262 Group, SH7264 Group
3.4 Rounding
In a floating-point instruction, rounding is performed when generating the final operation result
from the intermediate result. Therefore, the result of combination instructions such as FMAC will
differ from the result when using a basic instruction such as FADD, FSUB, or FMUL. Rounding is
performed once in FMAC, but twice in FADD, FSUB, and FMUL.
Which of the two rounding methods is to be used is determined by the RM bits in FPSCR.
FPSCR.RM[1:0] = 00: Round to Nearest
FPSCR.RM[1:0] = 01: Round to Zero
(1) Round to Nearest
The operation result is rounded to the nearest expressible value. If there are two nearest
expressible values, the one with an LSB of 0 is selected.
If the unrounded value is 2
Emax
(2 – 2
–P
) or more, the result will be infinity with the same sign as the
unrounded value. The values of Emax and P, respectively, are 127 and 24 for single-precision, and
1023 and 53 for double-precision.
(2) Round to Zero
The digits below the round bit of the unrounded value are discarded.
If the unrounded value is larger than the maximum expressible absolute value, the value will
become the maximum expressible absolute value.