ENABLE byte are organized in decreasing priority, left to right, i.e., BSUN is the highest
priority, and INEXl is the lowest priority. The only multiple exception possibilities are:
SNAN and INEXl
OPERR and INEX2
OPERR and INEXl
OVFL and INEX2 and/or INEXl
UNFL and INEX2 and/or INEXl
INEX2 AND INEXl
2.2.2 FPCR Mode Control Byte
The mode control byte (MODE), shown in Figure 2-3, controls the user-selectable rounding
modes and rounding precisions. A zero in this byte selects the IEEE defaults.
The rounding mode specifies how inexact results are rounded. "Round to the nearest"
specifies that the nearest number to the infinitely precise result should be selected as the
rounded value. In case of a tie, the even result is selected. "Round towards zero" truncates
the result. "Round towards plus infinity" always rounds numbers towards plus infinity.
"Round toward minus infinity" always rounds numbers towards minus infinity. See 6.1.7
Inexact Result for a detailed description of the rounding algorithm used.
The rounding precision selects where rounding of the mantissa occurs. For extended pre-
cision, the result is rounded to a 64-bit boundary. A single precision result is rounded to
a 24-bit boundary, and a double precision result is rounded to a 53-bit boundary.
The single and double rounding precisions are provided for emulation of machines that
only support those precisions. When the FPCP performs any operation, the calculation is
carried out using extended precision inputs and the intermediate result is calculated as if
to produce infinite precision. After the calculation is complete, this intermediate result is
rounded to the selected precision and stored in the destination.
if the destination is a floating-point data register, the stored value is in the extended
precision format rounded to the precision specified by the PREC bits. Thus, all mantissa
bits beyond the selected precision are zero after the rounding operation. Also, if the single
7 G 5 4 3 2 1 0
1
ROUNDING MOOE:
OQ TQ NEAREST
01 TOWARO ZERO
10 TOWARD MINUS INFINITY
11 TOWARD PLUS INFINITY
ROUNDING PRECISION:
O0 EXTENDED
01 SINGLE
10 DOUBLE
1 ! (UNDEFINEO. RESERVED)
Figure 2-3. MC68881/MC68882 FPCR Mode Control Byte
MC68881/MC68882 USER'S MANUAL FREESCALE
2-3
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