80C187
270640–4
Precision Control
00Ð 24 Bits (Single Precision)
01Ð (Reserved)
10Ð 53 Bits (Double Precision)
11Ð 64 Bits (Extended Precision)
Rounding Control
00Ð Round to Nearest or Even
01Ð Round Down (toward
b%
)
10Ð Round Up (toward
a %
)
11Ð Chop (Truncate toward Zero)
*The ‘‘infinity control’’ bit is not meaningful to the 80C187. To maintain compatibility with the 8087, this bit can be
programmed; however, regardless of its value, the 80C187 treats infinity in the affine sense (
b%
k
a %
).
Figure 5. Control Word
15 7 0
CONTROL WORD
a
0
STATUS WORD
a
2
TAG WORD
a
4
INSTRUCTION POINTER
15..0
a
6
IP
19..16
0 OPCODE
10..0
a
8
OPERAND POINTER
15..0
a
A
OP
19..16
000000000000
a
C
Figure 6. Instruction and Data Pointer Image in Memory
10
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